A
absence seizure
A seizure characterized by a brief lapse of consciousness (of about 10 seconds or less) and a cessation of motor activity without loss of posture.
accessory olfactory bulb
A brain region adjacent to the olfactory bulb; it is the axonal projection target of sensory neurons from the vomeronasal organ. (Figure 6-19)
accessory olfactory system (vomeronasal system)
An anatomically and biochemically distinct system from the main olfactory system; it detects and analyzes nonvolatile chemicals and peptides such as pheromones and cues from predators. (Figure 6-19)
acetylcholine (ACh)
The first discovered neurotransmitter; it is used by vertebrate motor neurons at the neuromuscular junction and autonomic nervous system. It is also used in the CNS as an excitatory or modulatory neurotransmitter. In some invertebrates, such as Drosophila, it is the major excitatory neurotransmitter in the CNS. (Figure 3-1; Table 3-2)
acetylcholine receptor (AChR)
Receptor for the neurotransmitter acetylcholine. Nicotinic AChRs (nAChRs) are nonselective cation channels; they are the postsynaptic receptor at the vertebrate neuromuscular junction and function as excitatory receptors at some CNS synapses. Metabotropic AChRs (muscarinic AChRs or mAChRs) are G-protein-coupled receptors that play a modulatory role. (Figure 3-20 for nAChR)
acetylcholinesterase
An enzyme that degrades acetylcholine, enriched in the cholinergic synaptic cleft.
acquisition (of memory)
The initial formation of a memory as a consequence of experience and learning.
action potential
An elementary unit of nerve impulses that axons use to convey information across long distances. It is all-or-none, regenerative, and propagates unidirectionally in an axon. It is also called a spike. (Figure 2-18; Figure 2-19)
active electrical property
A membrane property due to voltage-dependent changes in ion conductances. It can reduce or eliminate attenuation of electrical signals across a distance that occurs due to passive electrical properties.
active transport
Movement of a solute across a membrane against its electrochemical gradient via a transporter that uses external energy, such as ATP hydrolysis, light, or movement of another solute down its electrochemical gradient. (Figure 2-10)
active zone
An electron-dense region of the presynaptic terminal containing clusters of synaptic vesicles docked at the presynaptic membrane, ready for release. (Figure 3-3; Figure 3-10)
activity-dependent transcription
The process by which neuronal activity regulates gene expression.
adaptation (in evolution)
Genetic or phenotypic changes that render an individual and its progeny more likely to survive and reproduce in a particular environment.
adaptation (in sensory systems)
An adjustment of a system’s sensitivity according to background levels of sensory input.
adaptive (in evolution)
Adjective for adaptation (in evolution).
adeno-associated virus (AAV)
A DNA virus widely used to deliver transgenes into postmitotic neurons. It has a capacity to carry up to about 5 kb of foreign DNA. (Table 14-1)
adenylate cyclase
A membrane-associated enzyme that synthesizes cyclic AMP (cAMP) from ATP. (Figure 3-33)
adult neurogenesis
The production of new neurons in the adult brain. This occurs in song-production nuclei in the songbird and in limited regions of the mammalian brain.
afferent
An axon that projects from peripheral tissue into the CNS. It can also be generalized to describe an input axon to a particular neural center within the CNS.
agonist
A molecule that activates a biological process by interacting with a receptor, often mimicking the action of an endogenous molecule.
agrin
A protein secreted by motor neurons that induces aggregation of acetylcholine receptors in the skeletal muscle. (Figure 7-25)
AgRP neuron
A neuron in the hypothalamic arcuate nucleus that releases the orexigenic peptides agouti-related protein (AgRP) and neuropeptide Y. (Figure 9-14)
AII amacrine cell
A type of amacrine cell that links rod bipolars to the pathways that process cone signals. (Figure 4-35)
AKAP (A kinase anchoring protein)
An anchoring protein associated with protein kinase A.
allele
A specific version of a gene.
allelic exclusion
A phenomenon in which mRNAs of a gene are transcribed exclusively from one chromosome of a homologous pair. See also allele.
allodynia
A phenomenon wherein gentle touch or innocuous temperature causes pain when applied to inflamed or injured tissue. (Figure 6-72)
all-or-none
Having the property of being binary in occurrence. It applies to action potentials, which have the same amplitude and waveform regardless of the strength of the inducing stimulus, as long as the stimulus is above threshold.
allosteric agonist
A molecule that facilitates binding of an endogenous ligand to its receptor. An allosteric agonist binds to a site on a receptor that is different from the site that binds the endogenous ligand.
Alzheimer’s disease (AD)
A neurodegenerative disorder prevalent in the aging population; it is defined by the combined presence of abundant amyloid plaques and neurofibrillary tangles in postmortem brains, with symptoms including gradual loss of memory, impaired cognitive and intellectual capabilities, and reduced ability to cope with daily life.
amacrine cell
An inhibitory retinal neuron whose actions influence the signals transmitted from bipolar cells to retinal ganglion cells. (Figure 4-25)
AMPA receptor
A glutamate-gated ion channel that conducts mostly Na+ and K+ and can be selectively activated by the drug AMPA (2-amino-3-hydroxy-5-methylisoxazol-4-propanoic acid). It is a usually heterotetramer containing two or more kinds of subunits (GluA1, GluA2, GluA3, and GluA4) encoded by four genes. (Figure 3-24; Figure 3-25)
amygdala
An almond-shaped structure underneath the temporal lobe best studied for its role in processing emotion-related information. (Figure 1-8; Figure 11-42)
amyloid
β (Aβ) hypothesis The hypothesis that an increase in amyloid β (Αβ) protein production or accumulation is a common cause of Alzheimer’s disease.
amyloid plaque
An extracellular deposit consisting primarily of aggregates of amyloid β protein. (Figure 12-2)
amyloid precursor protein (APP)
A single-pass transmembrane protein from which the amyloid β protein is derived by proteolytic processing. (Figure 12-3)
amyloid
β protein (Αβ) A major component of amyloid plaques in Alzheimer’s disease; it is a 39- to 43-amino-acid peptide with a strong tendency to form aggregates rich in β-pleated sheets. (Figure 12-4)
amyotrophic lateral sclerosis (ALS)
A rapidly progressing motor neuron disease that is usually terminal within a few years after symptoms emerge. It is also known as Lou Gehrig’s disease.
analog signaling
Signaling that uses continuous values to represent information.
androgen
A male sex hormone, such as testosterone.
androgen receptor
A cytosolic protein that, upon binding of an androgen such as testosterone, translocates to the nucleus, where it acts as a transcription factor. (Figure 10-19)
anions
Negatively charged ions such as Cl–.
anosmic
Unable to perceive odors.
antagonist
A molecule that counters the action of an endogenous molecule.
antagonistic muscles
Muscles that perform opposite actions, such as an extensor and a flexor that control the same joint. (Figure 8-8)
antennal lobe
The first olfactory processing center in the insect brain. (Figure 6-24)
anterior cingulate cortex (ACC)
A neocortical area located near the midline of the frontal lobe. It has extensive connections with the hippocampus and is implicated in long-term memory storage.
anterior pituitary
See pituitary.
anterior–posterior
See rostral–caudal.
anterograde
From the cell body to the axon terminal.
anterograde tracer
A molecule used to trace axonal connections; it is taken up primarily by neuronal cell bodies and dendrites and travels down axons to label projection sites. (Figure 14-30)
anterolateral column pathway
An axonal pathway from the spinal cord to the brainstem; it consists of axons from lamina I dorsal horn projection neurons on the contralateral side of the spinal cord. It mainly relays pain, itch, and temperature signals to the brain. (Figure 6-71)
anteroventral periventricular nucleus (AVPV)
A hypothalamic nucleus in the preoptic area that plays a pivotal role in regulating the female ovulatory cycle.
antidromic spike
An action potential that propagates from the axon terminal to the cell body in artificial situations in which experimenters electrically stimulate an axon or its terminal.
antiporter
A coupled transporter that moves two or more solutes in opposite directions. Also called an exchanger. (Figure 2-10)
antisense oligonucleotide (ASO)
Short nucleotide that binds to an endogenous target RNA with a complementary sequence, leading to functional inhibition or degradation of the target RNA.
anxiety disorders
A group of psychiatric disorders that includes generalized anxiety disorders (characterized by persistent worries about impending misfortunes), phobias and panic disorders (characterized by irrational fears), and obsessive–compulsive disorder.
AP5 (2-amino-5-phosphonovaleric acid)
A widely used selective NMDA receptor antagonist.
apolipoprotein E (ApoE)
A high-density lipoprotein in the brain involved in lipid transport and metabolism. A specific polymorphic isoform (ε4) is a major risk factor for Alzheimer’s disease.
apoptosis
A form of cell death in which a cell kills itself by initiating a cell-death program.
Arc
A cytoskeletal protein present at the postsynaptic density that regulates trafficking of glutamate receptors. It is a product of the immediate early gene Arc.
archaerhodopsin
A light-activated outward proton pump in archaea; it can be used to silence neuronal activity in a heterologous system by light. See also optogenetics.
arcuate nucleus
A ventromedial hypothalamic nucleus that regulates food intake and energy expenditure. (Figure 9-6)
area X
A basal ganglia structure in the songbird essential for song learning. (Figure 10-16)
aromatase
An intracellular enzyme that converts testosterone to estradiol. (Figure 10-19)
arrestin
A protein that binds to a phosphorylated G-protein-coupled receptor and competes with the receptor’s binding to Gα proteins. It can therefore terminate GPCR signaling. It can also transduce signals of its own.
ascending arousal system
A neural system consisting of parallel projections from the brainstem and hypothalamus to the forebrain that are essential for maintaining wakefulness. It includes cholinergic projections from the tegmental nuclei, norepinephrine projections from the locus coeruleus, serotonin projections from the raphe nuclei, histamine projections from the tuberomammillary nucleus, and hypocretin projections from the lateral hypothalamus. (Figure 9-29)
association cortex
Cortical areas that integrate information from multiple sensory areas and link sensory systems to motor output.
associative learning
A type of learning involving formation of an association between two events, such as formation of an association between an unconditioned stimulus and a conditioned stimulus in classical conditioning or formation of an association between a behavior and a reinforcer in operant conditioning.
associativity (of LTP)
A property of long-term potentiation (LTP) whereby activation of a synapse that alone would be too weak to produce LTP can nonetheless lead to LTP if it coincides with the strong, LTP-inducing activation of a different synapse onto the same postsynaptic cell. (Figure 11-8)
astrocyte
A glial cell present in gray matter. It plays many roles, including in synaptic development and function. (Figure 1-9)
asymmetric cell division
A cell division in which the two daughter cells are of different types from birth.
ataxia
An abnormality in coordinated muscle contraction and movement.
attention
A cognitive function in which a subset of sensory information is subjected to more processing at the expense of other information.
attractant
A molecular cue that guides axons toward its source. (Figure 5-11)
attractor
A state in a Hopfield network in which the “energy” function is minimized. It also refers to a stable state in a generic neural network to which neural dynamics converge. (Figure 14-63)
auditory cortex
The part of the cerebral cortex that analyzes auditory signals. It is located in the temporal lobe. (Figure 1-23)
auditory fear conditioning
A classical conditioning procedure in which aversive, fear-inducing stimuli, such as electric shocks, are paired with sound stimuli during training; animals subsequently exhibit fear responses, such as freezing, in response to sound stimuli alone.
auditory nerve
A bundle of axons from spiral ganglion neurons that transmits auditory information to the brainstem. It also contains efferents from the brainstem that synapse primarily onto outer hair cells. (Figure 6-47)
auricle
The external part of the mammalian ear; it collects and focuses sound through the ear canal on the ear drum.
autism spectrum disorder (ASD)
A class of neurodevelopmental disorders characterized by deficits in reciprocal social interactions. Patients also exhibit restricted interests and repetitive behaviors.
autocrine
Of or related to a form of signaling in which a recipient cell receives a signal produced by itself.
autonomic nervous system
Collection of parts of the nervous system that regulate the function of internal organs, including the contraction of smooth and cardiac muscles, as well as secretion and excretion from glands.
autophagy
An ordered cellular process that removes unnecessary or damaged cellular components through the lysosomal pathway.
autosomal dominant
Of a mutation, having a Mendelian inheritance pattern in which mutation of only one allele of a gene located on an autosome is sufficient to produce a phenotype. It can result from a toxic gain-of-function effect of the mutant allele or a loss-of-function effect due to an insufficient amount of the normal gene product being produced by the remaining wild-type allele. (Figure 12-33)
autosomal recessive
Of a mutation, having a Mendelian inheritance pattern in which mutation of both alleles of a gene located on an autosome is required to produce a phenotype. It usually results from a loss-of-function effect of the mutation. (Figure 12-33)
autosome
A non-sex chromosome.
axon
A long, thin process of a neuron; it often extends far beyond the soma and propagates and transmits signals to other neurons or muscle at its presynaptic terminals. (Figure 1-9)
axon guidance molecules
Extracellular cues and cell surface receptors that guide axons along their paths toward the appropriate targets. (Figure 5-11)
axon hillock
See axon initial segment
axon initial segment (axon hillock)
The segment of the axon closest to the neuronal cell body; it is usually the site of action potential initiation.
axon myelination
The process in which glial cells wrap their cytoplasmic extensions around axons to increase conduction velocity. (Figure 2-27)
B
bacteriorhodopsin
A light-driven proton pump in archaea.
ball-and-chain
A model of voltage-gated channel inactivation in which a cytoplasmic portion of the channel protein (“ball”), connected to the rest of the channel by a polypeptide chain, blocks the channel pore after the ion channel opens. (Figure 2-31)
barrel
A discrete anatomical unit in layer 4 of the rodent primary somatosensory cortex that represents a whisker. The cortical region containing barrels for all whiskers is called the barrel cortex. The corresponding discrete units in the brainstem and thalamus are called barrelettes and barreloids, respectively. (Figure 5-26)
barrel cortex
See barrel.
basal ganglia
A collection of nuclei underneath the cerebral cortex; it includes the striatum, globus pallidus, subthalamic nucleus, and substantia nigra and is essential for motor initiation and control, habit formation, and reward-based learning. (Figure 1-8; Figure 8-21)
basilar membrane
An elastic membrane at the base of hair cells in the cochlea. (Figure 6-43)
basket cell
A type of GABAergic neuron; it wraps its axon terminals around the cell bodies of pyramidal cells in the cerebral cortex and Purkinje cells in the cerebellar cortex. (Figure 1-15; Figure 3-46)
basolateral amygdala
A brain region consisting of lateral and basal amygdala. It receives input from the thalamus, cortex, and hippocampus and sends output to the central amygdala and other brain regions. It is involved in regulating emotion-related behavior. (Figure 11-42)
battery
An electrical element that maintains a constant voltage, or electrical potential difference, across its two terminals and that can thus serve as an energy source. (Figure 2-13)
Bayes’ rule
The relationship between conditional probabilities that event A occurs given B is true and event B occurs given A is true, expressed as: P(A|B) = P(B|A) × P(A) / P(B). See also conditional probability.
BDNF (brain-derived neurotrophic factor)
See neurotrophins and Trk receptors.
bed nucleus of the stria terminalis (BNST)
A sexually dimorphic brain region that receives direct input from accessory olfactory bulb mitral cells. Its diverse functions include regulation of male courtship behavior. (Figure 10-26)
benzodiazepines
A class of drugs that act as allosteric agonists of GABAa receptors. They are widely used to treat anxiety, pain, epilepsy, and sleep problems. (Figure 12-27)
biased random walk
A strategy employed in bacteria chemotaxis. When swimming away from an attractant, bacteria exhibit frequent tumbles (reorientation); when swimming toward an attractant, they tumble less frequently. It is also employed by C. elegans for chemotaxis. (Figure 13-15)
bilaterians
Animals that are bilaterally symmetrical and have three germ layers. They include all vertebrates and most invertebrate species alive today. (Figure 13-2)
binary classification
A task in which observed events are classified into one of two classes. (Figure 14-59)
binary expression
Expression of a transgene using a strategy in which the cis-regulatory elements and coding sequence are separated into two transgenes. (Figure 14-12)
binocular vision
A form of vision involving integration of inputs from the two eyes that carry information about the same visual field location. It is important for depth perception.
binomial distribution
A discrete probability distribution that describes the frequency (f) with which k events occur in n independent trials, given the probability p that an event occurs in each trial. f(k; n, p) = [n!/k!(n – k)!] pk (1 – p)n – k. (Box 3-1)
biomarker
A measurable characteristic indicator of normal biological processes, pathogenic processes, and responses to therapeutic interventions. (Figure 12-11)
bipolar (neuron)
Having two processes leaving the cell body.
bipolar cell (in retina)
An excitatory neuron that transmits information from photoreceptors to retinal ganglion cells and amacrine cells. (Figure 4-22; Figure 4-25)
bipolar disorder
A mood disorder in which patients alternate between manic phases characterized by feelings of grandiosity and tirelessness, and depressive phases characterized by feelings of sadness, emptiness, and worthlessness.
bitter
A taste modality that functions primarily to warn the animal of potential toxic chemicals. It is usually aversive.
blastula
The product of cleavage; it is an early-stage embryo consisting of a hollow ball of thousands of cells. (Figure 7-2)
blood–brain barrier (BBB)
Derived from endothelial cell tight junctions in the blood vessels of the brain, it prevents the exchange of many substances between the blood and brain tissues.
blue-ON bipolar cell
An ON bipolar cell that selectively connects with S-cones; it is activated by short-wavelength light and inhibited by longer-wavelength light. (Figure 4-34)
bone morphogenetic proteins (BMPs)
A family of secreted proteins that act as morphogens to pattern embryonic tissues, such as the tissues along the anterior–posterior axis of the telencephalon and the dorsal–ventral axis of the spinal cord.
border cell
A cell in the entorhinal cortex that fires when an animal is at a specific edge of an arena.
Boss
(Bride of sevenless) Originally identified from a mutation in Drosophila that lacks photoreceptor R7, it is a gene that acts cell nonautonomously in R8 to specify R7 fate. It encodes a transmembrane ligand for the Sevenless receptor tyrosine kinase. (Figure 5-35)
botulinum toxins
A family of proteases produced by Clostridium botulinum. Different isoforms cleave synaptobrevin, syntaxin, or SNAP-25 at distinct sites. (Box 3-2)
bradykinin
A peptide released during inflammation; it binds to specific G-protein-coupled receptors on the peripheral terminals of nociceptive neurons. (Figure 6-73)
brain
The rostral part of the central nervous system located in the head. It is the command center for most nervous system functions. (Figure 1-8)
brain slice
A fresh section of brain tissue (usually about a few hundred micrometers thick) that largely preserves the three-dimensional architecture for physiological studies of neuronal and local circuit properties in vitro.
brainstem
A structure made up of the midbrain, pons, and medulla. (Figure 1-8)
Broca’s area
An area in the left frontal lobe involved in language production. Patients with lesions in this area have difficulty speaking. (Figure 1-23)
α-bungarotoxln
A snake toxin from the venom of Bungarus that is a competitive inhibitor of the nicotinic acetylcholine receptor.
C
Ca2+ indicator
A molecule whose optical properties depend on the intracellular Ca2+ concentration. Ca2+ indicators are used as optical sensors of neuronal activity. (Figure 14-41)
CA3→ CA1 synapse
The synapse between the axons of the Schaffer collaterals of hippocampal CA3 neurons and the dendrites of CA1 neurons. It is a model synapse for investigating mechanisms of synaptic plasticity.
cable properties
See passive electrical properties.
cadherin
A Ca2+-dependent homophilic cell-adhesion protein.
calmodulin (CaM)
A Ca2+-binding protein that transduces Ca2+ signals to many effectors. (Figure 3-34)
calyx of Held
A giant synapse made by a presynaptic terminal of a ventral cochlear globular bushy cell wrapping around the cell body of a medial nucleus of the trapezoid body principal cell. It contains hundreds of neurotransmitter release sites and is responsible for rapid and reliable transmission of auditory signals in the mammalian brainstem sound localization system. (Figure 6-56)
Cambrian
A geological period between 542 and 488 million years ago when major phyla within the animal kingdom diversified, as evidenced by an abundance of corresponding fossils. (Figure 13-2)
CaM kinase II (CaMKII)
A Ca2+/calmodulin-dependent serine/threonine kinase that is highly enriched in the postsynaptic densities of excitatory synapses and that regulates synaptic plasticity, such as long-term potentiation. (Figure 3-34; Figure 11-11)
cAMP-dependent protein kinase
A serine/threonine kinase made up of two regulatory and two catalytic subunits. Binding of cAMP to the regulatory subunits leads to dissociation of the catalytic subunits, which can then phosphorylate their substrates. It is also called A kinase, protein kinase A, or PKA. (Figure 3-33)
capacitance (C)
The ability of a capacitor to store charge; it is defined as C = Q/V, where Q is the electric charge stored when the voltage across the capacitor is V.
capacitor
An electrical element composed of two parallel conductors separated by a layer of insulator. It is a charge-storing device. (Figure 2-13)
Capricious
A Drosophila transmembrane protein that contains extracellular leucine-rich repeats and instructs wiring specificity of axons and dendrites. (Figure 5-38; Figure 7-44)
cardiac muscle
Muscle that controls heartbeat.
Cas9
A key protein in the type II CRISPR system; it is an RNA-guided endonuclease containing two separate nuclease domains that generate double-strand breaks in DNA complementary to a bound RNA. It is used by bacteria for adaptive immunity and experimentally for genome editing. See also CRISPR and guide RNA. (Figure 14-8)
caspases
Proteases best known for triggering apoptosis, a form of programmed cell death.
castrated male
A male from which the testes have been removed.
catecholamines
A class of chemicals that includes the neurotransmitters dopamine, norepinephrine, and epinephrine. (Figure 12-20)
cations
Positively charged ions such as K+ and Na+.
CB1
A G-protein-coupled receptor originally identified as the receptor for cannabinoids from the marijuana plant. It serves as a receptor for endocannabinoids under physiological conditions.
