Home :: Glossary

Glossary

Jump to letter
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

abducens nerve
Cranial nerve VI, an efferent nerve that controls the lateral rectus muscle of the eye.
accommodation
Dynamic changes in the lens of the eye that enable the viewer to focus. When viewing distant objects, the lens is made relatively thin and flat; for near vision, the lens becomes thicker and rounder and has more refractive power.
acetylcholine
Neurotransmitter at motor neuron synapses, in autonomic ganglia, and in a variety of central synapses. Binds to two types of acetylcholine receptors (AChRs), either ligand-gated ion channels (nicotinic receptors) and G-protein-coupled receptors (muscarinic receptors).
acetylcholinesterase (AChE)
Enzyme in the synaptic cleft that clears the cleft of acetylcholine released by the presynaptic cell. AChE hydrolyzes ACh into acetate and choline; the choline is then transported back into nerve terminals, where it is used to resynthesize ACh.
action potential
The electrical signal conducted along axons (or muscle fibers) by which information is conveyed from one place to another in the nervous system.
activation
The time-dependent opening of ion channels in response to a stimulus, typically membrane depolarization.
active transporters
Transmembrane proteins that actively move ions into or out of cells against their concentration gradients. Their source of energy may be ATP or the electrochemical gradients of various ions. See also co-transporters; ion exchangers.
adaptation
The phenomenon of sensory receptor adjustment to different levels of stimulation; critical for allowing sensory systems to operate over a wide dynamic range.
addiction
A persistent disorder of brain function in which the use of drugs or alcohol is compulsive for the afflicted individual despite serious negative physical and psychological consequences, and failure to take the substance results in a syndrome of negative physical and emotional symptoms loosely termed "withdrawal."
adenylyl cyclase
Membrane-bound enzyme that can be activated by G-proteins to catalyze the synthesis of cyclic AMP from ATP.
adhesion molecules
See cell adhesion molecules.
adrenaline
See epinephrine.
adrenal medulla
The central part of the adrenal gland that, under visceral motor stimulation, secretes epinephrine and norepinephrine into the bloodstream.
adrenergic
Refers to synaptic transmission mediated by the release of norepinephrine or epinephrine.
affective–motivational aspects of pain
The fear, anxiety, and autonomic nervous activation that accompany exposure to a noxious stimulus.
afferent
A neuron or axon that conducts action potentials from the periphery toward the central nervous system.
agnosia
The inability to name objects; literally means "not knowing."
allodynia
The induction of pain by a normally innocuous stimulus.
alpha (α) motor neurons
Neurons in the ventral horn of the spinal cord that innervate skeletal muscle.
amacrine cells
Retinal neurons that mediate lateral interactions between bipolar cell terminals and the dendrites of ganglion cells.
amblyopia
Diminished visual acuity as a result of the failure to establish appropriate visual cortical connections in early life.
amnesia
The pathological inability to remember or establish memories; retrograde amnesia is the inability to recall existing memories, whereas anterograde amnesia is the inability to lay down new memories.
AMPA receptors
See ionotropic glutamate receptors.
amphetamine
A synthetically produced central nervous system stimulant with cocaine-like effects; drug abuse may lead to dependence.
ampullae
The juglike swellings at the base of the semicircular canals that contain the hair cells and cupulae. See also cupulae.
amygdala
A nuclear complex in the temporal lobe that forms part of the limbic system; its major functions concern autonomic, emotional, and sexual behavior.
analgesia
Decreased perception of pain.
androgen insensitivity syndrome (AIS)
A condition in which, due to a defect in the gene that codes for the androgen receptor, testosterone cannot act on its target tissues. Also called testicular feminization.