CCK (cholecystokinin)
A neuropeptide produced in the small intestine in response to a rise in fatty acid concentrations. It acts as a satiety signal to inhibit eating. (Figure 9-15)
cDNA library
A collection of cloned complementary DNAs synthesized from mRNA templates prepared from a specific tissue or cell type.
cell adhesion molecule
A cell-surface protein that binds to its partners in opposing cells or the extracellular matrix to facilitate cell–cell or cell–matrix adhesion.
cell assembly
A group of interconnected neurons whose firing patterns collectively encode information, such as the location of an animal in an environment.
cell-attached patch recording (cell-attached recording)
A variant of the patch clamp recording method in which the patch pipette forms a high resistance (gigaohm) seal with the plasma membrane of an intact cell, allowing measurement of ion flow through a small number of channels or a single channel in the patch of membrane underneath the electrode. (Figure 2-29; Figure 14-40)
cell autonomous
Of a gene, acting in the cell that produces the gene product.
cell fate
The outcome of the developmental decision as to what type of cell it is.
cell lineage
The developmental history of a cell, comprising the identities of the progenitors from which a cell was derived.
cell nonautonomous
Of a gene, acting on a cell that does not produce the gene product.
cell-replacement therapy
A treatment strategy in which cells differentiated in vitro are transplanted into the body to replace dying cells, such as dopamine neurons in Parkinson’s disease.
cell-surface receptor
A membrane protein that binds to extracellular ligands and subsequently sends a signal into the recipient cell. (Figure 3-38)
cell theory
The idea that all living organisms are composed of cells as their basic units.
center–surround (receptive field)
A property of a visual system neuron’s receptive field, in which light in the receptive field center and light just outside of the receptive field center are antagonistic. (Figure 4-20)
central amygdala
The output nucleus of the amygdala complex; it receives input from the basolateral amygdala and sends GABAergic output to brainstem nuclei, the autonomic nervous system, the hypothalamus, and neuromodulatory systems to regulate diverse physiological functions, including emotion-related behavior. (Figure 11-42; Figure 11-45)
central dogma
The principle that genetic information flows from DNA to RNA to protein. (Figure 2-2)
central pattern generator (CPG)
A CNS circuit capable of producing rhythmic output for coordinated contraction of different muscles without sensory feedback. (Figure 8-12; Figure 8-13)
cerebellar nuclei
The output nuclei of the cerebellum; they receive input from Purkinje cell axons and the collaterals of mossy and climbing fibers, and send information to diverse regions including the thalamus and brainstem. (Figure 8-26)
cerebellum
A structure located dorsal to the pons and medulla that plays an important role in motor coordination, motor learning, and cognitive functions. (Figure 1-8; Figure 8-26)
cerebral cortex
The outer layer of the neural tissue in the rostral part of the mammalian brain. It is associated with higher functions, including sensory perception, control of voluntary movement, and cognition. (Figure 1-8; Figure 1-23)
CGRP (calcitonin gene-related peptide)
A peptide that promotes inflammation when released by the peripheral terminals of sensory neurons. It is also produced in the CNS, for example in a subset the parabrachial nucleus neurons activated by threat and dietary satiety. (Figure 6-73; Figure 9-15)
chandelier cell
A type of GABAergic neuron in the cerebral cortex that forms synapses onto the axon initial segments of cortical pyramidal cells. (Figure 3-46)
channel
A transmembrane protein or protein complex that forms an aqueous pore, allowing specific solutes to pass directly through when it is open. (Figure 2-8)
channelopathies
Diseases caused by mutations in ion channels.
channelrhodopsin
A member of a class of light-activated cation channels in single-celled green algae used for chemotaxis. See also channelrhodopsin-2 (ChR2). (Figure 13-21)
channelrhodopsin-2 (ChR2)
A light-activated cation channel from single-celled green algae; it is widely used to activate neurons in heterologous systems by light. See also optogenetics. (Figure 13-21; Figure 14-47)
characteristic frequency
The sound frequency to which a given cell in the auditory system is most sensitive. (Figure 6-47)
Charcot–Marie–Tooth (CMT) disease
A PNS demyelinating disease characterized by progressive deficits in sensation and movement that preferentially affects neurons with longer axons.
chemical gradient
A concentration difference of a solute over two sides of a membrane, which contributes to the direction and magnitude of solute movement across the membrane. If the solute is not charged, the chemical gradient alone determines the movement direction: from higher to lower concentration. (Figure 2-9)
chemical synapse
A specialized junction between two neurons or a neuron and a muscle cell where communication between cells occurs via neurotransmitter release and reception. It comprises a presynaptic terminal and a postsynaptic specialization separated by a synaptic cleft. (Figure 1-14; Figure 3-3)
chemoaffinity hypothesis
Proposed by Roger Sperry, it posits that growing axons use cell-surface molecules to determine their paths and connect to appropriate synaptic partners.
chemogenetics
An approach using chemicals to activate or silence neurons expressing receptors specifically engineered to be sensitive to those chemicals. (Figure 14-44)
chemotaxis
Movement toward or away from a chemical source.
chlorpromazine
A first-generation antipsychotic drug; it is an antagonist of the D2 dopamine receptor.
chordates
Animals with a notochord. (Figure 13-2)
chromophore
The light-absorbing portion of a molecule.
ciliary type
A type of photoreceptor in which opsins are packed into the primary cilium-derived outer segment. (Figure 13-22)
circadian pacemaker neuron
A neuron whose activity in isolation oscillates in a circadian fashion (i.e., with a period of close to 24 hours).
circadian rhythms
Self-sustained oscillations in an organism’s behavior, physiology, and biochemistry, with a period of close to 24 hours.
circuit motif
A common configuration of a neural circuit allowing the connection patterns of individual neurons to execute specific functions. (Box 1-2)
cis-regulatory element
A DNA element, such as a transcriptional enhancer, repressor, or insulator, that regulates expression of genes on the same chromosome.
clade
A branch in the tree of life, consisting of an ancestor species and all of its descendant species.
cladistic analysis
The study of the emergence and change of traits of organisms in the context of their phylogenetic relationships.
classical conditioning
A form of learning in which repeated pairing of a conditioned stimulus (CS) with an unconditioned stimulus (US) causes a subject to exhibit a novel conditioned response (CR) to the CS. It is also called Pavlovian conditioning. (Figure 11-21)
Cl– channel
An ion channel that allows selective passage of Cl–.
cleavage
A series of rapid cell divisions in early embryogenesis that converts a single large zygote cell into thousands of smaller cells. (Figure 7-2)
climbing fiber
An axon that climbs the dendritic trees of Purkinje cells and originates from a neuron in the inferior olive. (Figure 8-26)
Clock
A gene identified from a forward genetic screen in mice for mutations that produce circadian rhythm deficit; it encodes a transcriptional activator, CLOCK, that positively regulates expression of genes whose products feedback to negatively regulate CLOCK function. Its fly homolog serves a similar function. (Figure 9-21)
clonal analysis
A method of analyzing the relationships between cells by lineage; it involves labeling a progenitor such that all of its progeny are also labeled.
closed-loop
Of a system, having input to the system that is modified by the output of the system. In the context of a behavioral paradigm, having environmental stimuli that induce a behavior in an animal to also change in response to the animal’s behavior.
cluster analysis
A form of unsupervised learning that divides a data set into subsets based on similarities in their key feature(s).
cnidarians
Radially symmetric animals such as hydra, jellyfish, and corals. (Figure 13-2)
CNS (central nervous system)
The brain and spinal cord in vertebrates; the brain and nerve cord in some invertebrates.
cochlea
A coiled structure in the inner ear containing fluid-filled chambers and the organ of Corti. (Figure 6-43)
cochlear nuclei
Brainstem nuclei at the termination of the auditory nerve, subdivided into the dorsal and ventral cochlear nuclei. (Figure 6-52)
coding space
A theoretical space used to describe the activity of a neuronal population. The firing rate of each neuron in the population constitutes one dimension/axis in this space, and the activity state of the entire population is represented as a point in this space. (Figure 6-27)
cognitive learning
A theory of learning that emphasizes learning as acquisition of new knowledge rather than just modification of behavior.
coincidence detector (in auditory system)
A cell maximally activated by simultaneous auditory signals from the left and right ears. (Figure 6-53)
coincidence detector (in synaptic transmission)
A receptor that opens only in response to concurrent neurotransmitter binding and postsynaptic depolarization, such as the NMDA receptor.
collateral
An axon branch.
color contrast (in vision)
The difference in light wavelengths between adjacent spaces.
color-opponent RGC
A retinal ganglion cell that differentiates signals from cones with distinct spectral sensitivities. The blue–yellow opponent RGC (in all mammals) differentiates short- and longer-wavelength light signals; the green–red opponent RGC (in trichromatic primates) differentiates two long-wavelength light signals. (Figure 4-34)
Comm (Commissureless)
A Drosophila protein that acts in the secretory pathway to downregulate cell-surface expression of Robo during midline guidance. (Figure 7-13)
commissural neuron
A neuron that projects its axon to the contralateral side of the body. Midline crossing of commissural neurons has been used as a model system for studying axon guidance.
compact myelin
Closely packed layers of glial plasma membranes wrapped around axons.
complement cascade
Part of an innate immune system that “complements” the ability of antibodies and phagocytic cells to clear pathogens and damaged cells from the organism.
complex cell
A functionally defined primary visual cortex neuron type. It has no mutually antagonistic ON and OFF regions and is excited by light bars on a dark background or dark bars on an illuminated background. The stimulus bars must be in a specific orientation but can fall on any part of the receptive field. (Figure 4-42)
conditional knockout
The process of disrupting a gene in a specific spatiotemporal pattern or an animal in which a gene has been disrupted in a specific spatiotemporal pattern. The most common strategy for generating conditional knockouts in mice utilizes Cre/loxP-based recombination. It usually involves inserting a pair of loxP elements into introns flanking (an) essential exon(s) of a gene of interest. The gene of interest is only disrupted in cells in which Cre has been active. (Figure 14-9)
conditional probability
The likelihood that event A occurs given that B is true, expressed as P(A|B). See also Bayes’ rule.
conditioned response (CR)
See classical conditioning.
conditioned stimulus (CS)
See classical conditioning.
conductance (g)
The degree to which an object or substance passes electricity; it is the inverse of resistance: g = 1/R.
conductor
An object or substance that passes electric current.
cone
A cone-shaped photoreceptor in the vertebrate retina; it contributes to high acuity, motion, and color vision. (Figure 4-2)
confocal fluorescence microscopy (confocal microscopy)
A fluorescence microscopy technique in which a detector pinhole is used to collect fluorescence emission originating only from a focal spot restricted in all three dimensions. By scanning the laser across a plane to record fluorescence emission from many focal spots, it can produce a thin optical section of whole-mount tissue or a thick tissue section. (Figure 14-23)
connectome
A representation of the complete set of synaptic connections among a group of neurons of interest. (Figure 7-30; Figure 14-2)
connexin
A protein component of gap junctions in vertebrates. (Figure 3-48)
consolidation (of memory)
A step in the process of memory formation that occurs between acquisition and storage, during which a newly acquired memory is stabilized for long-term use.
contextual fear conditioning
A learning procedure in which a rodent is subjected to aversive fear-inducing stimuli, such as electric shocks, in a specific environment (context). When subsequently placed in the same context, the animal will exhibit a fear response, such as freezing. It requires both the hippocampus and amygdala.
continuous map
A type of neural map in which nearby neurons in the input field connect with nearby neurons in the target field, as exemplified by the relationship between the retina and the tectum. (Figure 7-36)
contralateral
Of the other side of the midline. For example, a contralateral axonal projection is an axon that crosses the midline and terminates on the side of the nervous system opposite the soma.
convergent evolution
The independent evolution of similar features in animals of different clades.
convergent excitation
A circuit motif in which several excitatory neurons synapse onto the same postsynaptic neuron. (Figure 1-20)
convolutional neural network (CNN)
A class of deep neural network that exhibits space invariant properties; it was originally developed for image analysis.
cooperativity (of LTP)
A property of long-term potentiation (LTP) whereby LTP can be induced at a synapse if the presynaptic cell releases neurotransmitter while the postsynaptic cell is in a depolarized state, even if transmitter release from the presynaptic cell alone (in the absence of postsynaptic depolarization) is insufficient to induce LTP. (Figure 11-8)
copy number variation (CNV)
A deletion or duplication of a chromosome segment that can vary in length from 500 base pairs to several megabases and may contain coding sequences that range from a small fraction of a single gene to many genes.
coronal section
A section plane perpendicular to the rostral–caudal axis; also called cross or transverse sections. (Figure 1-8)
corpus callosum
A structure composed of axon bundles linking the two cerebral hemispheres. (Figure 7-12)
cortical amygdala
Part of the olfactory amygdala complex that receives direct mitral cell input. (Figure 6-16)
courtship conditioning
The process by which a normal Drosophila male learns to reduce his attempts at courtship following repeated rejections by mated females.
CRE (cAMP-response element)
See CREB.
CREB (cAMP-response element binding protein)
A transcription factor that binds the cAMP response element (CRE), a DNA cis-regulatory element in the promoter regions of target genes. It is a substrate for several kinases, including cAMP-dependent protein kinase. (Figure 3-41)
CreER
A fusion of the Cre recombinase with the portion of the estrogen receptor responsible for ligand-dependent nuclear translocation. CreER enters the nucleus only in the presence of tamoxifen, an estrogen analog, and thereby catalyzes recombination in a tamoxifen-dependent manner. (Figure 14-14)
Cre recombinase
A bacteriophage-derived enzyme that catalyzes recombination between two sequence-specific DNA elements called loxP sites. (Figure 14-9; Figure 14-12)
Creutzfeldt–Jakob disease (CJD)
See prion diseases.
CRISPR
Acronym for clustered regularly interspaced short palindromic repeat; it is a genomic locus in some bacteria and archaea containing repeated DNA elements derived from the genomes of invading pathogens. It is used by bacteria for adaptive immunity. Components of the CRISPR system are used experimentally for genome editing. See also Cas9 and guide RNA. (Figure 14-8)
critical period
A sensitive period during development when experience plays an important role in shaping the wiring and function of the nervous system.
cryo-EM (cryogenic electron microscopy)
A technique used to determine the atomic structures of macromolecules.
cryptochrome
A protein that acts as a negative regulator of circadian gene expression in mice but as a light sensor for entrainment of circadian rhythms in flies. (Figure 9-22)
cued fear conditioning
A fear conditioning paradigm in which an electric shock is applied at the end of a cue presented during training. See also auditory fear conditioning.
curare
A plant toxin that is a competitive inhibitor of the nicotinic acetylcholine receptor.
current clamp
A mode of whole-cell patch clamp recording used to measure membrane potential changes while holding the current at a set level.
cyclic AMP (cAMP)
An intracellular second messenger synthesized from ATP by adenylate cyclase. (Figure 3-33)
cyclic GMP (cGMP)
A cyclic nucleotide derived from GTP, one of its functions is to activate the cyclic nucleotide-gated cation channel in vertebrate photoreceptors in the absence of light. (Figure 4-7; Figure 4-8)
cyclic nucleotide-gated (CNG) channels
Nonselective cation channels whose gating is regulated by the concentration of a specific intracellular cyclic nucleotide. (Figure 2-34)
D
DCC/Unc40
Homologous proteins in vertebrates (DCC, for deleted in colon cancer) and C. elegans (Unc40) that act as receptors for netrin/Unc6 and mediate attraction in the absence of Unc5. The Drosophila homolog is Frazzled. (Figure 5-12)
decoding (in neural information processing)
The process of deducing stimuli from spike trains of a neuron or a neuronal ensemble.
deep brain stimulation (DBS)
A treatment strategy used for some neurological and psychiatric conditions in which electrodes are surgically implanted to stimulate neurons and axons in specific brain nuclei.
deep learning
A class of supervised machine learning techniques that employ a few to a few dozen intermediate layers of neural network between the input and output layers.
delay line
A thin axon fiber that carries auditory signals to target neurons at different locations along the axon with different time delays. (Figure 6-53)
Delta
A transmembrane ligand that activates the Notch receptor. (Figure 7-9)
demyelinating disease
A disease in which damage to the myelin sheath decreases the axonal membrane resistance between nodes of Ranvier, leading to disruption of ion channel organization in the nodal region and reduction in action potential conduction speed.
dendrites
Thick, bushy processes of a neuron that receive and integrate synaptic inputs from other neurons. (Figure 1-9)
dendritic spine
A small protrusion on a dendrite of some neurons that receives synaptic input from a partner neuron. The thin spine neck creates chemical and electrical compartments for each spine such that they can be modulated independently of neighboring spines. (Figure 1-9; Figure 3-45)
dendritic tiling
A phenomenon in which the dendrites of certain neuronal types collectively cover an entire receptive field exactly once so they can sample the field without redundancy. For example, certain types of retinal neurons collectively cover the retina exactly once. Certain types of somatosensory neurons cover the body surface exactly once. (Figure 4-26)
dendrodendritic synapse
A synapse between dendritic processes of two neurons. The reciprocal synapses between olfactory bulb granule cell dendrites and mitral cell secondary dendrites were the first discovered example of dendrodendritic synapses. (Figure 6-15)
de novo
mutation A mutation produced in the parental germline that is present in all of the offspring’s cells.
dense-core vesicle
An intracellular vesicle containing neuropeptides; they are larger and more electron dense than small molecule neurotransmitter-containing synaptic vesicles.
dentate gyrus
The part of the hippocampus that receives external input, consisting of granule cells and their dendrites, as well as axons from the entorhinal cortex. (Figure 11-5)
depolarization
A change in the electrical potential inside a cell toward a less negative value.
depolarization-induced suppression of inhibition (DSI)
A transient reduction of inhibitory input to a postsynaptic neuron induced by depolarization of the postsynaptic neuron. Originally described in hippocampal CA1 pyramidal neurons, DSI was found to result from endocannabinoid signaling.
depressing synapse
A synapse at which successive presynaptic action potentials trigger progressively smaller postsynaptic responses. (Figure 3-15)
deuterostomes
Animals in which the anus appears before the mouth during development. They include all vertebrates. See also protostomes. (Figure 13-2)
developmental axon degeneration
The process by which axons fragment into pieces that are subsequently engulfed by surrounding glia during normal development.
diacylglycerol (DAG)
A lipid second messenger that binds to and activates protein kinase C (PKC). (Figure 3-34)
diffusion tensor imaging (DTI)
A magnetic resonance imaging technique that allows noninvasive imaging of axon bundles in white matter based on the direction of water diffusion in a given volume. (Figure 14-29)
digital signaling
Signaling that uses discrete values (0s and 1s) to represent information.
direction-selective retinal ganglion cell (DSGC)
A retinal ganglion cell whose firing pattern is influenced by the direction of motion of a stimulus. (Figure 4-28)
direct pathway (in basal ganglia)
An axonal projection from a subset of spiny projection neurons that link the striatum directly to the basal ganglia output nuclei, GPi and SNr. (Figure 8-21)
discrete map
A type of neural map in which input or target neurons or their processes are spatially organized into discrete units (such as glomeruli or layers) representing different qualities (such as cell types). (Figure 7-36)
disinhibition
A reduction of the inhibitory output of an inhibitory neuron. (Figure 1-20)
divergent excitation
A circuit motif in which an excitatory neuron synapses onto multiple postsynaptic targets via branched axons. (Figure 1-20)
dizygotic twins
Nonidentical (fraternal) twins who share only 50% of their genes because they originated from two different eggs fertilized by two different sperm.
DNA (deoxyribonucleic acid)
Double-stranded chains of nucleotides consisting of the sugar deoxyribose, a phosphate group, and one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or thymidine (T).
DNA microarray
A solid substrate containing up to millions of immobilized spots of different oligonucleotides or gene-specific probes. Labeled nucleic acid samples can be hybridized to a DNA microarray to quantify the abundance of different species of nucleic acid molecules in samples.
DNA shuffling
A process by which part or all of the protein-coding sequence of one gene is fused to that of another gene, usually following chromosomal duplication or translocation. The specific type of DNA shuffling that occurs when translocational breakpoints are within introns of two genes is called exon shuffling. (Figure 13-6)
l-dopa
The intermediate metabolite between tyrosine and dopamine in the catecholamine biosynthetic pathway. (Figure 12-20)
dopamine
A monoamine modulatory neurotransmitter derived from the amino acid tyrosine. (Figure 12-20; Table 3-2)
Doppler effect
A phenomenon wherein the sound frequency detected by an observer increases if the sound-emitting object moves toward the observer and decreases if the sound-emitting object moves away from the observer.
dorsal column pathway
An axonal pathway from the spinal cord to the brainstem; it consists of ascending branches of proprioceptive neurons and Aβ-LTMRs, as well as axons of some dorsal horn projection neurons. (Figure 6-70)
dorsal cortex
The evolutionary precursor to the mammalian neocortex in reptiles. It consists of three thin layers (as opposed to the six-layered structure of the mammalian neocortex).
dorsal horn
The dorsal part of the spinal gray matter devoted to processing somatosensory information. (Figure 6-71)
dorsal horn projection neuron
A neuron located in the dorsal horn of the spinal cord that projects its axon into the brainstem to relay touch signals. (Figure 6-71)
dorsal root
The place where somatosensory axons enter the spinal cord. (Figure 8-6)
dorsal root ganglia
(DRG) Clusters of primary somatosensory neurons located along an axis parallel to the spinal cord used for sensation of the body (as opposed to the face). (Figure 6-63)
dorsal stream
A visual processing pathway from primary visual cortex to parietal cortex; it processes motion and depth signals. It is also called the “where” stream. (Figure 4-51)
dorsal–ventral
Of a body axis, from back (dorsal) to belly (ventral). (Figure 1-8)
Doublesex
(Dsx) A Drosophila gene encoding sex-specific transcription factors produced by sex-specific alternative splicing. The Dsx isoform determines sex-specific somatic structures and also regulates sexual behavior. (Figure 10-5)
Down syndrome
A syndrome caused by the presence of an extra copy of chromosome 21. It is the most common form of intellectual disability with an established genetic etiology.
doxycycline (dox)
A tetracycline analog that readily diffuses across cell membranes and the blood–brain barrier; it is widely used for temporal regulation of gene expression through the tTA/rtTA/TRE system. (Figure 14-12)
drive
A motive for action to maintain homeostasis.
drive reduction theory
A psychological theory of motivated behavior positing that deviations from homeostasis (such as dehydration) create aversive drives (such as thirst) that motivate animals to perform actions (such as searching for and consuming water) to reduce the aversive drives.
driver transgene
In binary expression systems, it is the transgene that expresses a transcription factor or recombinase under the control of a tissue-specific or temporally regulated promoter. (Figure 14-12)
driving force
The force that pushes an ion into or out of a cell; it equals the difference between the membrane potential of the cell and the equilibrium potential of the ion.
drug addiction
Compulsive drug use that persists despite long-term negative consequences. It is often associated with loss of self-control and propensity to relapse.
Dscam (Down syndrome cell adhesion molecule)
Encoded by a gene on human chromosome 21, it is an evolutionarily conserved cell adhesion protein. Insect DSCAMs exhibit extraordinary molecular diversity due to alternative splicing. (Figure 7-21)
dye coupling
The diffusion of small-molecule dyes from one cell to another through gap junctions. It is used as a criterion to identify the presence of gap junctions between two cells.
dynamical state
A point in a coding space, representing the status of a dynamical system at a given time. See also coding space. (Figure 8-35)
dynamical system
A physical system whose future state is a function of its current state, its input, and some noise. It can model time-dependent changes of neural states in a coding space. See also coding space. (Figure 8-35)
dynamic range
In sensory systems, the ratio between the largest and smallest values of a given dimension of sensory stimuli that can be detected and distinguished.
dynamin
A protein localized at the neck of vesicles undergoing fission. It is essential for clathrin-mediated endocytosis of synaptic vesicles and ultrafast endocytosis at the presynaptic membrane. It is encoded by the gene Shibire in Drosophila. (Figures 3-14 and 14-27)
dynein
A minus-end-directed, microtubule-based motor protein. (Figure 2-6)
E
eardrum
A membrane at the intersection of the mammalian outer ear and middle ear whose vibrations are transmitted by the bones in the middle ear to the cochlea in the inner ear. (Figure 6-43)
echolocation
The ability of certain species to use echoes of their own ultrasonic sound pulses to locate objects.
ectoderm
The outer germ layer, which gives rise to skin and the nervous system. (Figure 7-2)
efference copy
A copy of motor commands or execution signals, which can be used to predict the sensory consequences of motor actions.
efferent
An axon that projects from the CNS to peripheral targets. It can also be generalized to describe an output axon from a particular neural center within the CNS.
efficacy of synaptic transmission (synaptic efficacy)
The strength of a synaptic connection; it is usually measured by the mean magnitude of the postsynaptic response to a defined presynaptic stimulus.
E–I balance
The relative strength of synaptic excitation versus synaptic inhibition.
electrical circuit
Connected electrical elements containing at least one closed current path.
electrical gradient
Electrical potential difference between two sides of a membrane; it promotes movement of a charged solute toward the side with the opposite charge. (Figure 2-9)
electrical synapse
A cell–cell junction enriched in gap junction channels. It transmits (usually bidirectionally) both depolarizing and hyperpolarizing signals between the two cells. See also gap junction. (Figure 1-14)
electrochemical gradient
A combination of chemical and electrical gradients; it determines the direction and magnitude of movement of a charged solute across a membrane. (Figure 2-9)
electroencephalography (EEG)
A method for recording the electrical potential differences between surface electrodes placed on specific locations of the scalp. It reports the collective electrical activities of many cortical neurons underneath the surface electrodes. The resulting record is called an electroencephalogram, also abbreviated as EEG. (Figure 9-28; Figure 12-43)
electromotility
A property of the cochlear outer hair cells whereby hyperpolarization causes the cells to lengthen, and depolarization causes them to shorten, along their long axis. (Figure 6-50)
electron microscopy
A microscopic technique that uses beams of electrons to create an image of a specimen. It has much higher resolution than light microscopy and can resolve structures separated by a nanometer or less. See also transmission electron microscopy, scanning electron microscopy, and cryo-EM.
electroporation
A procedure in which DNA containing a transgene is introduced into cells by applying electrical current to facilitate the transfer of negatively charged DNA molecules into cells. In animals, this can be achieved by placing a micropipette containing the DNA near the cells of interest and applying electrical current.
embryonic stem (ES) cells
Pluripotent cells derived from early embryos that can be propagated indefinitely in vitro and give rise to all cell types of an embryo in vivo. (Figure 7-6)
encoding (in neural information processing)
The process of representing stimuli using spike trains of a neuron or a neuronal ensemble.
endocannabinoids
Endogenous cannabinoids, which are lipophilic molecules such as anadamide and 2-arachidonylglycerol. They can be produced in response to a rise in intracellular Ca2+ concentrations in certain postsynaptic neurons and diffuse across the synapse to affect presynaptic neurotransmitter release by binding to the CB1 G-protein-coupled receptor.
endocrine
Of or related to a form of signaling in which a recipient cell receives a signal produced by a remote source and delivered via systemic circulation.
endocrine system
A system consisting of glands that release hormones into the bloodstream to affect hormone receptor-expressing cells remotely.
endocytosis
The process by which cells retrieve, via budding of intracellular vesicles from the plasma membrane, fluid and proteins from the extracellular space and transmembrane proteins from cells’ plasma membranes. (Figure 2-2)
endoderm
The inner germ layer, which gives rise to a variety of tissues, such as the liver, the inner linings of the gut, and the respiratory tract. (Figure 7-2)
endoplasmic reticulum (ER)
A network of membrane-enclosed compartments in eukaryotic cells where secreted and transmembrane proteins are synthesized and into which secreted and transmembrane proteins translocate. It also serves as a store for intracellular Ca2+. (Figure 2-2)
end-plate current
The current that crosses a muscle cell membrane in response to release of acetylcholine from a presynaptic motor neuron.
end-plate potential (EPP)
Depolarization produced in a postsynaptic muscle cell by acetylcholine released from a presynaptic motor neuron in response to an action potential. (Figure 3-1)
endosome
A membrane-enclosed organelle produced via endocytosis; it carries newly internalized extracellular materials and transmembrane proteins. (Figure 2-2)
engram
Physical substrate for memory; it is also called a memory trace.
enteric nervous system
A division of the autonomic nervous system that is associated with the gastrointestinal tract and that regulates digestion rather independently of the rest of the autonomic nervous system.
enteroendocrine cells
Mechano- or chemosensitive cells in the gastrointestinal tract that produce hormones; these cells may also directly release neurotransmitters to activate afferent axon fibers of visceral sensory neurons. (Figure 9-6)
entorhinal cortex
The part of the temporal cortex overlying the hippocampus. It provides major input to and receives output from the hippocampus. It plays a key role in representing spatial information. (Figure 11-5)
entrainment
The process by which a stimulus, such as light, resets the phase of the circadian clock.
Eph receptors
Receptor tyrosine kinases that bind ephrins with their extracellular domains. Two Eph receptor subtypes, the EphA and EphB receptors, typically bind ephrin-As and ephrin-Bs, respectively. They can also serve as ligands during reverse signaling. (Figure 5-7; Figure 5-12)
ephrins
Cell-surface proteins that usually act as ligands for Eph receptors to mediate repulsive axon guidance. The ephrin family consists of two subfamilies: ephrin-As are attached to the extracellular face of the plasma membrane by GPI, and ephrin-Bs are transmembrane proteins. They can also serve as receptors during reverse signaling. (Figure 5-7; Figure 5-10)
epigenetic modifications
Molecular modifications to DNA and chromatin, such as DNA methylation and various forms of post-translational histone modifications. They do not modify DNA sequences but can alter gene expression.
epilepsy
A medical condition characterized by recurrent seizures. See also seizure.
epinephrine
A hormone produced primarily by chromaffin cells in the adrenal gland that mediates systemic responses to extreme conditions, such as the systemic responses associated with fright, fight, and flight. It also acts as a modulatory neurotransmitter in a small group of neurons in the brainstem. (Figure 12-20)
epithelial Na+ channel (ENaC)
A member of a class of Na+ channels involved in Na+ reabsorption by epithelial cells; it is also essential in mammals for the taste of low concentrations of salts. Its invertebrate homologs participate in mechanotransduction. (Figure 6-41)
EPSC (excitatory postsynaptic current)
An inward current produced by binding of excitatory neurotransmitters to their receptor(s). (Figure 3-23)
EPSP (excitatory postsynaptic potential)
A transient depolarization of a postsynaptic cell associated with an excitatory postsynaptic current (EPSC). (Figure 3-23)
equilibrium potential
The membrane potential at which there is no net flow of an ion across a membrane, when the electrical and chemical forces are equal in magnitude but opposite in direction.
Erev
See reversal potential.
estradiol
A steroid hormone produced by the ovaries of sexually mature females. It is produced in vivo by the action of the enzyme aromatase on testosterone. In conjunction with progesterone, estradiol regulates female sexual behavior and reproduction. (Figure 10-19)
estrogen
A female sex hormone, such as estradiol.
estrogen receptor
A cytosolic protein that upon binding of an estrogen (such as estradiol) translocates to the nucleus, where it acts as a transcription factor. (Figure 10-19)
eukaryote
Organism consisting of cell(s) with a nuclear membrane separating the genetic material from the rest of the cellular components.
eumetazoan
A taxon that includes cnidarians, bilaterians, and the most recent common ancestor of cnidarians and bilaterians. (Figure 13-2)
exchanger
See antiporter.
excised patch
A patch clamp configuration in which the membrane patch underneath the electrode is excised from the cell and placed in a defined medium. It is often used to study the biophysical and biochemical properties of the ion channel(s) in the membrane patch. (Figure 14-40)
excitability
A property of a neuron that defines how readily it fires action potentials.
excitable cell
A cell that produces action potentials, such as a neuron or a muscle cell. It can also refer to any cell that uses electrical signaling to receive, integrate, propagate, or transmit information.
excitation–contraction coupling
A process by which action potentials in muscle cells lead to muscle contraction. It involves actin/myosin-mediated contraction triggered by a rise in intracellular Ca2+ concentration. (Figure 8-5)
excitatory neuron
A neuron that, when activated, depolarizes its postsynaptic target cells and makes them more likely to fire action potentials.
excitatory neurotransmitter
A neurotransmitter that depolarizes postsynaptic target cells and makes them more likely to fire action potentials.
excitotoxicity
Toxicity to neurons caused by excessive stimulation by excitatory neurotransmitters such as glutamate, which results in large or persistent increases in intracellular Ca2+ concentration.
exocrine system
A system consisting of glands that excrete fluids, such as sweat or tears, locally through specific ducts.
exocytosis
The process by which intracellular vesicles fuse with the plasma membrane to release secreted proteins into the extracellular space and deliver lipids and transmembrane proteins to the plasma membrane. (Figure 2-2)
exon
The part of a pre-mRNA molecule retained in mRNA after splicing. (Figure 2-2)
exon shuffling
See DNA shuffling.
explicit memory
A form of memory requiring conscious recall, such as memory for names, facts, and events. It is also called declarative memory. (Figure 11-3)
expression cloning
A strategy for cloning a gene by transfecting cells with pools of cDNAs and using a functional assay to identify the pool containing the cDNA of interest. The assay is repeated with progressively divided pools of cDNAs until a single cDNA is identified. (Figure 6-68)
extensor
A muscle whose contraction increases the angle of a joint. (Figure 8-8)
extinction (in classical conditioning)
A decrease in the conditioned response caused by repeated exposure to the conditioned stimulus without the unconditioned stimulus.
extinction (in operant conditioning)
A decrease in a reinforced action or an increase in a punished action when the action is repeatedly not reinforced or punished, respectively.
extracellular recording
A technique for recording voltage changes, such as action potentials from a single neuron or synaptic activity from a population of neurons, by placing an electrode (often made of a metal probe insulated everywhere except at its tip) at close range to a neuronal cell body or a synapse-rich region. (Figure 14-34)
Eyeless
A Drosophila transcription factor belonging to the Pax family contains a homeobox and a paired box; it is required for eye development. Its ectopic expression in other structures, such as the antenna or the wing precursors, can induce ectopic eye formation. See also Pax6.
F
facilitating synapse
A synapse at which successive presynaptic action potentials trigger progressively larger postsynaptic responses. (Figure 3-15)
familial advanced sleep phase
An extreme variant of the human circadian system characterized by very early morning waking and an early evening sleep onset.
familial Alzheimer’s disease (FAD)
A small subset of Alzheimer’s disease cases that follows a Mendelian autosomal dominant inheritance pattern.
fast axonal transport
Intracellular transport at a speed of 50–400 mm per day; cargos subject to fast axonal transport include organelles as well as transmembrane and secreted proteins. (Figure 2-4)
fear conditioning
See contextual fear conditioning and auditory fear conditioning.
feedback excitation
A circuit motif in which an excitatory neuron synapses onto its own presynaptic partner. (Figure 1-20)
feedback inhibition
A circuit motif in which an excitatory neuron both provides output to and receives input from an inhibitory neuron. (Figure 1-20)
feedforward excitation
A circuit motif in which serially connected excitatory neurons propagate information across multiple regions of the nervous system. (Figure 1-20)
feedforward inhibition
A circuit motif in which a postsynaptic neuron receives both direct excitatory input from a presynaptic neuron and disynaptic inhibitory input from the same excitatory neuron via an inhibitory interneuron. (Figure 1-20)
fertilization
The fusion of sperm and egg to create a genetically new organism. (Figure 7-2)
Fezf2
A transcription factor that specifies subcerebral projection neuron identity. (Figure 7-12)
fiber photometry
A technique that uses an optical fiber to record bulk fluorescence activity from a cell population that expresses a fluorescence indicator (e.g., a genetically encoded Ca2+ indicator).
fibroblast growth factor (FGF)
A member of a family of secreted growth factors that act as morphogens to pattern early embryos during development.
field excitatory postsynaptic potential (fEPSP)
Excitatory postsynaptic potentials recorded from a population of neurons near the tip of an extracellular electrode. fEPSPs evoked by stimulation of axonal inputs to a population are often used as a measure of the strength of synaptic transmission between the stimulated inputs and neurons near the recording electrode. (Figure 11-7)
filamentous actin (F-actin)
A major cytoskeletal element composed of two parallel helical strands of actin polymers. They are also called microfilaments. (Figure 2-5; Figure 8-3)
filopodia
Thin, protruding processes of a growth cone made of bundled F-actin. (Figure 5-16)
firing rate (spike rate)
Mean spike count per unit time.
fissure
A deep invagination of the cortical surface that separates areas of the cerebral cortex.
fitness
With respect to an allele (or phenotype), fitness is the ratio of the frequency of the allele (or phenotype) in a population after one generation of selection to the frequency of the allele (or phenotype) in the same population before the selection. With respect to an individual, fitness is the number of second-generation descendants the type of individual with a particular genome is expected to have.
fixed (of an allele)
The state of an allele when every member of a population is homozygous for the allele.
fixed action pattern
A concept in neuroethology, it refers to an instinctive sequence of behaviors that is usually invariant and runs to completion once triggered.
flavor
A synthesis of taste and olfaction with additional contribution from the trigeminal somatosensory system for special chemicals, temperature, and texture.
flexor
A muscle whose contraction decreases the angle of a joint. (Figure 8-8)
floor plate
A structure at the ventral midline of the spinal cord. (Figure 5-12; Figure 7-10)
FLP recombinase
A yeast-derived enzyme that catalyzes recombination between two sequence-specific DNA elements called FRT (FLP recognition target) sites. (Figure 14-9; Figure 14-26)
fluoxetine
A widely used antidepressant that acts as a selective serotonin reuptake inhibitor. Its brand name is Prozac. See also SSRI. (Figure 12-26)
Fmr1
See fragile-X syndrome.
FMRP
See fragile-X syndrome.
focal seizures
Seizures that affect a relatively small, discrete region of the brain.
follicle-stimulating hormone (FSH)
See gonadotropins.
forebrain
The rostral-most division of the three divisions of the embryonic brain. It gives rise to the cerebral cortex, basal ganglia, hippocampus, amygdala, thalamus, and hypothalamus. (Figure 1-8; Figure 7-3)
forward genetic screen
A procedure for identifying genes necessary for a biological process. It usually involves (1) inducing mutations in a population of experimental animals so that each animal carries a different set of random mutations in a small number of genes or a single gene, and (2) identifying mutations that disrupt the biological process of interest based on the phenotypes exhibited by the offspring of the mutagenized animals. (Figure 14-5)
forward model (in motor systems)
A model that uses efference copy and expected sensory feedback signals to provide online tuning of motor output.
Fos
An immediate early gene encoding a transcription factor. Its expression is commonly used as an indicator of recently active neurons.
fovea
The central part of the primate retina, which has a high density of cones. (Figure 4-13)
fragile-X syndrome (FXS)
A leading cause of inherited intellectual disability; it is caused by expanded trinucleotide repeats in the 5′ untranslated region of the Fmr1 gene, which encodes an RNA binding protein called the fragile X mental retardation protein (FMRP).
Frazzled
See DCC/Unc40.
frequency tuning
The property whereby a cell in the auditory system is best activated by sounds of a particular frequency. It is usually represented as a V-shaped curve on a frequency-intensity plot. (Figure 6-47)
frontal eye field (FEF)
A neocortical area that receives extensive feedforward connections from both the dorsal and the ventral streams of visual information and sends feedback projections to many visual cortical areas. (Figure 4-51)
frontal lobe
One of the four cerebral cortex lobes; it is located at the front of the brain rostral to the central sulcus. (Figure 1-23)
FRT
See FLP recombinase.
Fruitless (Fru)
A Drosophila gene that regulates all aspects of male courtship rituals. The splicing of one of its transcripts is regulated by a hierarchy of sex-determining splicing factors: females express a nonfunctional splice isoform of the protein while males express a functional form of the protein (FruM), which acts as a transcription factor. (Figure 10-5)
functional architecture
The physical arrangement of neurons in a brain region based on their functional properties.
functional magnetic resonance imaging (fMRI)
A noninvasive functional brain imaging technique; it monitors signals originating from changes in blood flow that are correlated with local neuronal activity. It is also called BOLD (blood-oxygen-level dependent) fMRI.
fundamental frequency
The frequency of the lowest frequency component of a periodic waveform.
fura-2
A small molecule Ca2+ indicator whose optimal excitation wavelength shifts from 380 nm to 350 nm when Ca2+ is bound. The ratio of fluorescence intensity measured at excitation wavelengths of 350 nm and 380 nm can be used as a sensitive measure of Ca2+ concentration. (Figure 14-41)
fusiform face area
A specific area of human temporal cortex preferentially activated by images of human faces.
G
Gα, Gβ, Gγ
See trimeric GTP-binding protein.
GABA
A glutamate derivative that is the predominant inhibitory neurotransmitter in vertebrates and invertebrates. (Figure 3-16; Table 3-2)
GABAa receptor
An ionotropic receptor that is gated by GABA and mediates fast inhibition. (Figure 3-21; Figure 12-27)
GABAB receptor
A metabotropic receptor that is activated by GABA and mediates slow inhibition.
gain control
Modulation of the slope of a system’s input–out function; it is often used to restrict output to a limited dynamic range.
gain-of-function experiment
An experiment in which a specific component is added to a system; it is often used to test whether the added component is sufficient for the system to function in a specific context.
GAL4
A yeast transcription factor that binds to a DNA element called a UAS (upstream activation sequence) in the promoter regions of genes to activate transcription of those genes. (Figure 14-12)
ganglion
A cluster of neurons located in the peripheral nervous system.
ganglionic eminences
Developing ventral telencephalon structures including the medial, caudal, and lateral ganglionic eminences (MGE, CGE, LGE); they are the birthplaces of cortical GABAergic neurons (MGE and CGE), GABAergic interneurons in the basal ganglia and amygdala (MGE and CGE), and olfactory bulb interneurons and most GABAergic projection neurons in the striatum (LGE). (Figure 7-5)
gap junction
The morphological correlate of the electrical synapse, which usually contains hundreds of closely clustered channels that bring the plasma membranes of two neighboring cells together and allow passage of ions and small molecules between the two cells. (Figure 3-48)
gastrin-releasing peptide receptor (GRPR)
A G-protein-coupled receptor activated by gastrin-releasing peptide and involved in processing itch signals.
gastrula
The product of gastrulation; it is an embryo with a three-layered structure consisting of ectoderm, mesoderm, and endoderm. (Figure 7-2)
gastrulation
The process by which an embryo is transformed from a ball of cells into a structure with three distinct layers: ectoderm, mesoderm, and endoderm. (Figure 7-2)
GCaMP
A GFP-based genetically encoded Ca2+ indicator; its fluorescence increases in response to a rise in Ca2+ concentration. (Figure 14-41)
gene
A segment of DNA that carries the instructions for how and when to make specific RNAs and proteins. (Figure 2-2)
gene duplication
A major step in evolutionary innovation. Along with associated diversification, it is the primary means of production of new genes. (Figure 13-6)
generalized seizures
Seizures that affect multiple, bilateral brain regions.
gene therapy
The use of DNA and/or genome modification to treat diseases caused by genetic defects.
genetically encoded Ca2+ indicators
Proteins whose fluorescence properties change before and after binding to Ca2+. See also Ca2+ indicator. (Figure 14-41)
genetic drift
Changes in allele frequency in small populations due to random events. An allele can be lost because it is not passed from parents to progeny due to random allele sampling or because the progeny carrying the allele die or fail to reproduce by chance. The loss of an allele increases the prevalence of the remaining allele(s).
genetic mosaic
An animal containing cells of more than one genotype. (Figure 14-10)
genetic susceptibility locus
A genomic locus with variant(s) that increase the probability of carriers developing a trait (such as a disease).
genome editing
The general process of altering the genome at predetermined loci, such as by deleting a piece of endogenous DNA, inserting a piece of foreign DNA, or creating a specific base-pair change.
genome-wide association study (GWAS)
A strategy for identifying genes associated with a specific trait by comparing genome-wide DNA samples collected from many people with or without the trait; it is also used to identify the single nucleotide polymorphisms most strongly linked with the trait.
ghrelin
A neuropeptide produced by stomach-associated glands in response to reduced glucose levels; it acts as a hunger signal to stimulate eating. (Figure 9-15)
Gi (inhibitory G protein)
A Gα variant that binds to adenylate cyclase and inhibits its activity.
gill-withdrawal reflex
A reflex in the sea slug Aplysia in which it withdraws its gill into its mantle shelf when a tactile stimulus is applied to its siphon. It has been used as a model system to investigate the mechanisms underlying simple forms of learning and memory. (Figure 11-24)
glia
Nonneuronal cells of the nervous system; they play essential roles in the development and function of the nervous system (Figure 1-9)
glomerulus
A discrete, ball-like structure in the vertebrate olfactory bulb or insect antennal lobe where axons of olfactory receptor neuron form synapses with the dendrites of their postsynaptic target neurons. (Figure 6-3; Figure 6-24)
GluN1
See NMDA receptor.
GluN2
See NMDA receptor.
glutamate
An amino acid that is the predominant excitatory neurotransmitter in vertebrates. (Figure 3-16; Table 3-2)
glutamic acid decarboxylase (GAD)
An enzyme that converts glutamate into GABA.
glycine
An amino acid that is an inhibitory neurotransmitter released by a subset of brainstem and spinal cord neurons in vertebrates. (Figure 3-16; Table 3-2)
glycine receptor
An ionotropic receptor that is gated by glycine and mediates fast inhibition. (Figure 3-21)
Goldman–Hodgkin–Katz (GHK) equation
An equation that relates the membrane potential at equilibrium to the membrane permeabilities and concentrations of multiple ions on the two sides of a membrane. A variant of the GHK equation relates the membrane potential at equilibrium to the equilibrium potential and conductance of each ion.
Golgi cell
A type of GABAergic neuron in the cerebellar cortex that receives input from granule cells and sends inhibitory output back to granule cells. (Figure 8-26)
Golgi outpost
A fragment of the Golgi apparatus located in neuronal dendrites. (Figure 7-19)
Golgi stain
A histological staining method; it uses solutions of silver nitrate and potassium dichromate, which react to form a black precipitate (microcrystals of silver chromate) that accumulates stochastically in a small fraction of nerve cells so that these cells, and most or all of their elaborate extensions, can be visualized against unstained tissue.
gonadotropin-releasing hormone (GnRH)
A prehormone released by hypothalamic neurons (called GnRH neurons) that stimulates release of gonadotropins by anterior pituitary endocrine cells. (Figure 10-22)
gonadotropins
A family of hormones including luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Released by anterior pituitary endocrine cells, these hormones stimulate the maturation of male testes and female ovaries during puberty. In adults, they stimulate the testes to release testosterone and the ovaries to release estradiol. (Figure 10-22)
GPCR (G-protein-coupled receptor)
A member of a receptor family with seven transmembrane helices that, upon ligand binding, activate trimeric G proteins, which in turn activate intracellular signaling cascades.