anencephaly
A congenital defect of neural tube closure, in which much of the brain fails to develop.
anopsia
A large deficit in the visual field resulting from pathological changes in some component of the primary visual pathway.
anosmia
Loss of the sense of smell; can be total or restricted to a single odorant.
anterior
Toward the front; sometimes used as a synonym for rostral, and sometimes as a synonym for ventral.
anterior commissure
A small midline fiber tract that lies at the anterior end of the corpus callosum; like the callosum, it serves to connect the two hemispheres.
anterior hypothalamus
Region of the hypothalamus containing nuclei that mediate sexual behaviors; not to be confused with region in rodent called the medial preoptic area, which lies anterior to hypothalamus and also contains nuclei that mediate sexual behavior (most notably the sexually dimorphic nucleus).
anterograde
Signals or impulses that travel "forward," e.g., from the cell body to the axon terminal, from the presynaptic terminal to the postsynaptic cell, or from the CNS to the periphery.
anterolateral pathway
Ascending sensory pathway in the spinal cord and brainstem that carries information about pain and temperature to the thalamus.
antiserum
Serum harvested from an animal immunized to an agent of interest.
aphasia
The inability to comprehend and/or produce language as a result of damage to the language areas of the cerebral cortex (or their white matter interconnections). See also Broca's aphasia; Wernicke's aphasia.
apoptosis
Cell death resulting from a programmed pattern of gene expression; also known as "programmed cell death."
aprosodia
The inability to infuse language with its normal emotional content. See also prosody.
arachnoid mater
One of the three coverings of the brain that make up the meninges; lies between the dura mater and the pia mater.
areflexia
Loss of reflexes.
association cortex
Defined by exclusion as those neocortical regions that are not involved in primary sensory or motor processing.
associational systems
Neural cell circuits that are not part of the relatively defined sensory (input) and motor (output) systems; they mediate the most complex and least well defined brain functions.
associativity
In the hippocampus, the enhancement of a weakly activated group of synapses when a nearby group is strongly activated.
astrocytes
One of the three major classes of glial cells found in the central nervous system; important in regulating the ionic milieu of nerve cells and, in some cases, transmitter reuptake.
astrotactin
A cell surface molecule that causes neurons to adhere to radial glial fibers during neuronal migration.
athetosis
Slow, writhing movements seen primarily in patients with disorders of the basal ganglia.
ATPase pumps
Membrane pumps that use the hydrolysis of ATP to translocate ions against their electrochemical gradients.
atrophy
The physical wasting away of a tissue, typically muscle, in response to disuse or other causes.
attention
The selection of a particular sensory stimulus or mental process for further analysis.
auditory meatus
Opening of the external ear canal.
auditory space map
Topographic representation of sound source location, as occurs in the inferior colliculus.
autonomic nervous system
The components of the nervous system (peripheral and central) concerned with the regulation of smooth muscle, cardiac muscle, and glands. Also known as the visceral motor system; sometimes called the "involuntary" nervous system. Consists of sympathetic and parasympathetic divisions.
axial plane
See horizontal plane.
axon
The neuronal process that carries the action potential from the nerve cell body to a target.
axon hillock
Point at the cell body that is the site of an action potential's initiation.
axoplasmic transport
The process by which materials are carried from nerve cell bodies to their terminals (anterograde transport), or from nerve cell terminals to the neuronal cell body (retrograde transport).

B

Babinski sign
An abnormal response (extension as opposed to flexion of the toes) to the stroking of the sole of the foot; indicative of damage to descending motor neuron pathways.
baroreceptors
Sensory receptors in the visceral motor system that respond to changes in blood pressure.
basal ganglia
A group of nuclei lying deep in the subcortical white matter of the frontal lobes that organize motor behavior. The caudate, putamen, and globus pallidus are the major components of the basal ganglia; the subthalamic nucleus and substantia nigra are often included.
basal lamina
A thin layer of extracellular matrix material (primarily collagen, laminin, and fibronectin) that surrounds muscle cells and Schwann cells. Also underlies all epithelial sheets. Also called the basement membrane.
basilar membrane
The membrane that forms the floor of the cochlear duct, on which the cochlear hair cells are located.
basket cells
Inhibitory interneurons in the cerebellar cortex whose cell bodies are located within the Purkinje cell layer and whose axons make basketlike terminal arbors around Purkinje cell bodies.
bHLH proteins
Neurogenic transcription factors (named for a shared basic helix-loop-helix amino acid motif that defines their DNA-binding domain) that have emerged as central to the differentiation of distinct neural and glial fates.
binocular
Referring to both eyes.
binocular field
The two symmetrical, overlapping visual hemifields. The left hemifield includes the nasal visual field of the right eye and the temporal visual field of the left eye; the right hemifield includes the temporal field of the right eye and the nasal field of the left eye.
biogenic amines
Category of small-molecule neurotransmitters; includes the catecholamines (epinephrine, norepinephrine, dopamine), serotonin, and histamine.
bipolar cells
Retinal neurons that provide a direct link between photoreceptor terminals and ganglion cell dendrites.
blastomere
A cell produced when the egg undergoes cleavage.
blastula
An early embryo during the stage when the cells are typically arranged to form a hollow sphere.
blind spot
The region of visual space that falls on the optic disk; due to the lack of photoreceptors in the optic disk, objects that lie completely within the blind spot are not perceived.
blood–brain barrier
A diffusion barrier between the brain vasculature and the substance of the brain formed by tight junctions between capillary endothelial cells.
bone morphogenetic proteins (BMPs)
Peptide hormones that play important roles in neural induction and differentiation.
bouton (synaptic bouton)
A swelling specialized for the release of neurotransmitter that occurs along or at the end of an axon.
bradykinesia
Pathologically slow movement.
brain-derived neutrophic factor (BDNF)
One member of a family of neutrophic factors, the best known constituent of which is nerve growth factor (NGF).
brainstem
The portion of the brain that lies between the diencephalon and the spinal cord; comprises the midbrain, pons, and medulla.
Broca's aphasia
Difficulty producing speech as a result of damage to Broca's area in the left frontal lobe. Also called motor, expressive, or production aphasia.
Broca's area
An area in the left frontal lobe specialized for the production of speech.