GPe (globus pallidus external segment)
An intermediate nucleus in the basal ganglia indirect pathway; it contains GABAergic neurons that project to the GPi, SNr, and STN. (Figure 8-21)
GPi (globus pallidus internal segment)
One of the two major output nuclei of the basal ganglia; it contains GABAergic neurons that project to the thalamus and brainstem. (Figure 8-21)
Gq
A Gα variant that activates phospholipase C, in turn leading to activation of the inositol-phospholipid signaling pathway. (Figure 3-34)
graded potentials
Membrane potentials that can change in continuous values, as opposed to the all-or-none property of action potentials. (Figure 2-18)
granule cells
Neurons that are granular in appearance because they are densely packed; there are three prominent types. (1) Cerebellar granule cells are the most numerous type of neuron in the brain; their cell bodies and dendrites reside in the granular layer of the cerebellar cortex, where they receive mossy fiber input, and their axons ascend into the molecular layer, where each bifurcates to become a parallel fiber that sends glutamatergic output to Purkinje cells. (Figure 8-26) (2) Dentate gyrus granule cells in the hippocampus receive input from the entorhinal cortex via the perforant path and send glutamatergic output to CA3 pyramidal neurons. (Figure 1-12; Figure 11-5) (3) Olfactory bulb granule cells constitute a large subtype of olfactory bulb interneurons that receive input from the secondary dendrites of mitral cells and send GABAergic output back to mitral cells. (Figure 6-14)
gray matter
The parts of the CNS enriched in neuronal cell bodies, dendrites, axon terminals, and synapses and that appear gray in histological sections.
green fluorescent protein (GFP)
A jellyfish protein that emits green fluorescence when excited by blue light. It is widely used as a marker of gene expression and in live imaging.
grid cell
A cell in the entorhinal cortex whose activity depends on an animal’s location in an arena, with peak firing rate occurring at the apices of an imaginary hexagonal grid superimposed on the arena floor. (Figure 11-32)
growth cone
A dynamic structure at the tip of a developing neuronal process; it enables the extension of the process and guides its direction. (Figure 1-13; Figure 5-16)
Gs (stimulatory G protein)
A Gα variant that binds to adenylate cyclase and stimulates its activity. (Figure 3-33)
GTPase
An enzyme that hydrolyzes GTP, converting it into GDP.
GTPase activating protein (GAP)
A protein that switches GTPases off by accelerating the GTPases’ endogenous activity, which converts GTP into GDP. (Figure 3-32)
guanine nucleotide exchange factor (GEF)
A protein that switches GTPases on by catalyzing the exchange of GDP for GTP. (Figure 3-32)
guanylate cyclase
An enzyme that produces cGMP from GTP.
guide RNA
In the CRISPR/Cas9 system, an RNA molecule that brings Cas9 to a target DNA sequence, where Cas9 generates a double-strand break. The guide RNA must contain sequences that base-pair with the target DNA. See also CRISPR and Cas9. (Figure 14-8)
gustatory nerve
A bundle of axons originating from the basal ends of taste receptor cells, the nerve projects to the nucleus of the solitary tract in the brainstem and relays taste information from the tongue to the brain. (Figure 6-32)
H
habituation
A decrease in the magnitude of responses to stimuli presented repeatedly.
hair cell
The primary sensory cell for audition; it converts mechanical stimuli—movement of stereocilia at its apical end—into electrical signals. (Figure 6-43; Figure 6-45)
halorhodopsin
A light-activated inward chloride pump in archaea; it can be used to silence neuronal activity in heterologous systems by light. See also optogenetics. (Figure 14-47)
HCN (hyperpolarization-activated cyclic nucleotide-gated) channels
Nonselective cation channels activated by hyperpolarization; their gating is influenced additionally by the concentration of intracellular cyclic nucleotides. (Figure 2-34)
head direction cell
A cell that fires when an animal’s head is facing a specific direction, regardless of the animal’s location in its environment. (Figure 11-32)
Hebbian synapse
A synapse whose strength can be enhanced by co-activation of pre- and postsynaptic partners.
Hebb’s rule
A postulate by Donald Hebb that describes how learning can be transformed into a lasting memory. It states: “When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased.” (Figure 5-24)
hedonic value
The degree to which something is pleasant or unpleasant, which usually correlates with the degree to which something is potentially beneficial or harmful to an animal.
hemisphere
One of the two sides of the brain.
heritability
A measure of the contribution of genetic differences to trait differences within a population. It can be measured in twin studies as 2 × (the correlation of the trait between pairs of monozygotic twins – the correlation of the trait between pairs of dizygotic twins).
herpes simplex virus (HSV)
A DNA virus used to deliver transgenes into postmitotic neurons. It has the capacity to encode ~150 kb of foreign DNA. (Table 14-1)
heterophilic binding
Binding of two different proteins, usually two different membrane proteins expressed from adjacent cells across a cellular junction, such as a synapse.
heterophilic cell adhesion protein
A protein that facilitates adhesion between cells via direct binding to a distinct partner from apposing cells. The partner can be a different member of the same protein family or from different protein families.
hindbrain
The caudal-most division of the three divisions of the embryonic brain. It gives rise to the pons, medulla, and cerebellum. (Figure 1-8; Figure 7-3)
hippocampus
A structure underneath the cortical surface of the temporal lobe; it has been most studied for its role in spatial representation and acquisition of explicit memory. (Figure 1-8; Figure 11-5)
histamine
A monoamine modulatory neurotransmitter derived from the amino acid histidine. (Figure 3-16; Table 3-2)
histological sections
Slices of frozen or chemically fixed tissue produced by microtomes, with thicknesses ranging from several to several hundred micrometers. They can be stained using a number of different methods and examined under a light microscope.
homeobox
See homeodomain.
homeodomain (homeobox)
Originally discovered in proteins whose disruption causes transformation of one body part into another, it is a DNA-binding domain shared by all Hox proteins and many other transcription factors.
homeostasis
The maintenance of a steady state of a physiological parameter—such as blood pressure, body temperature, or nutritional level—by feedback physiological and behavioral responses. (Figure 9-6)
homeostatic synaptic scaling
A plasticity phenomenon with a negative feedback mechanism such that a change of a certain parameter, such as the firing rate of a neuron or the strength of a synapse, triggers a compensatory process such that the parameter returns to the original set point.
homeotic transformation
Transformation of one body part into another, such as the transformation of a pair of antennae into a pair of legs in Drosophila antennapedia mutants.
homologous recombination
Exchange of nucleotide sequences between two identical or highly similar DNA molecules; it occurs naturally in certain cells due to its role in specific biological processes, such as in germline cells during meiotic crossing over. It is also used experimentally for genome editing, such as in generation of knockout and knock-in alleles. (Figure 14-6)
homophilic binding
Binding of two identical proteins, usually two membrane proteins expressed from adjacent cells across a cellular junction such as a synapse.
homophilic cell adhesion protein
A protein that facilitates adhesion between cells via direct binding of the same proteins from apposing cells.
Hopfield network
A recurrent neural network in which neurons do not form synapses onto themselves and the synaptic weights of each reciprocal pair are the same. It has the property that network dynamics will converge onto a local minimum of an “energy” function. See attractor.
horizontal cell
An inhibitory neuron in the vertebrate retina; it regulates signal propagation from photoreceptors to bipolar cells through lateral inhibition. (Figure 4-23)
horizontal gene transfer
Gene transfer from one organism to another through mechanisms other than reproduction, such as via viral transduction.
horizontal sections
A section plane perpendicular to the dorsal–ventral axis. (Figure 1-8)
hormone
A chemical or peptide produced by an organism that circulates in tissue fluids such as blood to affect the physiology of recipient cells at a distance.
Hox gene
A member of a family of evolutionarily conserved genes that are arranged in genomes in clusters and encode homeobox-containing transcription factors. Hox genes define the anterior–posterior body axes of most invertebrates and all vertebrates and also regulate neuronal fate at later developmental stages. (Figure 13-32)
5-HT
See serotonin.
HTMR (high-threshold mechanoreceptor)
A mechanosensory neuron that senses strong and noxious mechanical stimuli. (Figure 6-64)
huntingtin
See Huntington’s disease.
Huntington’s disease (HD)
A dominantly inherited disease that usually strikes patients during midlife; it is characterized initially by depression or mood swings and subsequently by abnormal movement due to degeneration of striatal neurons. It is caused by expanded polyglutamine repeats in the huntingtin protein.
HVC (high vocal center)
A dorsal forebrain nucleus in the songbird essential for song production. (Figure 10-16)
hyperalgesia
An enhanced response to noxious (painful) stimuli. (Figure 6-72)
hyperpolarization
A change in the electrical potential inside a cell toward a more negative value.
hypocretin (orexin)
A neuropeptide expressed by specific lateral hypothalamus neurons; it is important for regulating sleep and eating.
hypogonadotropic hypogonadism
A disorder characterized by delayed, reduced, or absent puberty due to reduced gonadotropin levels.
hypothalamus
A collection of nuclei ventral to the thalamus; it controls many bodily functions, including eating, digestion, metabolic rate, drinking, salt intake, reproduction, body temperature, emergency responses, and circadian rhythms. It executes many of these functions by regulating the autonomic nervous system and neuroendocrine system. (Figure 1-8; Figure 9-6)
I
identified neuron
A neuron that can be recognized across individuals of the same species due to its stereotyped location, size, and/or shape.
Ig CAM (immunoglobulin cell adhesion molecule)
A cell adhesion molecule that contains immunoglobulin domains on its extracellular side.
imipramine
A tricyclic antidepressant that inhibits the plasma membrane monoamine transporters. (Figure 12-26)
immediate early genes (IEGs)
A class of genes whose transcription is rapidly induced by external stimuli without requiring new protein synthesis.
immuno-EM
A combination of immunostaining and electron microscopy used to visualize the distribution of individual proteins at an ultrastructural level. (Figure 14-27)
immunostaining
A staining method that uses antibodies to visualize the distributions of specific proteins in histological sections or whole-mount tissues.
implicit memory
A form of memory in which previous experience aids in the performance of a task without conscious recall. It is also called nondeclarative memory or procedural memory. (Figure 11-3)
inactivation (of ion channels)
A decrease in ion conductance through a channel after its opening. The ion channel, when inactivated, is in a distinct state from when it is closed.
incentive salience theory
A psychological theory of motivated behavior positing that food and water are inherently rewarding to animals; hunger and thirst amplify their reward value.
inclusion bodies
Intracellular foci into which aggregated proteins are sequestered.
indirect pathway (in basal ganglia)
An axonal projection from a subset of spiny projection neurons that terminates in the GPe and STN. (Figure 8-21)
induced pluripotent stem (iPS) cells
Pluripotent cells produced experimentally from differentiated cells by a variety of means, such as forced expression of key transcription factors involved in maintaining the pluripotency of embryonic stem cells. (Figure 7-6)
induction
A mechanism for determining cell fate in which a cell is born with the same potential to develop into different cell types as its sibling or cousins, and its fate is acquired by receiving external signals (i.e., the cell’s fate is “induced” by external cues).
inferior colliculus
A midbrain nucleus that integrates auditory signals from brainstem nuclei. It sends auditory output to the thalamus and the nearby superior colliculus/tectum. (Figure 6-52)
inferior olive
A nucleus in the medulla containing neurons whose axonal projections to the cerebellum form climbing fibers. (Figure 8-26)
inhibitory neuron
A neuron that, when activated, hyperpolarizes its postsynaptic target cells and makes them less likely to fire action potentials.
inhibitory neurotransmitter
A neurotransmitter that hyperpolarizes postsynaptic target cells and makes them less likely to fire action potentials.
innate
A trait or behavior that is genetically programmed and is thus with an organism from birth rather than acquired by experience.
innate song
The song a songbird would sing if raised in acoustic isolation during the sensory stage of song learning.
innexin
A protein component of gap junctions in invertebrates.
inositol 1,4,5-triphosphate (IP3)
A second messenger that binds to the IP3 receptor on the endoplasmic reticulum (ER) membrane to trigger the release of ER-stored Ca2+ into the cytosol. (Figure 3-34)
input specificity (of LTP)
A property of long-term potentiation (LTP) whereby LTP occurs only at synapses that have experienced an LTP-inducing stimulus and not at unstimulated synapses on the same postsynaptic neuron. (Figure 11-8)
in situ hybridization
A method for determining mRNA distribution in tissues by hybridizing labeled gene-specific nucleic acid probes to fixed histological sections or whole-mount tissues.
instrumental conditioning
See operant conditioning.
insular cortex
A part of the cerebral cortex that represents taste, pain, and interoception, among other functions; it is located internal to the part of the somatosensory cortex representing the tongue depicted in the sensory homunculus. (Figure 6-32; Figure 9-3)
insulator
An object or substance that does not allow electric current to pass. It is equivalent to a resistor with infinite resistance.
insulin
A peptide hormone produced by the pancreas in response to a rise in blood glucose levels after meals; it regulates carbohydrate metabolism throughout the body and regulates food intake through its actions on target neurons in the brain. (Figure 9-15)
intellectual disability
A condition characterized by deficits in general mental abilities such as reasoning, problem solving, planning, abstract thinking, judgment, and learning.
interaural level difference (ILD)
The level difference of a sound received in the left and right ears, used for sound localization.
interaural time difference (ITD)
The difference in the arrival time of a sound at the left and right ears, used for sound localization.
intermediate progenitor
A progenitor cell produced by division of a radial glial cell. It divides further to give rise to postmitotic neurons. (Figure 7-4)
interneuron
A neuron with its axon confined to the specific CNS region that houses the neuron’s cell body; it can also be called a local neuron in this context. It may also refer to any neuron that is not a motor or sensory neuron.
interoception
The sense of the state of internal organs.
interoceptive system
The sensory system that senses the state of internal organs.
intersectional methods (in genetics)
Strategies that use two orthogonal binary expression systems to refine patterns of transgene expression. (Figure 14-13)
interstitial branching
The extension of a collateral from the side of a growing process. (Figure 7-20)
intracellular recording
A procedure for measuring the membrane potential of a cell using an electrode inserted into or continuous with its cytoplasm. (Figure 14-34)
intracellular vesicle
A small, membrane-enclosed organelle in the cytoplasm of a eukaryotic cell. (Figure 2-2)
intrinsically photosensitive retinal ganglion cell (ipRGC)
A subclass of retinal ganglion cells that expresses melanopsin and can be directly depolarized by light. (Figure 4-36)
intrinsic properties
The electrophysiological properties of a neuron determined by the composition, concentration, subcellular distribution, and biophysical properties of its ion channels.
intrinsic signal imaging
A method for measuring neuronal activity based on changes in the optical properties of tissue surrounding active neurons, primarily as a result of changes in blood oxygenation in those regions.
intron
The part of an RNA molecule that is removed during splicing. (Figure 2-2)
inward-rectifier K+ channels
A subfamily of K+ channels that preferentially passes inward currents over outward currents; these channels pass current at membrane potentials more hyperpolarized than EK but allow minimal outward currents at membrane potentials more positive than EK. (Figure 2-34)
ion channel
A specialized transmembrane protein that allows passage of one or more specific ions across the lipid bilayer.
ionotropic receptor
A neurotransmitter receptor that functions as a neurotransmitter-gated ion channel to allow rapid (within a few milliseconds) membrane potential changes in response to neurotransmitter binding. (Figure 3-21)
iontophoresis
A technique by which ions or charged chemicals are locally applied from a micropipette via a current pulse.
IP3 receptor
An IP3-gated Ca2+ channel on the ER membrane. (Figure 3-34)
iproniazid
The first antidepressant, discovered serendipitously in the 1950s; it is an inhibitor of monoamine oxidase. (Figure 12-26)
IPSC (inhibitory postsynaptic current)
An outward current produced by the binding of an inhibitory neurotransmitter to its receptor. The fast component is usually mediated by Cl– influx through the GABAa or glycine receptors.
ipsilateral
Of the same side of the midline. For example, an ipsilateral axonal projection is an axon that does not cross the midline and therefore terminates on the same side of the nervous system as its corresponding soma.
IPSP (inhibitory postsynaptic potential)
A transient hyperpolarization of a postsynaptic cell associated with an inhibitory postsynaptic current (IPSC).
I–V curve
A graphical representation of the relationship between the current that passes through a piece of ion-channel-containing membrane (I) and the voltage across the membrane (V). (Figure 3-17)
J
None.
K
kainate receptor
A glutamate-gated ion channel that conducts Na+ and K+ and can be selectively activated by the drug kainate (kainic acid).
K+ channels
Ion channels that allow selective passage of K+; they constitute the most diverse channel family. (Figure 2-34)
Kenyon cells
Principal cells of the insect mushroom body, which receive synaptic inputs mainly from olfactory projection neurons and play key roles in olfactory learning and memory. See also mushroom body.
kinase
An enzyme that adds phosphates to proteins.
kinesins
A family of microtubule-based motor proteins that are mostly plus-end-directed. (Figure 2-6; Figure 2-7)
kisspeptins
A family of neuropeptides encoded by the Kiss1 gene that play an important role in activating GnRH neurons. (Figure 10-22)
knee-jerk reflex
The involuntary forward movement of the lower leg due to contraction of the quadriceps femoris muscle (an extensor) and relaxation of the hamstring muscle (a flexor). (Figure 1-19)
knock-in
A variation of the knockout procedure in which a DNA construct—either a transgene or a variant of an endogenous gene—is inserted into a specific chromosomal locus; the procedure can produce changes to endogenous genes as small as a single base pair.
knockout
A genetic engineering procedure that disrupts a specific gene. In the mouse, it is traditionally achieved by homologous recombination in embryonic stem cells to create a mutation in the target gene. The resulting mutant mouse is called a knockout mouse for that particular gene. (Figure 14-6)
kuru
See prion diseases.
L
labeled line (in sensory systems)
A dedicated processing channel that carries specific information from the periphery to the brain (Figure 6-33)
lamellipodia
A veil-like meshwork in growth cones made of branched F-actin. (Figure 5-16)
lamina (in insect visual system)
The first neuropil layer underneath the retina in the insect compound eye. (Figure 4-31; Figure 5-34)
lamina terminalis
A set of nuclei in the anterior hypothalamus that are key regulators of water intake; it consists of the subfornical organ (SFO), the organum vasculosum of the lamina terminalis (OVLT), and the median preoptic nucleus (MnPO). (Figure 9-16)
landmark-based strategy
A navigational strategy in which animals use external cues to determine their locations.
large dense-core vesicle
Intracellular vesicle that is responsible for neuropeptide release; it is larger than synaptic vesicles and contains electron-dense materials.
laser-scanning two-photon microscopy
See two-photon microscopy.
late LTP
A long-lasting phase of long-term potentiation (LTP), usually lasting longer than 3 hours and requiring new protein synthesis and likely new gene expression.
lateral geniculate nucleus (LGN)
A thalamic nucleus that receives visual input from retinal ganglion cell axons and sends output to the primary visual cortex. (Figure 4-37; Figure 4-38)
lateral horn
A second-order olfactory center for odor-mediated innate behavior in the insect brain. It and the mushroom body are the two major output sites for olfactory projection neuron axons. (Figure 6-24; Figure 6-30)
lateral hypothalamus
A loosely organized hypothalamic region implicated in regulation of multiple functions, including eating, drinking, and sleep.
lateral inhibition (in cell fate determination)
The process by which neighboring cells are prevented from adopting identical fates through cell–cell interactions, such as those mediated by Notch/Delta. (Figure 7-9)
lateral inhibition (in information processing)
A circuit motif in which an inhibitory neuron receives excitatory input from one or several parallel streams of excitatory neurons and sends inhibitory output to many or all of the postsynaptic targets of these excitatory neurons. It is widely used in sensory systems. (Figure 1-20)
lateral intraparietal area (LIP)
A cortical area in the primate parietal lobe implicated in decision making to move eyes in a particular direction.
law of effect
A psychological principle positing that behavior followed by a reward will be repeated, whereas behavior followed by a punishment will diminish.
learning
The process by which experience changes the nervous system, enabling animals and humans to acquire new knowledge or skills and modify their behaviors.
length constant (space constant, λ)
A key parameter that defines the passive properties of electrical signaling; it is equal to the distance along a neuronal process over which the amplitude of a membrane potential change decays to 1/e, or about 37% of its original value.
lentivirus
A retrovirus that can infect postmitotic neurons. It has a capacity to carry ~8 kb foreign DNA. (Table 14-1)
leptin
A hormone secreted by fat tissues that negatively regulates food intake through its actions on specific neurons in the brain. (Figure 9-15)
Lewy bodies
Intracellular inclusions that are a defining pathological feature of most forms of Parkinson’s disease.
ligand
A molecule that binds to its receptor to exert a biological activity.
ligand-gated ion channel
A transmembrane protein complex that directly conducts ions in response to the binding of a neurotransmitter or other ligand.
light microscopy
The most widely used microscopic technique in biology; it uses beams of visible light (photons) to create an image of a specimen and, with the exception of super-resolution fluorescence microscopy, can only resolve structures greater than 200 nm apart.
light-sheet fluorescence microscopy
A fluorescence microscopy technique in which only the focal plane (a single plane in the z-dimension) is illuminated with a thin sheet of a laser beam from the side. All fluorescence emissions in the focal plane are collected simultaneously by a detector. (Figure 14-23)
LKB1
A protein kinase essential for determining axon fate during the establishment of neuronal polarity.
LMAN (lateral magnocellular nucleus of the anterior nidopallium)
A forebrain nucleus in the songbird essential for song learning but not song production. (Figure 10-16)
local field potential
Electrical potential at an extracellular recording site relative to a distal ground. Usually filtered to remove high-frequency signals, it reflects the collective dendritic and synaptic activities of many neurons near the electrode. (Figure 14-35)
local interneuron (LN) (in the insect olfactory system)
A neuron whose processes are restricted to the antennal lobe. (Figure 6-24)
local neuron
See interneuron.
local protein synthesis
Translation of mRNA into protein in a neuron’s dendrites and axons rather than in its cell body.
locus coeruleus
A brainstem nucleus consisting of norepinephrine neurons that project widely across the brain. (Figure 9-32)
long-range cue (in axon guidance)
A secreted protein that can act at a distance from its cell of origin. (Figure 5-11)
long-term depression (LTD)
A long-lasting decrease in synaptic efficacy that can be induced experimentally by specific stimulus conditions.
long-term memory
Memory that lasts hours to years. (Figure 11-3)
long-term potentiation (LTP)
A long-lasting enhancement of synaptic efficacy. It can be induced experimentally by a variety of stimuli, such as high-frequency stimulation of input axons. (Figure 11-7)
long-term synaptic plasticity
A change in the efficacy of synaptic transmission that lasts hours to the lifetime of the animal.
loose-patch recording
A technique in which a patch electrode is placed against a cell membrane without forming a gigaohm seal. It can be used to record only spiking activity (not subthreshold activity) but, unlike whole-cell recording, does not affect the intracellular content of the recorded cell.
lordosis
A posture that female rodents assume when sexually aroused. It facilitates sexual intercourse.
loss-of-function experiment
An experiment in which a specific component is disrupted, often used to determine if the missing component is necessary for the system to function.
loss-of-function mutation
A mutation that disrupts the function of a gene.
lower envelope principle
The idea that the limits of psychophysical performance are determined by the sensitivities of the most sensitive individual neurons. (Figure 6-74)
loxP
See Cre recombinase.
LRP4 (low-density lipoprotein receptor-related protein-4)
Along with MuSK, it is an agrin receptor in muscle. It also signals back to motor axons to trigger presynaptic differentiation in a MuSK-independent manner. (Figure 7-25)
LTMRs (low-threshold mechanoreceptors)
Touch-sensitive somatosensory neurons that innervate hair follicles, specialized epithelial cells, and encapsulated corpuscles in the skin. They respond to vibration, indentation, pressure, and stretch of the skin as well as to movement or deflection of hairs. (Figure 6-64)
luminance contrast (in vision)
A difference in light intensity between adjacent spaces.
luteinizing hormone (LH)
See gonadotropins.
lysosome
A membrane-enclosed organelle that contains enzymes for protein degradation. (Figure 2-2)
M
M1
See primary motor cortex (M1).
macular degeneration
A disease that causes photoreceptors in the fovea to die, impairing high-acuity vision.
major depression
A mood disorder characterized by persistent feelings of sadness, emptiness, and worthlessness.
major urinary protein (MUP)
A highly stable protein found in urine that is used by some species to mark an individual’s territory for a long duration.
MARCM (mosaic analysis with a repressible cell marker)
A genetic mosaic method in Drosophila used to label individual cells or groups of cells that share the same lineage and at the same time to delete an endogenous gene or express a transgene specifically in these labeled cells. (Figure 14-26)
Martinotti cell
A type of GABAergic neuron in the cerebral cortex that forms synapses onto the distal dendrites of pyramidal cells. (Figure 3-46)
massively parallel processing
An information processing method; it utilizes a large number of units to perform a set of coordinated computations in parallel. It is a key feature of the nervous system.
mass spectrometry
An analytic technique that can precisely determine the mass-to-charge ratio of electrically charged molecules; it is widely used to determine the identity of individual proteins in a protein mixture. (Figure 14-20)
maximum parsimony
A means of generating phylogenetic predictions by selecting the interpretation of the experimental data that posits the fewest number of evolutionary changes among all the potential interpretations.
MC4R
A G-protein-coupled receptor activated by α-MSH to reduce food intake. (Figure 9-14)
mechanosensitive channel
An ion channel gated by mechanical force.
mechanosensory neurons
Somatosensory neurons activated by mechanical force and responsible for proprioception, touch, and a subset of nociceptive sensations.
mechanotransduction
The process in sensory cells by which mechanical stimuli are converted into electrical signals.
MeCP2 (methyl-CpG-binding protein 2)
A nuclear protein that binds to DNA at methylated CpG sites (adjacent cytosine and guanine nucleotides). It is highly expressed in developing and adult neurons. See also Rett syndrome. (Figure 12-35)
medial amygdala
Part of the olfactory amygdala complex that receives direct input from accessory olfactory bulb mitral cells. It is sexually dimorphic and regulates male courtship behavior. (Figure 10-26)
medial geniculate nucleus
A thalamic nucleus that processes and relays auditory signals to the auditory cortex. (Figure 6-52)
medial–lateral
Of a body axis, from midline to side. (Figure 1-8)
medial preoptic area (MPOA)
A sexually dimorphic nucleus in the anterior hypothalamus that regulates male courtship behavior. (Figure 10-23; Figure 10-26)
median preoptic nucleus (MnPO)
Part of the lamina terminalis in the anterior hypothalamus; it is responsible for integrating dehydration signals sensed by the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT) and broadcasting the signals to downstream neurons. (Figure 9-16)
medulla
The caudal-most part of the brainstem, between the pons and the spinal cord. (Figure 1-8)
medulla (in insect visual systems)
A neuropil that lies beneath the lamina in the insect optic lobe. (Figure 4-31; Figure 5-34)
melanopsin
An opsin expressed by vertebrate intrinsically photosensitive retinal ganglion cells; it is a member of the c-opsin subfamily, whose members are most widely used in invertebrate visual systems.
membrane potential
The electrical potential difference between the inside of a cell and its extracellular environment.
memory
Lasting changes in the brain that retain learned information. It is typically divided into several distinct processes, including acquisition, consolidation, storage, and retrieval.
memory trace
See engram.
Merkel cell
A specialized epithelial cell at the junction of the dermis and epidermis; it is closely associated with the peripheral ending of the slowly adapting type I (SAI) LTMR. (Figure 6-63)
mesencephalic locomotor region (MLR)
A midbrain region wherein electrical stimulation evokes locomotor activity.
mesoderm
The middle germ layer, which gives rise to the skeletal system, connective tissues, muscle, and the circulatory system. (Figure 7-2)
messenger RNA (mRNA)
A mature RNA molecule that has undergone 5′ capping, 3′ polyadenylation, and splicing to remove introns and is exported to the cytoplasm to direct protein synthesis. (Figure 2-2)
metabotropic receptor
A neurotransmitter receptor that regulates ion channel conductance indirectly through intracellular signaling cascades, modulating membrane potential over a time scale of tens of milliseconds to seconds. (Figure 3-22)
microglia
A glial cell that functions as the resident immune cell of the nervous system. It engulfs damaged cells and debris. (Figure 1-9)
microneurography
A neurophysiological technique used to record neuronal activity in the peripheral nerves of awake human subjects.
microRNA
A short, noncoding RNA (21–26 nucleotides in length) widely used in eukaryotic organisms to regulate protein production. It triggers the degradation and inhibits the translation of mRNAs with complementary sequences. See also RNA interference.
microstimulation
Delivery of small currents through an extracellular electrode with the goal of activating a limited number of nearby neurons.
microtubule
A major cytoskeletal element composed of hollow cylinders of 13 parallel protofilaments made of α- and β-tubulin. (Figure 2-5)
midbrain
The rostral-most part of the brainstem. It includes the tectum (superior and inferior colliculus in mammals) dorsally and the tegmentum ventrally. It is also the middle part of the three divisions of the embryonic brain caudal to the forebrain and rostral to the hindbrain. It is also called the mesencephalon. (Figure 1-8; Figure 7-3)
middle temporal visual area (MT)
A high-order visual cortical area in the dorsal stream specialized for analyzing motion signals. (Figure 4-51)
midget ganglion cell
A type of retinal ganglion cell with a small receptive field used for high-acuity and green–red color vision in primates. (Figure 4-34)
miniature end-plate potential (mEPP)
Small depolarization of a muscle cell in response to spontaneous neurotransmitter release from a motor neuron. (Figure 3-2)
mirror neuron
Neuron in the premotor or posterior parietal cortex of primates, activated when an animal observes actions by another animal as well as when the animal performs the same actions it has observed.
mitogen-activated protein (MAP) kinase cascade
A kinase cascade that acts downstream of the small GTPase Ras and other signaling molecules; it consists of three serine/threonine kinases represented by Raf, Mek, and Erk. (Figure 3-39)
mitotic recombination
Exchange of a portion of homologous maternal and paternal chromosome during mitotic cell division; it can create daughter cells homozygous for alleles on portions of the paternal or maternal chromosomes. (Figure 14-10; Figure 14-26)
mitral cell
A second-order neuron in the vertebrate olfactory bulb; it receives input from ORNs and sends output to the olfactory cortex. It differs from a tufted cell, also a second-order neuron in the vertebrate olfactory bulb, in its cell body location in the olfactory bulb and axon termination pattern in the olfactory cortex. (Figure 6-16)
MnPO
See median preoptic nucleus.
modulatory neurons
Neurons that release modulatory neurotransmitters. They can act on excitatory, inhibitory, and other modulatory neurons to up- or downregulate their excitability or synaptic transmission.
modulatory neurotransmitter (neuromodulator)
A neurotransmitter that can bidirectionally change the membrane potential, excitability, or neurotransmitter release of its postsynaptic target neurons. It acts through G-protein-coupled receptors.
molecular clock
A technique that utilizes the rates of sequence changes, calibrated against fossil records, to estimate the times at which two species diverged.
monoamine neurotransmitter
A neurotransmitter derived from an aromatic amino acid, including serotonin, dopamine, norepinephrine, and histamine.
monoamine oxidase
An enzyme that oxidizes dopamine, norepinephrine, and serotonin, leading to their degradation. (Figure 12-23)
monozygotic (identical) twins
Twins produced from the same fertilized egg or zygote; they share 100% of their genomes.
morphine
The active ingredient of opiates and a powerful opioid receptor agonist.
morphogen
A diffusible signaling protein that can cause cells located at different distances from the source to adopt different fates.
Morris water maze
A navigation task in which rats and mice learn to locate a hidden platform in a pool of milky water using distant cues in the room.
Mosaic analysis
A method for analyzing the cell types in which the function of a gene is required by creating genetic mosaic animals containing both wild-type and mutant cells that are usually differentially marked.
mossy fiber
An axon that has elaborate terminal arborizations. The two most prominent types are found in the cerebellum and hippocampus. (1) The cerebellar mossy fiber is an axon that terminates in the granular layer of the cerebellar cortex, where it synapses onto granule cells. It originates from neurons residing in the pons, medulla, and spinal cord. (Figure 8-26) (2) The hippocampal mossy fiber is an axon of a dentate gyrus granule cell that synapses onto CA3 pyramidal neuron dendrites. (Figure 11-5)
motor homunculus
A map in the primary motor cortex that corresponds to movement of specific body parts. Nearby areas in the motor cortical areas represent movement control of nearby body parts. (Figure 1-25)
motor neuron
A type of neuron that extends dendrites within the CNS and projects its axon out of the CNS to innervate a muscle. (Figure 1-15; Figure 8-9)
motor pool
A cluster of motor neurons innervating the same muscle. (Figure 8-6)
motor protein
A protein that converts energy from ATP hydrolysis into movement along the cytoskeletal polymers.
motor system
The collected parts of the nervous system that control the contraction of skeletal muscles and thereby enable movement and maintain body posture.
motor unit
A motor neuron and the set of muscle fibers it innervates. (Figure 8-6)
motor unit size
The number of muscle fibers a motor neuron innervates.
mount
A posture that male rodents assume when sexually aroused. It facilitates sexual intercourse.
M pathway
A visual processing pathway from the retina to the visual cortex that originates from retinal ganglion cells with large receptive fields and engages lateral geniculate nucleus cells in the magnocellular layers; it carries information about luminance and has excellent contrast and temporal sensitivity. (Figure 4-51)
MrgprA3
A G-protein-coupled receptor activated by pruritogens such as chloroquine.
α-MSH (α-melanocyte-stimulating hormone)
A neuropeptide released by POMC neurons in the arcuate nucleus that reduces food intake.
mTOR (mammalian target of rapamycin)
A key protein in intracellular signaling pathways that plays an important role in regulating protein translation. (Figure 12-41)
Müller glia
A glial cell in the retina wherein the conversion of all-trans retinal to 11-cis retinal occurs to assist the recovery process in cones.
multielectrode array
A device used to record the spiking activities of many individual neurons. The electrodes can be arrayed either horizontally or vertically. (Figure 14-36)
multiple sclerosis (MS)
A common adult-onset CNS demyelinating disease characterized by inflammatory plaques in the white matter caused by immune cell attack of myelin.
multipolar (neuron)
Having more than two processes leaving the cell body.
muscarinic AChR
See acetylcholine receptor (AChR).
muscimol
A mushroom-derived toxin that is a potent activator of the GABAa receptor.
muscle fiber
A muscle cell.
muscle spindle
A special apparatus in muscle cells that senses muscles stretch. It has embedded endings of peripheral branches of proprioceptive somatosensory neurons. (Figure 1-19; Figure 6-63)
mushroom body
A second-order olfactory center for odor-mediated learning and memory in the insect brain; it and the lateral horn are the two major output sites for olfactory projection neuron axons. (Figure 6-24; Figure 11-29)
MuSK
A muscle-specific receptor tyrosine kinase; it acts together with LRP4 as an agrin receptor to promote acetylcholine receptor clustering. (Figure 7-25)
mutation
A change in DNA, including insertion, deletion, or alteration of one or more base pairs.
myelin sheath
Cytoplasmic extensions of oligodendrocytes and Schwann cells that wrap around axons with multilayered glial plasma membranes to increase resistance and decrease capacitance for action potential propagation. See also axon myelination. (Figure 2-26; Figure 2-27)
myofibril
A thread-like longitudinal structure in muscle cells composed of repeating sarcomeres and responsible for muscle contraction. (Figure 8-3)
myosin
An F-actin-based motor protein. (Figure 8-3)
N
β2 nAChR
A subunit of nicotinic acetylcholine receptors that, among other functions, is essential for cholinergic retinal wave propagation. See also acetylcholine receptor (AChR).
Na+–K+ ATPase
A pump that uses energy derived from ATP hydrolysis to pump Na+ out of a cell and K+ into a cell against their respective electrochemical gradients; it helps maintain the Na+ and K+ concentration differences across cellular membranes. (Figure 2-12)
narcolepsy
A disorder characterized by difficulty staying awake during the day, especially following moments of happiness or excitement, caused by a deficiency in the neuropeptidergic hypocretin signaling system or by dysfunction of hypocretin-expressing neurons.
nasal (in retinal maps)
In the direction of the nose.
natural selection
A key mechanism of evolution; it is the process by which genetic variations that confer individuals a better chance of reproductive success become more common in a population over time. (Figure 13-1)
nature (in context of nature versus nurture)
The contribution of genetic inheritance to brain function and behavior.
negative reinforcement
A type of learning in which an action occurs more often because it is followed by removal of a punishment.
negative selection
The process by which an allele that is detrimental to an organism becomes less prevalent in a population.
nematocin
The C. elegans ortholog of vertebrate oxytocin and vasopressin. (Figure 10-36)
neocortex
The largest part of the mammalian cerebral cortex; it typically contains six layers and is evolutionarily the newest part of the cerebral cortex.
Nernst equation
An equation that relates the equilibrium potential of an ion to the concentrations of the ion on the two sides of a membrane.
nerve
A discrete bundle of axons in the peripheral nervous system.
nerve growth factor (NGF)
A prototypical neurotrophin; it is a target-derived secreted protein that supports the survival and axon growth of sensory and sympathetic neurons. (Figure 7-34; Figure 7-35)
nerve impulse
The historical name for transient changes in membrane potential that propagate along axons; it is the same as an action potential.
netrin/Unc6
Homologous secreted proteins originally identified by biochemical purification (netrins in vertebrates) and genetic screening (Unc6 in C. elegans) in the context of midline guidance; they are widely used axon guidance cues. (Figure 5-12)
neural circuit
An ensemble of interconnected neurons that act together to perform specific functions.
neural crest cells
A special group of cells at the junction of the dorsal neural tube and the overlying epidermal cells. They migrate away from the neural tube to produce diverse cell types, including cells of the peripheral nervous system. (Figure 7-2)
neural plasticity
Changes of the nervous system in response to experience and learning.
neural plate
The layer of ectodermal cells overlaying the notochord that invaginates and gives rise to the neural tube during neurulation. (Figure 7-2)
neural progenitor
A dividing cell that gives rise to neurons and glia. In vertebrates, it is usually located near the ventricle in the developing vertebrate CNS. (Figure 7-4)
neural prosthetic device
A device that can substitute a sensory or motor function that has been disrupted due to injury or disease. For example, population activity of neurons in the motor cortex can be used to control an external device such as a robotic arm or a computer cursor to help patients who suffer from paralysis or motor neuron diseases. (Figure 8-40)
neural tube
A hollow tube surrounded by layers of neuroectodermal cells; it is the embryonic precursor to the vertebrate CNS. (Figure 7-2)
neuraxis
Axis of the CNS. The rostral–caudal neuraxis follows the curvature of the embryonic neural tube; the dorsal–ventral neuraxis is perpendicular to the rostral–caudal neuraxis. (Figure 1-8)
neurexin
A protein on the presynaptic membrane that mediates synaptic adhesion. A major binding partner is neuroligin. (Figure 7-26)
neuroblast
A neuronal progenitor.
neurodegenerative disorders
Disorders characterized by progressive neuronal dysfunction, including loss of synapses, atrophy of dendrites and axons, and death of neurons.
neuroendocrine system
The collected parts of the nervous system that control hormone secretion and thus regulate animals’ physiology and behavior in response to sensory stimuli and brain states.
neuroethology
A branch of science that emphasizes the study of animal behavior in the natural environment.
neurofibrillary tangle
An intracellular fibril consisting of an abnormal accumulation of hyperphosphorylated tau, a microtubule-binding protein. (Figure 12-2)
neurofilament
An intermediate filament (a cytoskeletal polymer with a diameter between F-actin and microtubules) in vertebrate neurons; it is concentrated in and provides stability to axons.
neurogenic inflammation
Inflammation triggered by release of neuropeptides such as substance P and calcitonin gene-related peptide from the peripheral terminals of sensory neurons.
neuroligin
A protein on the postsynaptic membrane that mediates synaptic adhesion. A major binding partner is neurexin. (Figure 7-26)
neuromodulator
See modulatory neurotransmitter.
neuromuscular junction
The synapse between a motor neuron’s presynaptic terminal and a skeletal muscle cell. (Figure 3-1)
neuron (nerve cell)
An electrically excitable cell that receives, integrates, propagates, and transmits information as the working unit of the nervous system.
neuronal polarity
The distinction between axons and dendrites.
neuronal process
Cytoplasmic extension of a neuron.
neuron doctrine
The proposition that the individual neuron is the working unit of the nervous system.
neuropeptide
Polypeptide that is a few to a few dozen amino acids in length and acts as a neurotransmitter.
neuropil
Region of neural tissue densely packed with neuronal and glial processes.
neuropilin-1 (Nrp1)
A co-receptor for semaphorins in vertebrates.
neurotransmitter reuptake
The process by which neurotransmitters in the synaptic cleft are transported either into nearby glial cells or back into the presynaptic cytosol and then into synaptic vesicles. (Figure 3-12)
neurotransmitters
Molecules stored in synaptic vesicles (or dense-core vesicles in the case of neuropeptides) in presynaptic terminals, released into the synaptic cleft (triggered by presynaptic depolarization), and activate ionotropic or metabotropic receptors on postsynaptic target cells. (Figure 3-16; Table 3-2)
neurotrophic hypothesis
The idea that the survival of developing neurons requires neurotrophins produced by the neurons’ postsynaptic targets.
neurotrophin-3 (NT3)
See neurotrophins.
neurotrophin-4 (NT4)
See neurotrophins.
neurotrophins
A family of secreted signaling proteins that regulate the survival, morphology, and physiology of target neurons through binding to specific receptors on those neurons. Mammalian neurotrophins include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), and neurotrophin-4 (NT4). See also Trk receptors and p75NTR. (Figure 3-39; Figure 7-35)
neurulation
The developmental process in vertebrate embryos leading to formation of the neural tube, which gives rise to the nervous system. (Figure 7-2)
nicotinic AChR
See acetylcholine receptor (AChR).
Nissl stain
A stain that labels RNA and thus highlights the rough endoplasmic reticulum in cytoplasm. It uses basic (i.e., positively charged) dyes such as cresyl violet that bind to negatively charged RNA molecules. (Figure 14-22)
NMDA receptor
A glutamate-gated ion channel that conducts Na+, K+, and Ca2+ and can be activated by the drug NMDA (N-methyl-d-aspartate). Its opening requires both binding of glutamate and postsynaptic depolarization. It is a heterotetramer of two GluN1 subunits encoded by a single gene, and two GluN2 subunits, of which there are four variants (GluN2A, GluN2B, GluN2C, GluN2D) each encoded by separate genes. (Figure 3-24)
nociception
The sense of noxious stimuli by the somatosensory system.
nociceptive neuron
A somatosensory neuron that detect noxious stimuli.
nodes of Ranvier
Periodic gaps in axonal myelination, usually 200 µm to 2 mm apart, where the axon surface is exposed to the extracellular ionic environment. They contain high concentrations of voltage-gated Na+ and K+ channels that regenerate action potentials. (Figure 2-26)
noise (in neural encoding and decoding)
The source of variability in spike trains unexplained by stimuli.
nonhomologous end joining
An endogenous DNA repair system; it rejoins the two ends of a DNA molecule with a double-strand break. It often creates a small deletion or duplication at the break point as a result of the repair process. (Figure 14-8)
non-spiking neuron
A neuron that uses graded potentials rather than action potentials to transmit information.
norepinephrine
A monoamine neuromodulator derived from dopamine. (Figure 12-20; Table 3-2)
northern blot
A method for determining the amount of a specific RNA in an RNA mixture. RNAs are separated by gel electrophoresis and transferred to a membrane; labeled nucleic acid probes are then hybridized to the membrane to visualize specific RNA molecules that hybridize to the probe. (Figure 6-8)
Notch
A transmembrane receptor widely used in diversifying cell fate during development. Binding of a ligand to Notch triggers proteolytic cleavage of Notch in its transmembrane domain, releasing the Notch intracellular domain, which can then enter the nucleus to regulate gene expression. (Figure 7-9)
notochord
A midline mesodermal structure in vertebrate embryos ventral to the spinal cord that produces secreted cues for patterning the spinal cord. (Figure 7-2; Figure 7-10)
NREM sleep
Non-rapid eye movement (NREM) sleep, or sleep stages other than REM sleep. (Figure 9-28)
nucleus accumbens
The major part of the ventral striatum; it is involved in processing reward signals and receives input from VTA dopamine neurons and the prefrontal cortex, thalamus, hippocampus, and amygdala. (Figure 12-29)
nucleus laminaris (NL)
A brainstem nucleus in the barn owl that analyzes interaural time differences; it is analogous to the medial superior olivary nucleus in mammals.
nucleus of the solitary tract (NTS)
A nucleus in the brainstem that receives input from the taste system as well as sensory information from internal organs. (Figure 6-32; Figure 9-4)
null direction
The direction of stimulus motion that elicits the lowest firing rate of a direction-sensitive visual system neuron.
Numb
A Drosophila protein that segregates asymmetrically in daughter cells during sensory organ precursor and neuroblast divisions; it is essential for conferring different fates to the two daughter cells of an asymmetric division. (Figure 7-8)
nurture (in context of nature versus nurture)
The contribution of environmental factors to brain function and behavior.
O
occipital lobe
One of the four cerebral cortex lobes; it is located at the caudal-most part of the neocortex. (Figure 1-23)
octopamine
A neurotransmitter in some invertebrate nervous systems that is chemically similar to norepinephrine in vertebrates.
ocular dominance
Preference for receiving and representing visual input from one eye over the other. In the primary visual cortex of some mammals, such as cats and monkeys, cells in the same vertical columns share the same ocular dominance, thus producing ocular dominance columns. (Figure 5-18)
ocular dominance column
See ocular dominance.
odds ratio
In genetics, a measure of the effect of a genetic variant on the likelihood of having a particular trait, such as a disease. It is calculated by dividing the probability of having the trait among people with the genetic variant by the probability of having the trait among people without the genetic variant.
odorant
A molecule that elicits olfactory perception; it is usually volatile. (Figure 6-6)
odorant receptor
A receptor on the surface of olfactory cilia that binds odorants. (Figure 6-8)
OFF bipolar
A bipolar cell that expresses ionotropic glutamate receptors and is depolarized by glutamate release from photoreceptors; its membrane potential changes follow the sign of photoreceptors, such that it is hyperpolarized by light. (Figure 4-22)
Ohm’s law
An equation that relates current (I) to voltage (V) and resistance (R); I = V/R.
olfactory bulb
The first olfactory processing center in the vertebrate brain. (Figure 6-3; Figure 6-14)
olfactory cilium
A dendritic branch of an olfactory receptor neuron enriched in odorant receptors. (Figure 6-3)
olfactory cortex
Brain regions that receive direct input from olfactory bulb mitral/tufted cells, including the anterior olfactory nucleus, piriform cortex, olfactory tubercle, cortical amygdala, and entorhinal cortex. (Figure 6-16)
olfactory epithelium
The epithelial layer in the nose that houses olfactory receptor neurons. (Figure 6-3)
olfactory processing channel
A discrete information-processing unit in the olfactory system consisting of olfactory receptor neurons (ORNs) expressing a given odorant receptor, the glomerular targets of those ORNs, and second-order neurons that send dendrites to the same glomeruli.
olfactory receptor neuron (ORN)
A primary sensory neuron in the olfactory system; it converts odorant binding to odorant receptors into electrical signals relayed to the brain via its axon. Also called an olfactory sensory neuron (OSN). (Figure 6-3)
oligodendrocyte
A glial cell in the CNS that wraps axons with its cytoplasmic extensions to form a myelin sheath. (Figure 1-9)
ommatidium
A repeating unit of the arthropod compound eye. In Drosophila, each ommatidium contains eight photoreceptors. (Figure 5-34)
ON bipolar
A bipolar cell that expresses metabotropic glutamate receptors and is inhibited by glutamate release from photoreceptors; its membrane potential changes are opposite in sign to those of photoreceptors, such that it is depolarized by light. (Figure 4-22)
open probability
The proportion of time an individual ion channel is open and able to conduct current.
operant conditioning (instrumental conditioning)
A form of learning in which a subject associates performance of a specific action (e.g., pressing a lever) with a particular outcome, such as delivery of a reinforcer (e.g., food) or punishment (e.g., an electrical shock).
opioid receptors
A subfamily of G-protein-coupled receptors that serve as receptors for opioids, including morphine and endogenous opioid neuropeptides. They are widely distributed across the nervous system.
opioids
Molecules with similar effects to opiates such as morphine. They include opiates from the opium poppy and endogenous neuropeptides such as enkephalin, endorphin, and dynorphin.
opsin
A member of a family of G-protein-coupled receptors expressed in photoreceptors of multicellular organisms; it is associated with retinal and converts photon absorption into activation of a trimeric GTP-binding protein. In microbes, it is a member of a family of light-induced channels or pumps that are not G-protein-coupled receptors. (Figure 4-6; Figure 13-20)
optical imaging
An approach that uses changes in fluorescence or other optical properties as indicators of neuronal activity.
optic ataxia
A condition in which patients cannot guide their hand toward an object using visual information, even though other aspects of their movement and vision are less affected.
optic chiasm
The midline structure where a fraction of retinal ganglion cell axons cross to the side of the brain contralateral to the eye of origin. (Figure 4-37; Figure 5-13)
optic lobe
The part of the insect brain that consists of the retina, lamina, medulla, and lobula complex and is used to analyze visual signals. (Figure 5-34)
optic nerve
The bundle of retinal ganglion cell axons; it sends visual information from the eye to the brain. (Figure 4-37)
optic tract
The bundles of retinal ganglion cell axons distal to the optic chiasm. (Figure 4-37)
optogenetics
The set of methods used to manipulate neuronal activity by using light to activate genetically encoded effectors, most commonly microbial opsins (e.g., channelrhodopsin-2, archaerhodopsin, halorhodopsin). (Figure 14-47)
orexin
See hypocretin.
organization–activation model
A central principle in endocrinology; it proposes that sex hormones have two different types of effects: organizational effects during development, which configure the brain in a sex-typical manner, and activational effects in adults, which stimulate male- or female-typical sexual behaviors. (Figure 10-20)
organ of Corti
An organ in the cochlea consisting of hair cells, the surrounding support cells, and the basilar membrane. (Figure 6-43)
organoids
Miniature structures that resemble certain aspects of specific organs, produced from pluripotent stem cells in three-dimensional cultures in vitro.
otolith organ
A sensory organ in the vestibular system that senses linear acceleration and stationary head tilts. (Figure 6-59)
outer radial glia (oRG)
A type of radial glia whose cell bodies are located in the subventricular zone. They serve, along with ventricular zone radial glia, as neural progenitors. They are greatly expanded in number in humans compared to mice and likely contribute to increased neuronal production in mammals with large neocortices. See also radial glia. (Figure 13-37)
outer segment
A cytoplasmic extension of a rod or a cone; it contains a highly specialized photon detection apparatus made of tightly stacked membrane disks enriched in opsins. (Figure 4-2)
outgroup
A group of organisms that is closely related to but falls outside of a set of organisms of interest. It is used as a reference group in determining the phylogenetic relationships among a set of organisms.
ovariectomized female
A female from which the ovaries have been removed.
OVLT (organum vasculosum of the lamina terminalis)
A part of the lamina terminalis in the anterior hypothalamus responsible for sensing dehydration signals from the blood, including osmolarity and angiotensin II levels. (Figure 9-16)
oxytocin
A hormone secreted by hypothalamic neurons in the posterior pituitary and a neuropeptide released by some hypothalamic neurons; it regulates maternal and social behavior.
P
P1 neurons
A cluster of ~20 Fru+/Dbx+ neurons in the male fly brain that integrates mating-related sensory signals and promote courtship behavior.
p75NTR
A 75 kilodalton neurotrophin receptor that has a low affinity for all neurotrophins and is also a receptor for all proneurotrophins. (Figure 7-35)
pacemaker cell
A cell that can produce rhythmic output in the absence of input.
PALM
See super-resolution fluorescence microscopy.
parabiosis
The joining of the circulatory systems of two animals such that they have limited exchange of substances in systemic circulation.
parabrachial nucleus
A brainstem nucleus that transmits ascending signals from the visceral sensory and pain somatosensory systems to the thalamus, amygdala, hypothalamus, and brainstem autonomic centers. (Figure 6-71; Figure 9-15)
paracrine
Of or related to a form of signaling in which a recipient cell receives a signal produced by nearby cells.
parallel fiber
The portion of the axon of a cerebellar granule cell that runs in parallel to the pial surface and crosses Purkinje cell dendrites at a right angle. (Figure 8-26)
parasol ganglion cell
A type of retinal ganglion cell in primates with a large receptive field and excellent contrast and temporal sensitivity used for motion vision.
parasympathetic system
A branch of the autonomic nervous system that facilitates energy conservation. Activation of the parasympathetic system decreases heart rate and blood flow, constricts airways in the lung, and stimulates salivation and digestion. (Figure 9-1; Figure 9-2)
paraventricular hypothalamic nucleus (PVH)
A hypothalamic nucleus involved in multiple physiological functions, including release of oxytocin and vasopressin into the bloodstream through axonal projections in the posterior pituitary and descending control of autonomic nervous system functions. (Figure 9-6)
parietal lobe
One of the four cerebral cortex lobes; it is located behind the frontal lobe and above the occipital lobe. (Figure 1-23)
parietal reach region (PRR)
A region in monkey’s posterior parietal lobe that is selectively associated with arm reaching.
Parkinson’s disease (PD)
A common neurodegenerative disease caused by death of substantia nigra dopamine neurons; it primarily affects movement control, with symptoms including shaking, rigidity, slowness, and difficulty walking.
parthenogenesis
A reproductive strategy in which embryos develop from unfertilized eggs without exchange of genetic materials.
partial agonist
A drug that binds to a target receptor but elicits only a partial biological effect.
passive electrical properties
Membrane properties in the absence of voltage-dependent conductances. Two salient examples: (1) due to membrane capacitance, a sharp change in electrical signal (e.g., a current pulse) becomes more diffuse temporally as the signal travels along a neuronal process; (2) due to membrane conductance, the magnitude of electrical signal becomes attenuated over distance. They are also called cable properties. (Figure 2-16)
passive transport
Movement of a solute across a membrane down its electrochemical gradient via a channel or transporter. (Figure 2-8)
patch clamp recording
An electrophysiological recording technique that utilizes a glass electrode (patch pipette) to form a high-resistance seal with a membrane. It has several variants, including cell-attached patch, excised patch, and whole-cell recordings. (Figure 14-40)
patch pipette
See patch clamp recording.
path-integration strategy
A navigational strategy in which animals use the speed, duration, and direction of their own movement to calculate their current position with respect to their starting position.
pattern separation
A theoretical neuroscience concept that describes the ability to separate similar neural states, such as firing patterns of neuronal populations, or similar memories.
Pax6
A member of the Pax family of transcription factors; it contains a homeobox and a paired box. It regulates patterning of the cerebral cortex and spinal cord and is required for eye development in mammals. Its Drosophila homolog is Eyeless. See also Eyeless.
pC1 neurons
A cluster of Fru–/Dbx+ neurons in the female fly brain that integrates mating-related sensory signals and promotes courtship receptivity.
PDZ domain
Acronym for a domain shared by PSD-95, Discs large (a Drosophila protein implicated in cell proliferation and associated with postsynaptic densities), and ZO-1 (an epithelial tight junction protein). It is a protein–protein interaction domain that binds to a specific protein sequence motif present at the C-terminal end of many transmembrane receptors.
percept
A specifically perceived object or the brain representation of the object.
perceptron
An artificial neural network with a feed-forward architecture; it can be used to discriminate visual objects through changes in connection weights (usually in a single layer) after viewing examples and their correct labels.
perforant path
The pathway taken by axons of neurons in the superficial layer of the entorhinal cortex that project to the hippocampus. (Figure 11-5)
periaqueductal gray (PAG)
A midbrain gray matter structure surrounding the cerebral aqueduct; it serves many functions, including descending control of pain and execution of defensive behaviors such as freezing. (Figure 6-71; Figure 11-45)
periglomerular cell
A member of diverse types of interneurons in the olfactory bulb that receive direct input from olfactory receptor neuron (ORN) axons or apical dendrites of mitral cells and send (mostly inhibitory) output to targets within the same glomerulus or in nearby glomeruli. (Figure 6-14)
Period
A fruit fly gene discovered based on mutations that speed up, slow down, or disrupt circadian rhythms. It encodes a protein that participates in negative regulation of its own transcription, and its mammalian homologs serve a similar function. (Figure 9-21)
peristimulus time histogram (PSTH)
A graph that plots firing rates of neurons as a function of time after stimulus onset.
permeability
The ability of a membrane to conduct specific ions, determined principally by the number of open channels capable of conducting those ions.
perturbation experiment
An experiment in which key parameters in a biological system are altered, usually under the experimenter’s control, in order to study the consequences.
pharmacodynamics
The effects of a drug in the body, including intended effects on target molecules and processes as well as unintended side effects.
pharmacokinetics
The effects of the body’s biological processes on a drug, including the drug’s absorption, distribution, metabolism, and excretion.
phase locking
A property whereby the spikes of auditory neurons occur at a specific phase of each cycle of a sound wave. (Figure 6-49)
phasic
Of a neuronal firing pattern, bursts of action potentials in response to specific stimuli.
pheromone
A substance produced by an individual to elicit a specific reaction from other individuals of the same species.
phosphodiesterase (PDE)
An enzyme that hydrolyzes cyclic AMP (cAMP) to AMP, or cGMP to GMP.
phospholipase C (PLC)
A membrane-associated enzyme that is activated by Gq and cleaves inositol-phospholipids to produce inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). (Figure 3-34)
photoreceptor
A cell that converts light into electrical signals. (Figure 4-2; Figure 13-22)
phototaxis
Movement toward or away from a light source.
phototransduction
The biochemical reactions triggered by photon absorption. (Figure 4-8)
phrenology
A discipline in the 1800s with the goal of mapping the functions of brain areas by studying the shape and size of bumps and ridges on the skull, which were thought to be correlated with an individual’s talents and character traits. (Figure 1-22)
phylogenetic tree
A branching diagram showing the relationships among different organisms; it is constructed based on the similarities and differences of different organisms’ traits, such as nucleotide and protein sequences. (Figure 13-2)
picrotoxin
A plant toxin that is a potent blocker of the GABAa receptor.
Piezo
A mechanosensitive channel whose conductance is affected by mechanical force. (Figure 6-65)
pigment cell
A cell in the pigment epithelium layer of the retina adjacent to the outer segments of photoreceptors; it absorbs scattered photons and converts all-trans retinal back into 11-cis retinal to assist the recovery process in rods. (Figure 4-2)
piriform cortex
The largest olfactory cortical region; it is a three-layered cortex separated from the more dorsally located neocortex by the rhinal sulcus. (Figure 6-16)
pituitary
The endocrine center of the brain; it is located ventral to the hypothalamus. The posterior pituitary contains axon terminals of hypothalamic neurons that directly release hormones into the bloodstream. The anterior pituitary contains endocrine cells that release hormones into the bloodstream in response to prehormones originating from hypothalamic neurons and transmitted by specialized portal vessels. (Figure 9-6)
placebo effect
In the context of pain perception, the phenomenon whereby the perception of pain can be reduced in some patients by the mistaken belief that they have received a treatment thought to reduce pain.
place cell
A hippocampal cell that fires maximally when the animal is at a particular place in an environment. (Figure 11-31)
place field
The physical location in an environment that elicits maximal firing of a particular place cell. (Figure 11-31)
plasma membrane dopamine transporter (DAT)
See plasma membrane monoamine transporters.
plasma membrane monoamine transporters (PMATs)
A family of proteins on the presynaptic plasma membrane that transport serotonin (serotonin transporter [SERT]), dopamine (dopamine transporter [DAT]), or norepinephrine (norepinephrine transporter [NET]) from the synaptic cleft into the presynaptic cytosol. See also plasma membrane neurotransmitter transporter. (Figure 12-23)
plasma membrane neurotransmitter transporter
A transmembrane protein on the presynaptic or glial plasma membrane that transports neurotransmitters from the extracellular space into the cell, usually using energy from the co-transport of Na+ down its electrochemical gradient. (Figure 3-12)
plexin
A member of a class of proteins that serve as receptors for the axon guidance cue semaphorins.
pluripotent stem cell
A cell that has the potential to develop into all cell types of an embryo.
PNS (peripheral nervous system)
Neural tissue and cells outside the CNS, including the nerves that connect the CNS with the body and internal organs as well as isolated ganglia outside the CNS.
Poisson distribution
A discrete probability distribution in which the frequency (f) that k events occur can be determined by a single parameter λ (the mean frequency of occurrence, which equals the product of n and p in the binomial distribution): f(k; λ) = (λk / k!) e-λ. It is an approximation of the binomial distribution when n is large and p is small. (Box 3-1)
polymerase chain reaction (PCR)
A highly sensitive DNA amplification technique that uses a pair of oligonucleotide primers to amplify the DNA segment between the sequences corresponding to the primers through cycles of DNA replication.
polymodal neuron (in the somatosensory system)
A neuron that responds to stimuli of more than one sensory modality.
polymorphism (in genetics)
DNA sequence variation among individuals of the same species.
POMC neuron
A neuron in the arcuate nucleus that expresses pro-opiomelanocortin (POMC), a precursor protein for multiple peptides, including the anorexigenic peptide α-melanocyte-stimulating hormone (α-MSH). (Figure 9-14)
pons
The middle part of the brainstem caudal to the midbrain and rostral to the medulla. (Figure 1-8)
pontine nuclei
Nuclei located in the basal pons that receive input from the cerebral cortex and send output to the cerebellum.
population vector (in movement control)
The sum of the preferred direction vectors of a population of neurons weighted by the firing rate of each neuron. The preferred direction of a motor system neuron is a vector in a three-dimensional space pointing in the direction toward which movement elicits the highest firing rate of the neuron. (Figure 8-31)
positional cloning
A molecular genetic technique that uses molecular and genetic markers on specific chromosomes to identify a gene that causes a particular phenotype or disease.
positive reinforcement
A type of learning in which an action occurs more often because it is followed by receipt of a reward.
positive selection
The process by which an allele that is beneficial to an organism becomes more prevalent in a population.
positron emission tomography (PET)
A noninvasive three-dimensional imaging technique for measuring the distribution of positron-emitting probes introduced into the body.
posterior pituitary
See pituitary.
postganglionic neuron
A neuron whose cell body is located in a sympathetic or parasympathetic ganglion in the peripheral nervous system and whose axon innervates effectors such as smooth muscle, cardiac muscle, or glands. (Figure 9-1; Figure 9-2)
postsynaptic density
See postsynaptic specialization.
postsynaptic specialization
A structure on a postsynaptic target cell adjacent to a presynaptic terminal; it is enriched in neurotransmitter receptors and signaling and scaffold molecules. It is also called a postsynaptic density because it is electron dense in electron microscopic images.
Potocki–Lupski syndrome
A neurodevelopmental disorder characterized by mild intellectual disability and autistic symptoms; it is caused by duplication of a chromosome segment including Rai1, reciprocal to the deletion that causes Smith–Magenis syndrome.
power stroke
The process by which myosin and actin filaments move relative to each other; it involves conversion of chemical energy from ATP hydrolysis into mechanical force by the myosin motor. (Figure 8-4)
P pathway
A visual processing pathway from the retina to the visual cortex that originates from retinal ganglion cells with small receptive fields and engages lateral geniculate nucleus cells in the parvocellular layers; it carries information about high-acuity color vision. (Figure 4-51)
pre-Bötzinger complex (preBötC)
A caudal brainstem region that generates inspirational breathing rhythms.
precedence effect
The ability of a first-arriving sound to suppress perception of later-arriving sounds.
preferred direction (in the visual system)
The direction of stimulus motion that elicits the highest firing rate of a direction-sensitive visual system neuron.
prefrontal cortex
A neocortical area in the anterior frontal lobe; it is an executive control center that integrates multisensory information, mediates working memory, and performs complex executive functions such as goal selection and decision making.
preganglionic neuron
A neuron whose cell body is located within the CNS and whose axon synapses onto postganglionic neurons in the sympathetic or parasympathetic ganglia. (Figure 9-1; Figure 9-2)
premotor cortex
Area of motor cortex anterior to the primary motor cortex; its neurons send axons primarily to primary motor cortex.
premotor neuron
A spinal cord or brainstem neuron that is presynaptic to motor neurons and thereby participates directly in controlling the firing of motor neurons. (Figure 8-10)
preparatory activity
Neural activity preceding the onset of motor commands and often predictive of upcoming motor actions; it is abundant in neurons in the frontal and parietal lobes, in particular the premotor cortex.
presenilin
A member of a family (consisting of presenilin-1 and presenilin-2) of multi-pass transmembrane proteins that function as subunits of the γ-secretase complex. They were originally identified based on mutations that cause familial Alzheimer’s disease. (Figure 12-5)
prestin
A protein that mediates electromotility in cochlear outer hair cells.
presynaptic facilitation
The process by which neurotransmitter release from cell A onto the presynaptic terminal of cell B leads to an increase in neurotransmitter release from cell B.
presynaptic inhibition
The process by which neurotransmitter release from cell A onto the presynaptic terminal of cell B leads to a decrease in neurotransmitter release from cell B.
presynaptic terminal
A structure at the end (or along the trunk) of an axon that is specialized for releasing neurotransmitters onto target cells. (Figure 1-9)
pretectum
A brainstem structure that receives retinal ganglion cell axon input and regulates pupil, lens, and eye movement reflexes. (Figure 4-37)
primary antibody
An antibody that selectively recognizes a specific antigen (usually a protein).
primary auditory cortex (A1)
The part of the cerebral cortex that first receives auditory sensory information.
primary cilium
A single short, non-motile cilium that projects from the surface of many animal cell types and is often used as a signaling center.
primary motor cortex (M1)
The part of the cerebral cortex that sends descending axons to spinal cord (and in some species directly to motor neurons) to control muscle contraction. (Figure 1-25)
primary somatosensory cortex
The part of the cerebral cortex that first receives somatosensory information from the body. (Figure 1-25)
primary visual cortex (V1)
The part of the cerebral cortex that first receives visual input from the lateral geniculate nucleus. (Figure 4-37; Figure 4-47)
principal component analysis (PCA)
A statistical method used to reduce the dimensionality of a data set. The axes of the reduced data set are called principal components; their orientations in the nonreduced space are selected to maximize the spread of the data along each principal component—data are most spread along the axis of the first principal component, followed by the axis of the second principal component, and so forth.
prion
proteinaceous infectious particle.
prion diseases
Diseases characterized by propagation across the brain of prion protein (PrP) that adopts a specific conformation (PrPSc), which aggregates and causes massive neurodegeneration and neuronal death. They include scrapie in sheep and goats, mad cow disease in cows, kuru (a human disease that occurred in certain tribes that observed ritual cannibalism), and Creutzfeldt–Jakob disease (CJD; a human disease in which mutations in the Prp gene make PrPC more prone to adopt the PrPSc conformation spontaneously). (Figure 12-13)
prion hypothesis
The idea that the infectious agent in scrapie is solely proteinaceous in nature.
progesterone
A steroid hormone that regulates female sexual behavior and reproduction in conjunction with estradiol.
projection neuron
A neuron with an axon that projects outside the CNS region that houses the neuron’s cell body. In the insect olfactory system, it is a second-order neuron (PN) that receives input from olfactory receptor neuron (ORN) axons and sends output to higher olfactory centers, analogous to a vertebrate mitral/tufted cell. (Figure 6-24)
prokaryote
A single-cell organism without a nucleus. Prokaryotes are members of one of two domains of life: eubacteria and archaea.
proprioception
The sense of body position and movement.
proprioceptive neurons
Somatosensory neurons with peripheral endings embedded in muscle spindles, tendons, and joints to sense muscle stretch and tension. (Figure 6-63)
prostaglandin
A lipid released during inflammation; it binds to specific G-protein-coupled receptors on the peripheral terminals of nociceptive neurons. (Figure 6-73)
protein
Chain of amino acids with a specific sequence linked by peptide bonds.
protein kinase A (PKA)
See cAMP-dependent protein kinase.
protein kinase C (PKC)
A serine/threonine kinase with diverse substrates; it is activated by binding of both diacylglycerol and Ca2+. (Figure 3-34)
proteinopathy
A disease caused by altered protein conformations, interactions, and/or homeostasis.
protein phosphatase
An enzyme that removes phosphates from phosphorylated proteins, thus counteracting the actions of kinases.
proteome
The collection of all proteins in a specimen.
protocadherin
A member of a class of cell adhesion molecules in vertebrates whose structures and biochemical properties resemble those of cadherins.
protostomes
Animals in which the mouth appears before the anus during development. They include most invertebrate phyla. See also deuterostomes. (Figure 13-2)
proximity labeling
A technique for labeling macromolecules (proteins or RNAs) based on their proximity to a specific protein tagged with an enzyme that catalyzes the labeling reaction.
pruriception
The sense of itch.
pruritogen
A chemical that causes the sensation of itch.
PSD-95 (postsynaptic density protein of 95 kilodalton)
A postsynaptic scaffold protein highly enriched at glutamatergic synapses. (Figure 3-27; Figure 7-26)
pseudogene
A gene rendered nonfunctional by stop codons in its coding sequence or by other disrupting mutations. Such disrupting mutation(s) are prevalent in a given species.
psychometric function
The quantitative relationship between a parameter of a physical stimulus and the response or perception of a subject.
psychophysics
An experimental approach for characterizing the relationship between physical stimuli and the sensations or behaviors they elicit.
psychosis
A mental state characterized by hallucinations and/or delusions.
psychostimulant
A drug that transiently produces euphoria and suppresses fatigue.
pump
A transporter that uses external energy, such as ATP hydrolysis or light, to actively move a solute across a membrane against its electrochemical gradient. (Figure 2-10)
Purkinje cell
GABAergic neuron of the cerebellar cortex with a highly branched planar dendritic tree; it receives excitatory input from parallel fibers (axons of cerebellar granule cells) and climbing fibers from inferior olive neurons and sends output to the cerebellar nuclei. (Figure 1-11; Figure 8-26)
pyramidal neuron
A type of glutamatergic neuron that has a pyramid-shaped cell body with an apical dendrite and several basal dendrites that branch further; it is abundant in the mammalian cerebral cortex and hippocampus. (Figure 1-15)
Q
quantal content
The number of synaptic vesicle exocytosis events in response to a single action potential.
quantal hypothesis of neurotransmitter release
The idea that neurotransmitters are released in discrete packages of relatively uniform size.
R
RA (robust nucleus of the arcopallium)
A dorsal forebrain nucleus in the songbird essential for song production; it functions downstream of the HVC. (Figure 10-16)
Rab
A member of a family of small monomeric GTPases involved in intracellular vesicle trafficking.
rabies virus
A neurotropic RNA virus that spreads within the nervous system of its host naturally by crossing synapses. It has been modified for retrograde trans-synaptic tracing. (Figure 14-33)
radial glia
Progenitor cell in the ventricular zone that extends two radial processes—one to the ventricle and the other to the pial surface of the developing cortex. These radial processes serve as substrates for neuronal migration. (Figure 7-4)
random mutagenesis
See forward genetic screen.
random X-inactivation
A process in which one of the two X chromosomes in female mammals is randomly inactivated in each cell during early development.
raphe nuclei
Brainstem nuclei enriched in serotonin neurons that project widely across the brain. (Figure 9-32)
Ras
A member of a family of small monomeric GTPases involved in signaling pathways required for cell growth and differentiation.
R-C circuit
A circuit containing both resistors and capacitors. (Figure 2-14)
readily releasable pool
A small subset of synaptic vesicles docked at the active zone and primed by an ATP-dependent process to achieve a high-energy configuration that includes preassembled SNARE complexes.
receiver operating characteristic (ROC)
A curve in binary classification where the rate of true positives is plotted against the rate of false positives as the discriminating threshold is systematically changed.
receptive field
In the visual system, the area of the visual field that influences the activity of a given neuron. In the somatosensory system, the area of the body where stimuli can influence the firing of a neuron. Generally, the region of space from which an appropriate stimulus can influence the activity a given neuron in a sensory system.
receptor
A protein that binds and responds to a specific signaling molecule.
receptor potential
A type of graded potential induced at the peripheral endings of sensory neurons by sensory stimuli.
receptor tyrosine kinase (RTK)
A transmembrane protein with an N-terminal extracellular ligand-binding domain and a C-terminal intracellular tyrosine kinase domain. Upon ligand binding, receptor tyrosine kinases add phosphates to tyrosine residues of target proteins.
recombinase
An enzyme that catalyzes recombination between two sequence-specific DNA elements. See also Cre recombinase and Flp recombinase.
reconsolidation (of memory)
A repeated process of memory consolidation after retrieval of a previously consolidated memory.
recording electrode
An electrode used to measure membrane potential changes.
recovery (photoreceptor)
The process by which light-activated photoreceptor cells return to the dark state. (Figure 4-11)
recurrent (cross) excitation
A circuit motif in which two parallel excitatory pathways mutually excite each other. (Figure 1-20)
recurrent (cross) inhibition
A circuit motif in which two parallel excitatory pathways mutually inhibit each other via inhibitory interneurons. (Figure 1-20)
refractory period
A time window after an action potential during which another action potential cannot be initiated. (Figure 2-25)
regenerate (axons)
Have the ability to reextend and connect with their synaptic partners after damage.
regenerative (action potentials)
Propagating without attenuation in amplitude. (Figure 2-25)
regulator of G protein signaling (RGS)
A protein that acts as a GTPase activating protein for a trimeric GTP-binding protein.
reinforcement learning
A type of machine learning wherein an agent interacts with an environment by repeatedly performing an action and receiving a reward as a consequence of its action.
release probability
The probability that an active zone will release one or more synaptic vesicles following an action potential.
releaser
A concept in neuroethology; it refers to the essential features of a stimulus that activate a fixed action pattern.
Remak Schwann cell
A type of Schwann cell whose cytoplasm extends between individual unmyelinated axons, forming a Remak bundle. (Figure 2-27)
REM sleep
A stage of sleep characterized by rapid eye movement. (Figure 9-28)
repellent
A molecular cue that guides axons away from its source. (Figure 5-11)
reserpine
A first-generation antipsychotic drug; it is an inhibitor of monoamine oxidase.
resistance (R)
The degree to which an object or substance opposes passage of electrical current; it is the inverse of conductance: R = 1/g.
resistor
An electrical element through which passage of current is limited. Current flow through a resistor produces a voltage difference across its two terminals. (Figure 2-13)
responder transgene
In binary expression systems; it is the transgene containing the coding sequence for the protein or RNA of interest, along with binding or recombinase sites for the transcription factor or recombinase, respectively, encoded by the driver transgene. (Figure 14-12)
resting potential
The membrane potential of a neuron at rest (i.e., in the absence of action potentials or synaptic input), which is typically between –50 and –80 millivolts relative to the extracellular fluid. (Figure 2-11)
reticular theory
The idea that the processes of nerve cells fuse and form a giant net constituting the working unit of the nervous system. It has been mostly disproven (with the possible exception of electrical synapses, which allow limited exchange of ions and small molecules between partner neurons).
retina
A layered structure at the back of the vertebrate eye with five major neuronal classes (photoreceptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells) and support cells. Together, these cells convert light into electrical signals, extract biologically relevant features from the outputs of photoreceptors, and transmit such information to the brain. (Figure 4-2)
retinal
A chromophore covalently linked to opsins; it changes its configuration upon photon absorption. (Figure 4-6; Figure 13-20)
retinal ganglion cell (RGC)
The output cell class of the retina; it transmits information from the eyes to the brain. (Figure 4-2; Figure 4-25)
retinal wave
The spread of spontaneous excitation across the developing retina. (Figure 5-21)
retinotopy
The topographical arrangement of cells in the visual pathway according to the position of the retinal ganglion cells that transmit signals to them.
retrieval (of memory)
The recall of a memory.
retrograde
From the axon terminal to the cell body.
retrograde flow
The flow of F-actin from the leading edge of the growth cone to its center powered by myosin motors. It contributes to growth cone dynamics. (Figure 5-16)
retrograde tracer
A molecule used to trace axonal connections; it is taken up primarily by axon terminals and transported back to cell bodies. (Figure 14-30)
retrograde trans-synaptic tracing
See trans-synaptic tracing.
Rett syndrome
A neurodevelopmental disorder in girls caused by disruption of the X-linked gene encoding methyl-CpG binding protein 2 (MeCP2). Patients usually develop normally for the first 6–18 months. Their development then slows, arrests, and regresses, with severe deficits, including social withdrawal, loss of language, and motor symptoms. See also MeCP2.
reversal potential (Erev)
The membrane potential at which current passing through an ion channel changes direction.
reverse genetics
The strategy or process of disrupting a predesignated gene to identify its loss-of-function phenotypes. (Figure 14-4)
reward prediction error
A theoretical value representing the difference between a received reward and the predicted reward; it is represented by a population of midbrain dopamine neurons.
rhabdomeric type
A type of photoreceptor in which the apical surface folds into microvilli that house opsins. (Figure 13-22)
Rho
A member of a family of small monomeric GTPases involved in actin cytoskeleton regulation.
rhodopsin
A photosensitive molecule in the rod consisting of opsin covalently attached to the chromophore retinal. (Figure 4-6)
RNA (ribonucleic acid)
Chain of ribose-containing nucleotides consisting of the sugar ribose, a phosphate group, and one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or uracil (U).
RNA editing
Post-transcriptional modification that alters a nucleotide sequence of an RNA transcript after it is synthesized.
RNAi (RNA interference)
A genetic technique for knocking down expression of a gene of interest by producing a double-stranded RNA with a sequence corresponding to that of the gene of interest. (Figure 14-7)
RNA-seq
A technique in which RNA molecules from a given tissue are sequenced one by one in a massively parallel fashion using next-generation sequencing methods. It is used to obtain information about which genes are expressed and at what level at a transcriptome-wide level.
RNA splicing
The process by which introns are removed from RNA molecules. In the case of alternative splicing, a subset of exons is removed as well. (Figure 2-2)
Robo (Roundabout)
An axon guidance receptor for the ligand Slit. (Figure 7-13)
rod
A rod-shaped photoreceptor in the vertebrate retina; it is a very sensitive photon detector specialized for night vision. (Figure 4-2)
rostral–caudal (anterior–posterior)
Of a body axis, from head to tail. (Figure 1-8)
rtTA
See tTA.
ryanodine receptor
A Ca2+ channel on the ER membrane activated by an increase in intracellular Ca2+ concentrations and thus amplifies cytosolic Ca2+ signals. It is also activated by the plant-derived agonist ryanodine. (Figure 3-41)
S
saccade
A rapid movement of the eyes between fixation points.
sagittal section
A section plane perpendicular to the medial–lateral axis. (Figure 1-8)
saltatory conduction
The process by which an action potential in a myelinated axon “jumps” from one node of Ranvier to the next. (Figure 2-26)
salty
A taste modality that functions primarily to reveal the salt content of food; it is usually appetitive at low concentrations and aversive at high concentrations.
sarcomere
The contractile element of a myofibril composed of overlapping F-actin (thin filaments) and myosin (thick filaments). (Figure 8-3)
sarcoplasmic reticulum
A special endoplasmic reticulum derivative that extends throughout muscle cells. Ca2+ released from the sarcoplasmic reticulum mediates excitation-contraction coupling. (Figure 8-5)
Satb2
A transcription factor that specifies callosal projection neuron identity. (Figure 7-12)
savings
A phenomenon whereby less effort is required for an animal to relearn something it has previously learned and then forgotten.
scanning electron microscopy (SEM)
A form of electron microscopy that produces images by scanning the surface of a biological specimen and collecting information regarding the interaction of the electron beam with the surface areas.
Schaffer collateral
An axonal branch of a hippocampal CA3 pyramidal neuron that synapses onto CA1 pyramidal neurons. (Figure 11-5)
schizophrenia
A psychiatric disorder characterized by positive symptoms (such as hallucinations and delusions), negative symptoms (such as social withdrawal and lack of motivation), and cognitive impairment (such as deficiencies in memory, attention, and executive functions).
Schwann cell
A glial cell in the PNS that wraps axons with its cytoplasmic extensions to form myelin sheaths. (Figure 2-27)
scrapie
See prion diseases.
secondary antibody
An antibody that selectively recognizes primary antibodies made by specific animal species; it is usually conjugated to a fluorophore or an enzyme that produces a color substrate.
secondary dendrite
A mitral cell dendrite that extends laterally; it forms reciprocal synapses with granule cells and other olfactory bulb interneurons to spread signals to different olfactory processing channels. It is distinct from the primary (apical) dendrites of mitral cells, which extend into glomeruli. (Figure 6-14)
α-secretase
An extracellular protease that cleaves amyloid precursor protein (APP) in the middle of the amyloid β (Aβ) peptide and prevents production of pathology-associated Aβ . (Figure 12-3)
β-secretase
An extracellular protease that cleaves amyloid precursor protein (APP) at the N-terminus of amyloid β (Aβ ) to produce, along with γ-secretase, intact Aβ . (Figure 12-3)
γ-secretase
An intramembrane protease that cleaves α- or β-secretase-processed amyloid precursor protein (APP) at the C-terminus of Aβ . (Figure 12-3)
secreted protein
A protein destined for export from the cell. (Figure 2-2)
seizure
An episode involving abnormal synchronous firing of large groups of neurons. (Figure 12-43)
selection (in evolution)
The process by which genetic variants that confer higher/lower chances of reproductive success become more/less prevalent in future generations. See also positive selection and negative selection.
selectivity filter
The part of an ion channel pore responsible for discriminating between different ionic species so that only some species pass through the channel. (Figure 2-33)
self-avoidance
The process in which different axonal or dendritic branches from the same neuron repel each other to avoid overlap of processes from a single cell.
Sema1A, Sema2A, Sema2B (Semaphorin-1A, -2A, -2B)
Axon guidance molecules of the semaphorin family in invertebrates; Sema1A is a transmembrane isoform, while Sema2A and Sema2B are secreted isoforms.
Sema3A (Semaphorin-3A)
A secreted axon guidance molecule of the semaphorin family in vertebrates.
Sema3F
A secreted axon guidance molecule of the semaphorin family in vertebrates.
semaphorins
Evolutionarily conserved, widely used axon guidance cues. They consist of secreted and transmembrane variants and act mostly as repellents. Some transmembrane variants can also act as axon guidance receptors. (Figure 5-11)
semicircular canal
A sensory organ in the vestibular system that senses angular acceleration in a specific plane. (Figure 6-59)
sensitization
An increase in the magnitude of a response to a stimulus after a different kind of stimulus, often noxious, has been applied.
sensorimotor transformation
The process by which sensory information is transformed into motor commands, which often involves transformation of spatial coordinates from those of the sensory system into those of the motor system.
sensory homunculus
A map in the primary somatosensory cortex; it corresponds to sensation of specific body parts. Nearby somatosensory cortical areas represent sensation from nearby body surfaces. (Figure 1-25)
sensory neuron
A neuron that responds directly to external stimuli, such as light, sound, chemical, thermal, or mechanical stimuli.
sensory rhodopsin
A type I rhodopsin used in prokaryotes for phototaxis. (Figure 13-20)
serial electron microscopic (EM) reconstruction
A method in which consecutive electron micrographs of thin sections are aligned to produce a three-dimensional volume. (Figure 14-32)
serial processing
An information processing method in which processing units are arranged in sequential steps.
serine/threonine kinase
An enzyme that adds a phosphate onto specific serine or threonine residues of target proteins.
serotonin
A monoamine neurotransmitter derived from the amino acid tryptophan that primarily acts as neuromodulator. It is also called 5-HT for 5-hydroxytryptamine. (Figure 3-16; Table 3-2)
Sevenless
Originally identified from a mutation in Drosophila lacking photoreceptor R7; it encodes a receptor tyrosine kinase that acts cell autonomously in R7 to specify the R7 fate. (Figure 5-35)
sex chromosome
The chromosome whose presence or number determines the sex of an organism.
sex-linked
Of a mutation, having a Mendelian inheritance pattern characteristic of genes located on a sex chromosome. (Figure 12-33)
sex peptide
In Drosophila, a peptide transferred with sperm from males to females during mating. It reduces female receptivity to courtship.
sexually dimorphic
Of a trait, differing between females and males.
SFO
See subfornical organ.
Shaker
Identified as a mutation in Drosophila that causes defects in a fast and transient K+ current in muscles and neurons; it encodes a voltage-gated K+ channel.
sharp-wave ripple
Large amplitude rapid oscillations in local field potential in the hippocampus during sleep or during resting while awake. (Figure 11-40)
short-range cue (in axon guidance)
A cell-surface protein that can exert its guidance effects only when axons contact the cell that produces it. (Figure 5-11)
short-term memory
Memory that lasts seconds to minutes. (Figure 11-3)
short-term synaptic plasticity
A change in the efficacy of synaptic transmission that lasts milliseconds to minutes.
sign
In sensory physiology, the direction in which a neuron’s activity or membrane potential is changed by a stimulus (for example, the sign is positive if a neuron is depolarized by a stimulus, and the sign is negative if a neuron is hyperpolarized by a stimulus).
signal transduction
The process by which an extracellular signal is relayed via intracellular pathways to varied effectors to produce specific biological effects.
silent synapse
A glutamatergic synapse containing NMDA but not AMPA receptors on the postsynaptic membrane; it can be activated by presynaptic glutamate release that coincides with postsynaptic depolarization but not by presynaptic glutamate release alone.
simple cell
A functionally defined neuronal type enriched in layer 4 of the primary visual cortex; it is best excited by a bar of light in a specific orientation and has separate ON and OFF regions that, when stimulated together, cancel each other’s effects. (Figure 4-41)
single-cell RNA-seq
A technique using high-throughput sequencing methods to identify and quantify all mRNAs expressed in individual cells. See also RNA-seq. (Figure 14-17)
single channel conductance (γ)
The conductance of a single ion channel when open.
single nucleotide polymorphism (SNP)
A single nucleotide of DNA in the genome that varies between members of a species.
single-unit recording
An extracellular recording of the firing pattern of an individual neuron. See also extracellular recording. (Figure 14-34)
siRNA (short interfering RNA)
Double-stranded RNA with a length similar to microRNA (21-26 nucleotides); it directs a protein complex to degrade target mRNA through base pairing. See also RNAi.
size principle
The idea that within a motor pool, motor neurons with smaller motor unit sizes (with smaller axon diameters and cell bodies) fire before motor neurons with larger motor unit sizes during muscle contraction. (Figure 8-7)
Slit
A secreted protein best characterized as a repulsive ligand involved in midline axon guidance in many species, from insects to vertebrates. (Figure 7-13)
slow axonal transport
Intracellular transport at a speed of 0.2–8 mm per day. Cargos subject to slow axonal transport include cytosolic proteins and cytoskeletal components. (Figure 2-4)
small bistratified RGC
A blue–yellow color opponent retinal ganglion cell. See also color-opponent RGC. (Figure 4-34)
Smith–Magenis syndrome
A neurodevelopmental disorder characterized by mild to moderate intellectual disability, delayed speech, sleep disturbances, impaired impulse control, and other behavioral problems. It is caused by mutations that disrupt the function of one copy of a single gene called Rai1 (retinoic acid induced 1) or loss of one copy of a chromosome segment including Rai1.
smooth muscle
Muscle that controls movement of tissue within the digestive, respiratory, vascular, excretory, and reproductive systems.
SM protein
A protein related to yeast Sec1 and mammalian Munc18; it binds SNAREs and is essential for vesicle fusion. See also SNAREs.
SNAP-25
A t-SNARE attached to the plasma membrane via lipid modification. See also SNAREs. (Figure 3-8)
SNAREs (soluble NSF-attachment protein receptors)
Proteins on intracellular vesicles and target membranes that are part of a complex mediating membrane fusion. (Figure 3-8)
SNc (substantia nigra pars compacta)
A midbrain nucleus containing dopamine neurons that project mainly to the dorsal striatum. (Figure 8-21)
SNr (substantia nigra pars reticulata)
One of the two major output nuclei of the basal ganglia; it contains GABAergic neurons projecting to the thalamus, superior colliculus, and brainstem motor control nuclei. (Figure 8-21)
solute
A water-soluble molecule, such as an inorganic ion, nutrient, metabolite, or neurotransmitter.
soma
The cell body of a neuron or any other cell.
somatic mutation
A mutation that occurs in a progenitor cell and thus affects only the cells derived from that progenitor.
somatosensory system
The collected parts of the nervous system that process bodily sensation.
Sonic Hedgehog (Shh)
A morphogen that determines cell fate by regulating expression of specific transcription factors in many developmental contexts. For instance, floor plate–derived Shh is responsible for determining the different fates of neuronal progenitors located at different positions along the dorsal–ventral axis of the ventral spinal cord. It is also used as a midline attractant for commissural axons. (Figure 7-10)
sour
A taste modality that functions primarily to warn animals of potentially spoiled food; it is usually aversive.
Southern blot
A method for determining the amount of a specific DNA in a DNA mixture. DNA molecules are separated by gel electrophoresis and transferred to a membrane; labeled nucleic acid probes are then hybridized to the membrane to visualize specific DNA molecules that hybridize to the probe.
space constant
See length constant.
spatial integration (in dendrites)
The summation of postsynaptic potentials produced by synchronous activation of synapses located at different spatial locations on the postsynaptic neuron. (Figure 3-43)
spectral sensitivity
The relationship between a response (e.g., of a photosensitive cell or molecule) and the wavelength of the stimulus light.
spike
See action potential.
spike rate
See firing rate.
spike-timing-dependent plasticity (STDP)
A change of synaptic efficacy induced when pre- and postsynaptic neurons repeatedly fire within a restricted time window: synaptic efficacy is potentiated if the presynaptic neuron fires before the postsynaptic neuron and depressed if the presynaptic neuron fires after the postsynaptic neuron.
spike train
action potential firing pattern.
spinal cord
The caudal part of the vertebrate CNS enclosed by the vertebral column. (Figure 1-8)
spinal muscular atrophy (SMA)
A neurodegenerative disease that cause motor neuron death due to homozygous disruption of the Smn1 (survival motor neuron 1) gene; it is a leading genetic cause of infant mortality.
spinocerebellar ataxia
One of a collection of neurodegenerative diseases that share motor defects such as ataxia and are caused by polyglutamine expansion in a number of proteins. (Table 12-1)
spinocervical tract pathway
An axonal pathway from the dorsal spinal cord to the lateral cervical nucleus that relays a subset of touch signals, particularly from hairy skin.
spiny projection neuron
The most numerous type of neuron in the striatum; it is a GABAergic neuron that projects either directly or indirectly to the output nuclei of the basal ganglia. It is also called a medium spiny neuron. (Figure 8-21)
spiral ganglion neuron
A bipolar neuron whose peripheral axon receives auditory information from a hair cell in the cochlea and whose central axon transmits information to the brainstem as part of the auditory nerve. (Figure 6-47)
spontaneous activity
Firing of neurons in the absence of environmental stimuli.
sporadic
Of a human disease, occurring in a patient without an identifiable family history of the disease.
Sry (Sex determining region Y)
A gene located on the Y chromosome in mammals; it encodes a transcription factor that determines testes differentiation and other male-specific characteristics.
SSRI (selective serotonin reuptake inhibitor)
An inhibitor of the plasma membrane serotonin transporter; it prolongs the action of serotonin in the synaptic cleft.
starburst amacrine cell (SAC)
A class of GABAergic inhibitory neurons in the retina that also release acetylcholine. It is a crucial cell type that shapes the responses of direction-selective retinal ganglion cells and participates in generating retinal waves essential for activity-dependent wiring of the visual system. (Figure 4-29)
starter cell
See trans-synaptic tracing.
STED
See super-resolution fluorescence microscopy.
stereocilium
A rigid bundled F-actin-based cylinder located on the apical surface of a hair cell. Stereocilia on the same hair cell are arranged in rows of increasing height like a staircase. (Figure 6-45)
stereotactic injection
Injection via a device positioned precisely in a three-dimensional coordinate system to target substances such as viruses to a small region.
stereotyped
Invariable among individual animals.
stereotyped axon pruning
The pruning of exuberant axons with an invariable outcome.
stereotypy
A trait or behavior that is largely invariant in different individual organisms of a species.
stimulating electrode
An electrode used to pass current into a neuron, usually with the goal of changing the membrane potential of a neuron or its processes.
stomatogastric ganglion (STG)
A crustacean ganglion that controls stomach contraction; it has been used as a model system for studying central pattern generators and rhythmic activity in neuronal circuits. (Figure 8-13)
storage (of memory)
A step in between memory acquisition and retrieval, in which a memory is encoded as a persistent representation in the nervous system.
STORM
See super-resolution fluorescence microscopy.
striatum
The part of the basal ganglia that receives convergent input from the cerebral cortex and thalamus. Also called the caudate-putamen because in some species, the striatum has two separate regions called the caudate and the putamen. (Figure 8-21)
subfornical organ (SFO)
Part of the lamina terminalis in the anterior hypothalamus; it is responsible for sensing dehydration signals from the blood, including osmolarity and angiotensin II levels. (Figure 9-16)
substance P
A neuropeptide that promotes inflammation when released by the peripheral terminals of sensory neurons. (Figure 6-73)
substantia nigra
A midbrain structure named after the high levels of melanin pigments present in the dopamine neurons of healthy human subjects. See also SNc and SNr. (Figure 12-16)
subthalamic nucleus (STN)
An intermediate nucleus in the basal ganglia indirect pathway; it contains glutamatergic neurons that project to the GPi and SNr. These neurons receive GABAergic input from the GPe and glutamatergic input from the cerebral cortex. (Figure 8-21)
subthreshold stimulus
A stimulus that is insufficient to cause a neuron to generate an action potential. (Figure 2-18)
subventricular zone
A cellular layer next to the ventricular zone in the developing nervous system; it contains intermediate progenitors and some radial glia.
superior colliculus
A multilayered midbrain structure in mammals that receives retinal ganglion cell axonal input as well as input from other sensory systems. Among many functions, it regulates head orientation and eye movement. It is analogous to the tectum in nonmammalian vertebrates. (Figure 4-37)
superior olivary nuclei
Brainstem nuclei in mammals where auditory signals from the left and right ears first converge. The medial superior olivary nucleus (MSO) analyzes interaural time differences, whereas the lateral superior olivary nucleus (LSO) analyzes interaural level differences. (Figure 6-55)
super-resolution fluorescence microscopy
A set of fluorescence microscopy techniques capable of imaging specimens at resolutions below the diffraction limit of light. For example: (1) STED (stimulated emission depletion microscopy) achieves super-resolution by exciting fluorophores in a region of tissue smaller than the diffraction limit by depleting fluorescence in an annulus surrounding a central focal spot. (2) STORM (stochastic optical reconstruction microscopy) and (3) PALM (photoactivated localization microscopy) achieve super-resolution by photoactivating a small random subset of photo-switchable fluorophores at any one time, such that the position of each fluorophore can be localized with precision below the diffraction limit; repeated rounds of imaging and deactivation enable reconstruction of the entire imaging field. (Figure 14-28)
supervised learning
A category of machine learning in which the task is to identify a function for mapping an input to an output after experiencing example input–output pairs.
suprachiasmatic nucleus (SCN)
A hypothamalic nucleus that is the master regulator of circadian rhythms and light entrainment in mammals. (Figure 9-6; Figure 9-24)
suprathreshold stimulus
A stimulus that can cause a neuron to generate an action potential. (Figure 2-18)
sweet
A taste modality that functions primarily to detect the sugar content of food; it is usually appetitive.
sympathetic system
A branch of the autonomic nervous system that facilitates energy expenditure, such as in the case of an emergency response. Activation of the sympathetic system increases heart rate and blood flow, relaxes airways in the lungs, inhibits salivation and digestion, and stimulates production of the hormone epinephrine (adrenaline) in the adrenal glands. (Figure 9-1; Figure 9-2)
symporter
A coupled transporter that moves two or more solutes in the same direction. (Figure 2-10)
synapse
A site at which information is transferred from one neuron to another neuron or a muscle cell; it consists of a presynaptic terminal and a postsynaptic specialization separated by a synaptic cleft.
synapse elimination
The process by which extra synapses are removed during development. It is best described at the vertebrate neuromuscular junction, where the innervation of muscle cells by multiple motor neurons is refined during early postnatal development so that each muscle cell is innervated by a single motor neuron in adults. (Figure 7-29)
synaptic cleft
A 20–100 nm gap that separates the presynaptic terminal of a neuron from its postsynaptic target cell. (Figure 1-14; Figure 3-3)
synaptic efficacy
See efficacy of synaptic transmission.
synaptic failure
An event in which an action potential in a presynaptic neuron does not produce a postsynaptic response.
synaptic plasticity
The ability to change the efficacy of synaptic transmission, usually in response to experience and neuronal activity.
synaptic potential
A graded potential produced at postsynaptic sites in response to neurotransmitter release by presynaptic partners.
synaptic tagging
The hypothesis that induction of LTP at a synapse causes production of a “tag” at the synapse and that newly synthesized macromolecules necessary for stabilization of LTP are selectively captured by the tag. The hypothesis explains how the input specificity of LTP is maintained despite the cell-wide distribution of newly synthesized macromolecules required for LTP. (Figure 11-20)
synaptic transmission
The process of neurotransmitter release from a presynaptic neuron and neurotransmitter reception by a postsynaptic neuron.
synaptic vesicle
A small, membrane-enclosed organelle (typically about 40 nm in diameter) localized at the presynaptic terminal; it is filled with neurotransmitters and, upon stimulation, fuses with the plasma membrane to release neurotransmitters into the synaptic cleft. (Figure 3-4; Figure 3-7)
synaptic weight matrix
A network of synapses between ensembles of input neurons and output neurons, where the strength (weight) of each synapse can vary between 0 (no connection) and 1 (maximal strength connection). (Figure 11-4)
synaptobrevin
A transmembrane SNARE on the synaptic vesicle (a v-SNARE). It is also called VAMP (vesicle-associated membrane protein). See also SNAREs. (Figure 3-8)
synaptotagmin
A Ca2+-binding transmembrane protein on the synaptic vesicle that serves as a Ca2+ sensor to trigger neurotransmitter release.
syndromic disorder
A disorder characterized by a defined constellation of behavioral, cognitive, and physical symptoms.
syntaxin
A transmembrane SNARE on the target plasma membrane (a t-SNARE). See also SNAREs. (Figure 3-8)
α-synuclein
A protein normally enriched in the presynaptic terminal; it is a major component of Lewy bodies, a defining pathological feature of most forms of Parkinson’s disease.
T
T1R1
A G-protein-coupled receptor and a subunit (along with T1R3) of the mammalian umami taste receptor. (Figure 6-39)
T1R2
A G-protein-coupled receptor and a subunit (along with T1R3) of the mammalian sweet taste receptor. (Figure 6-39)
T1R3
A G-protein-coupled receptor and a shared subunit of the mammalian umami and sweet taste receptors. (Figure 6-39)
T2Rs
A family of G-protein-coupled receptors that are the mammalian bitter taste receptors. (Figure 6-39)
tamoxifen
See CreER.
tastant
A nonvolatile, hydrophilic molecule in saliva that elicits taste perception.
taste bud
A cluster of tens of taste receptor cells, with their apical endings facing the surface of the tongue. (Figure 6-32)
taste pore
The collected apical endings of taste receptor cells in a taste bud. (Figure 6-32)
taste receptor cell (TRC)
A sensory neuron on the surface of the tongue and oral cavity; it converts tastant binding to taste receptor proteins into electrical signals that are transmitted to the peripheral terminals of the gustatory nerve. (Figure 6-32)
tau
A microtubule binding protein highly enriched in axons.
tauopathies
Neurodegenerative diseases characterized by the presence of neurofibrillary tangles, which consist of aggregates of hyperphosphorylated tau.
Tbr1
A transcription factor that specifies corticothalamic projection neuron identity. (Figure 7-12)
tectorial membrane
A membrane on the apical side of hair cells apposed to the stereocilia. (Figure 6-48)
tectum
A midbrain structure analogous to the mammalian superior colliculus; it is the major target of retinal ganglion cells in the brains of amphibians and lower vertebrates. (Figure 5-5)
telencephalon
The anterior part of the forebrain, including the olfactory bulb, cerebral cortex, hippocampus, and basal ganglia. (Figure 7-3)
temporal (in retinal map)
In the direction of the temples.
temporal integration (in dendrites)
The summation of postsynaptic potentials produced by activation of synapses within a finite time window. (Figure 3-43)
temporal lobe
One of the four cerebral cortex lobes; it is located at the lateral sides of the brain. (Figure 1-23)
teneurins
Evolutionarily conserved cell adhesion proteins that control synaptic partner matching and synaptic signaling.
testosterone
A steroid hormone that promotes the development of the male reproductive system (masculinization) and inhibits the development of the female reproductive system (de-feminization). In adults, it stimulates sexual and aggressive behaviors. It also serves as a precursor of estradiol. (Figure 10-19)
tetanus toxin
A protease produced by Clostridium tetani that cleaves synaptobrevin, thereby inhibiting neurotransmitter release.
tetraethylammonium (TEA)
A chemical that selectively blocks voltage-gated K+ channels.
tetrode
An extracellular recording electrode containing four wires that enable four independent recordings of spiking activities of neurons nearby the electrode tip. The firing patterns of up to ~20 neurons can be resolved based on their different action potential amplitudes and waveforms.
tetrodotoxin (TTX)
A toxin that potently blocks voltage-gated Na+ channels across animal species and is widely used experimentally to silence neuronal firing; it is produced by symbiotic bacteria in puffer fish, rough-skinned newt, and some octopi. (Figure 2-29)
thalamocortical axons (TCAs)
The axons of thalamic neurons that project to the cortex.
thalamus
A structure situated between the cerebral cortex and midbrain; it relays sensory and other signals to the cerebral cortex through extensive bidirectional connections. (Figure 1-8)
theory of dynamic polarization
The idea that every neuron has (1) a receptive component, the cell body and dendrites; (2) a transmission component, the axon; and (3) an effector component, the axon terminals. According to this theory, originally proposed by Ramón y Cajal, neuronal signals flow from dendrites and cell bodies down the axon to the axon terminals.
thermosensation
The sense of temperature.
thermosensory neuron
A somatosensory neuron that senses temperature.
threshold (of action potential)
The membrane potential above which an action potential is generated. (Figure 2-18)
thrombospondin (TSP)
A member of a family of secreted proteins with diverse functions; it can be produced by astrocytes to stimulate synapse formation.
time constant (τ)
The product of resistance and capacitance in an R-C circuit. It is a measure of the rate at which both a capacitor charges or discharges and the voltage across a resistor changes in response to changes in current. In neurons, τ corresponds to the time required for the membrane potential change to reach 63% (1 – 1/e) of its maximal value in response to a sudden change in current flow.
Timeless
A fruit fly gene discovered based on mutations that affect circadian rhythms; it encodes a protein that negatively regulates its own transcription. (Figure 9-22)
Timothy syndrome
A syndrome characterized by cardiac arrhythmia and autistic symptoms; it is caused by mutations in the gene encoding CaV1.2, a voltage-gated Ca2+ channel. (Figure 12-41)
tip link
The connection between adjacent stereocilia; it consists of cadherin-23 on the taller stereocilium and protocadherin-15 on the shorter stereocilium. (Figure 6-46)
tonic
Of a neuronal firing pattern, regularly timed and repetitive.
tonic–clonic seizure
A seizure associated with loss of consciousness and a predictable sequence of motor activity: patients first stiffen and extend all extremities (tonic phase) and then undergo full-body spasms during which muscles alternately flex and relax (clonic phase).
tonotopic map
The ordered arrangement of cells in the auditory system in physical space according to their frequency tuning. The cochlea and multiple brain regions contain tonotopic maps. (Figure 6-47)
topographic map
An ordered representation in the brain of features of either the external world or the animal’s interactions with the world. For examples, see retinotopy, sensory homunculus, and motor homunculus.
transcription
The process by which RNA polymerase uses DNA as a template to synthesize RNAs. (Figure 2-2)
transcription factor
A DNA-binding protein that regulates transcription of target genes.
transcription unit
The part of the gene that serves as a template for RNA synthesis. (Figure 2-2)
transcriptome
The collection of all expressed RNAs in a tissue or single cell.
transcytosis
The process by which transmembrane or extracellular proteins are first retrieved by endocytosis in one cellular compartment and then delivered for exocytosis at another cellular compartment.
transducin
A trimeric GTP-binding protein complex that links light-activated rhodopsin (or cone opsin) to phosphodiesterase activation in vertebrate photoreceptors. (Figure 4-8)
transgene
An in vitro engineered gene introduced into somatic cells or the germline of an organism.
transgenic organism
An organism containing a transgene, usually in the germline.
translation
The process by which an mRNA is decoded by ribosomes for protein synthesis. (Figure 2-2)
transmembrane AMPA receptor regulatory proteins (TARPs)
Transmembrane proteins associated with the AMPA receptors. They regulate trafficking, postsynaptic density anchoring, and physiological properties of the AMPA receptors. (Figure 3-26)
transmembrane protein
A protein destined to span the lipid bilayer of a membrane. (Figure 2-2)
transmission electron microscopy (TEM)
A form of electron microscopy in which high voltage electron beams transmitted through ultra-thin (typically under 100 nm) sections of biological specimens are used to create images.
transporter
A transmembrane protein or protein complex with two separate gates that open and close sequentially to allow solutes to move from one side of a membrane to the other. (Figure 2-8)
trans-synaptic tracing
A method for labeling the synaptic partners of a given neuron or neuronal population of interest (the starter cell or cells). A retrograde trans-synaptic tracer labels presynaptic partners of starter cells, whereas an anterograde trans-synaptic tracer labels postsynaptic partners of starter cells.
transverse section
See coronal section.
transverse tubules (T tubules)
An invagination of the plasma membrane that extends into the muscle cell interior, bringing the plasma membrane close to the sarcoplasmic reticulum, such that depolarization effectively triggers Ca2+ release from the sarcoplasmic reticulum throughout the entire large muscle cell. (Figure 8-5)
TRE (tetracycline response element)
The DNA sequence to which tTA or rtTA bind. (Figure 14-12). See also tTA.
TREM2 (triggering receptor expressed on myeloid cells 2)
A cell-surface receptor expressed in the myeloid lineage, including microglia in the brain. Loss-of-function alleles are major risk factors for late-onset Alzheimer’s disease. (Figure 12-9)
trichromat
An organism with three different cones for color vision—the S-, M-, and L-cones.
trigeminal ganglia
Clusters of somatosensory neurons near the brainstem involved in sensation of the face.
trimeric GTP-binding protein (G protein)
A GTP-binding protein complex composed of a Gα, a Gβ, and a Gγ subunit with intrinsic GTPase activity in Gα. It has many variants, which couple different GPCRs to diverse signaling pathways. See also Gs, Gi, and Gq.
Trk receptors
A family of neurotrophin receptors that function as receptor tyrosine kinases, including TrkA, TrkB, and TrkC. (Figure 3-39; Figure 7-35)
TRP channels
Nonselective cation channels that share sequence similarities with the Drosophila transient receptor potential (TRP) protein. (Figure 2-34)
TRPM8
A TRP channel activated by menthol and by temperatures <26°C. (Figure 6-68)
TRPV1
A TRP channel activated by capsaicin and by temperatures >43°C. (Figure 6-68)
t-SNARE
A SNARE located on the target membrane, such as syntaxin. See also SNAREs.
tTA (tetracycline-repressible transcriptional activator)
A bacterial transcription factor widely used in heterologous systems, including transgenic mice, to control transgene expression. It drives expression of target genes whose promoters contain a tetracycline response element (TRE), but its activity is repressed by tetracycline or its analog doxycycline. A variant called rtTA (reverse tTA) activates TRE-driven transgenes in the presence but not the absence of doxycycline. (Figure 14-12)
tuberomammillary nucleus
A hypothalamic nucleus rich in histamine neurons. (Figure 9-29)
tufted cell
See mitral cell.
two-photon microscopy
A microscopy technique that utilizes simultaneous absorption of two long-wavelength photons to excite fluorophores. Compared with confocal microscopy, it produces less photodamage because only at the focal plane is the density of photons high enough to cause substantial fluorescence emission. Like confocal microscopy, it relies on laser scanning of imaging spots across a plane to produce an optical section. (Figure 14-42)
type III neuregulin-1 (Nrg1-III)
An axonal cell-surface protein, the expression level of which determines the degree of axon myelination by Schwann cells.
tyrosine hydroxylase
An enzyme that converts l-tyrosine to l-dopa; it is the rate-limiting enzyme in the catecholamine biosynthetic pathway. (Figure 12-20)
tyrosine kinase
An enzyme that adds a phosphate onto specific tyrosine residues of target proteins.
U
UAS
See GAL4.
ubiquitin-proteasome system
A protein degradation system present in all eukaryotes.
umami
A taste modality that functions primarily to detect the amino acid content of food; it is usually appetitive.
Unc5
A co-receptor for netrin/Unc6 that acts together with DCC/Unc40 to mediate axon repulsion.
Unc6
See netrin/Unc6.
Unc40
See DCC/Unc40.
unconditioned response (UR)
See classical conditioning.
unconditioned stimulus (US)
See classical conditioning.
unipolar (neuron)
Having one process leaving the cell body that gives rise to both dendritic and axonal processes. (Figure 1-15)
unsupervised learning
A category of machine learning in which the task is to experience a data set and draw inferences about its structure without instruction.
V
V1
See primary visual cortex.
vagus nerve
A cranial nerve in the parasympathetic system that connects the brainstem with various internal organs. (Figure 9-2)
variation (in evolution)
The presence of differences in genes or heritable traits.
vasopressin
A hormone secreted by hypothalamic neurons in the posterior pituitary and a neuropeptide released by hypothalamic neurons; it regulates water balance and social behavior.
V-ATPase
A transmembrane protein on synaptic vesicles that pumps protons (H+) into vesicles against their electrochemical gradient using energy derived from ATP hydrolysis. (Figure 3-12)
ventral horn
The ventral part of the spinal gray matter where motor neurons reside. (Figure 8-6)
ventral nerve cord
An invertebrate CNS structure posterior to the brain; it is analogous to the vertebrate spinal cord. (Figure 7-13)
ventral root
The place where motor axons exit the spinal cord. (Figure 8-6)
ventral stream
A visual processing pathway from primary visual cortex to temporal cortex; it is responsible for analyzing form and color. It is also called the “what” stream. (Figure 4-51)
ventral tegmental area (VTA)
A midbrain nucleus containing dopamine neurons that project mainly to the ventral striatum (nucleus accumbens) and prefrontal cortex. (Figure 8-21; Figure 12-29)
ventricle
A cavity derived from the lumen of the neural tube; it is filled with cerebrospinal fluid. (Figure 7-5)
ventricular zone
A layer of cells adjacent to a ventricle. (Figure 7-4)
ventromedial hypothalamic nucleus (VMH)
A hypothalamic nucleus whose best characterized functions include regulation of female lordosis and male mounting and aggression. (Figure 10-26)
vesicular monoamine transporter (VMAT)
A transmembrane protein on synaptic vesicles that transports dopamine, norepinephrine, and serotonin from the presynaptic cytosol into synaptic vesicles. (Figure 12-23). See also vesicular neurotransmitter transporter.
vesicular neurotransmitter transporter
A transmembrane protein on synaptic vesicles that transports neurotransmitters from the presynaptic cytosol into vesicles using energy from the transport of protons down their electrochemical gradient. (Figure 3-12)
vestibular ganglion neuron
A bipolar neuron whose peripheral axon receives vestibular information from cells in an otolith organ or a semicircular canal and whose central axon transmits information to the brainstem as part of the vestibular nerve.
vestibular nerve
A collection of axons from vestibular ganglion neurons that transmits vestibular information to the brainstem. (Figure 6-59)
vestibular nuclei
Brainstem nuclei that the vestibular nerve innervates; they also receive input from other sensory systems, such as the somatosensory system. (Figure 6-60)
vestibular system
The collected parts of the nervous system that sense the movement and orientation of the head and use this information to regulate a variety of functions including balance, spatial orientation, coordination of head and eye movements, and perception of self-motion.
vestibulo-ocular reflex (VOR)
A reflexive eye movement that stabilizes images on the retina during head movement by moving the eyes in the direction opposite to the head movement. (Figure 6-61)
viral transduction
The process by which a virus infects a host cell, introducing its genome; it is widely used for transgene expression in somatic cells.
visceral afferent
The peripheral axon of a visceral sensory neuron.
visceral motor neurons
Pre- and postganglionic neurons of the autonomic nervous system.
visceral sensory neuron
A sensory neuron whose peripheral branch innervates an internal organ and whose central branch extends into the spinal cord or brainstem. (Figure 9-3; Figure 9-4)
visual cortex
The part of the cerebral cortex dedicated to analyzing visual information.
visual field
The portion of the external world that can be seen at a given time.
voltage clamp
An experimental technique used to measure the current passing through a membrane while holding (clamping) the membrane potential at a set level. (Figure 2-21)
voltage-gated Ca2+ channel
An ion channel that allows selective passage of Ca2+ and whose conductance is regulated by the membrane potential. (Figure 2-34)
voltage-gated ion channel
An ion channel whose conductance changes as a function of the membrane potential. (Figure 2-30)
voltage indicator
A molecule whose optical properties change in response to membrane potential changes.
volume transmission
The secretion of neurotransmitters (usually neuromodulators) into the extracellular space outside the confines of morphologically defined synapses, where they can affect multiple nearby cells.
vomeronasal organ (VNO)
A sensory organ located above the roof of the mouth that houses the sensory neurons of the accessory olfactory system. (Figure 6-19)
vomeronasal system
See accessory olfactory system.
VOR gain
The ratio of rotation of the eyes to the rotation of the head in the vestibulo-ocular reflex. See also vestibulo-ocular reflex (VOR).
v-SNARE
A SNARE located on a vesicle, such as synaptobrevin. See also SNAREs.
W
Wallerian degeneration
The process by which distal axons are eliminated after they are severed from their cell bodies.
Weber’s Law
In sensory perception, the property that the just-noticeable difference between two sensory stimuli is proportional to the magnitude of the stimuli.
Wernicke’s area
An area in the left temporal lobe involved in language comprehension. Patients with lesions in this area have difficulty comprehending language. (Figure 1-23)
western blot
A method for determining the amount of a specific protein in a protein mixture. Proteins are separated by gel electrophoresis and transferred to a membrane; labeled antibodies are then used to visualize specific proteins bound by the antibody. It can be used to determine protein expression patterns.
white matter
The parts of the CNS enriched in oligodendrocytes and myelinated axons and that thus appear white in histological sections, due to the high lipid content of myelin.
whole-cell patch clamp recording (whole-cell recording)
A form of intracellular recording in which a glass electrode forms a high-resistance seal with the plasma membrane of the recorded cell. After formation of the seal, the membrane underneath the patch electrode is ruptured, such that the interior of the patch electrode and the cytoplasm form a single compartment. See also patch clamp recording. (Figure 14-40)
whole-mount
A tissue specimen that has not been sectioned.
Wnts
A family of secreted proteins that act as morphogens to pattern embryonic tissues, such as the tissues along the anterior–posterior axes of vertebrates and C. elegans. They can also serve as cues in axon guidance and direct formation of synapses along an axon.
working memory
A form of explicit short-term memory; it maintains, updates, and manipulates information for a short period of time. (Figure 11-3)
X
None.
Y
None.
Z
zygote
A fertilized egg. (Figure 7-2)