C

cadherins
A family of calcium-dependent cell adhesion molecules found on the surfaces of growth cones and the cells over which they grow.
calcarine sulcus
The major sulcus on the medial aspect of the occipital lobe; the primary visual (striate) cortex lies largely within this sulcus.
cAMP response element binding protein (CREB)
A protein activated by cyclic AMP that binds to specific regions of DNA, thereby increasing the transcription rates of nearby genes.
cAMP response elements (CREs)
Specific DNA sequences that bind transcription factors activated by cAMP. See also cAMP response element binding protein.
carotid bodies
Specialized tissue masses found at the bifurcation of the carotid arteries in humans and other mammals that respond to the chemical composition of the blood (primarily the partial pressure of oxygen and carbon dioxide).
catecholamine
A term referring to molecules containing a catechol ring and an amino group; examples are the neurotransmitters epinephrine, norepinephrine, and dopamine.
cauda equina
The collection of segmental ventral and dorsal roots that extend from the caudal end of the spinal cord to their exit from the spinal canal.
caudal
Posterior, or "tailward."
caudate (caudate nucleus)
One of the three major components of the basal ganglia (the other two are the globus pallidus and putamen).
cell adhesion molecules (CAMs)
A family of molecules on cell surfaces that cause cells to stick to one another. See also fibronectin and laminin.
central nervous system (CNS)
The brain and spinal cord of vertebrates (by analogy, the central nerve cord and ganglia of invertebrates).
central pattern generators
Oscillatory spinal cord or brainstem circuits responsible for programmed, rhythmic movements such as locomotion.
central sulcus
A major sulcus on the lateral aspect of the cerebral hemispheres that forms the boundary between the frontal and parietal lobes. The anterior bank of the sulcus contains the primary motor cortex; the posterior bank contains the primary sensory cortex.
cephalic flexure
Sharp bend in the neural tube which during early neurulation balloons out to form the prosencephalon, which in turn will give rise to the forebrain and later to the cerebral hemispheres.
cerebellar ataxia
A pathological inability to make coordinated movements, associated with lesions to the cerebellum.
cerebellar cortex
The superficial gray matter of the cerebellum.
cerebellar peduncles
Three bilateral pairs of axon tracts that carry information to and from the cerebellum. The superior cerebellar peduncle, or brachium conjunctivum, is an efferent motor pathway; the middle cerebellar peduncle, or brachium pontis, is an afferent pathway arising from the pontine nuclei. The smallest but most complex is the inferior cerebellar peduncle, or restiform body, which encompasses multiple pathways.
cerebellum
Prominent hindbrain structure concerned with motor coordination, posture, and balance. Composed of a three-layered cortex and deep nuclei; attached to the brainstem by the cerebellar peduncles (see preceding entry).
cerebral achromatopsia
Loss of color vision as a result of damage to extrastriate visual cortex.
cerebral aqueduct
The portion of the ventricular system that connects the third and fourth ventricles. Also called the aqueduct of Sylvius.
cerebral cortex
The superficial gray matter of the cerebral hemispheres.
cerebral hemispheres
Either of the two symmetrical halves of the cerebrum.
cerebral peduncles
The numerous major axon tracts that connect the brainstem to the cerebral hemispheres. They include the important corticospinal and corticobulbar tracts.
cerebrocerebellum
The part of the cerebellar cortex that receives input from the cerebral cortex via axons from the pontine relay nuclei.
cerebrospinal fluid (CSF)
A normally clear and cell-free fluid that fills the ventricular system of the central nervous system; produced by the choroid plexus in the third ventricle.
cerebrum
The largest and most rostral part of the brain in humans and other mammals, consisting of the two cerebral hemispheres.
c-fos
A transcription factor, originally isolated from cellular feline osteosarcoma cells, that binds as a heterodimer, thus activating gene transcription.
chemical synapses
Synapses that transmit information via the secretion of chemical signals (neurotransmitters).
chemoaffinity (chemoaffinity hypothesis)
The idea that nerve cells bear chemical labels that determine their connectivity.
chemotaxis
The movement of a cell up (or down) the gradient of a chemical signal.
chemotropism
The growth of a part of a cell (axon, dendrite, filopodium) up (or down) a chemical gradient.
chimera
An experimentally generated embryo (or organ) comprising cells derived from two or more species (or other genetically distinct sources).
cholinergic
Referring to synaptic transmission mediated by acetylcholine.
chorea
Jerky, involuntary movements of the face or extremities associated with damage to the basal ganglia.
choreoathetosis
The combination of jerky, ballistic, and writhing movements that characterizes the late stages of Huntington's disease.
choroid plexus
Specialized epithelium in the ventricular system that produces cerebrospinal fluid.
chromosome
Nuclear organelle that bears the genes.
ciliary body
Two-part ring of tissue encircling the lens of the eye. The muscular component is important for adjusting the refractive power of the lens. The vascular component produces the fluid that fills the front of the eye.
cingulate cortex
Cortex of the cingulate gyrus that surrounds the corpus callosum; important in emotional and visceral motor behavior.
cingulate gyrus
Prominent gyrus on the medial aspect of the hemisphere, lying just superior to the corpus callosum; forms a part of the limbic system.
cingulate sulcus
Prominent sulcus on the medial aspect of the hemisphere.
circadian rhythms
Variations in physiological functions that occur on a daily basis.
circle of Willis
Arterial anastomosis on the ventral aspect of the midbrain; connects the posterior and anterior cerebral circulation.
cisterns
Large, cerebrospinal-fluid-filled spaces that lie within the subarachnoid space.
Clarke's nucleus
A group of relay neurons located in the medial aspect of the dorsal spinal column. Component of a cerebellar motor pathway important in processing proprioceptive input. Also called the dorsal nucleus of Clarke.
clathrin
The most important protein for endocytotic budding of vesicles from the plasma membrane; its three-pronged "triskelia" attach to the vesicular membrane to be retrieved.
climbing fibers
Axons that originate in the inferior olive, ascend through the inferior cerebellar peduncle, and make terminal arborizations that invest the dendritic tree of Purkinje cells.
clone
The progeny of a single cell.
cochlea
The coiled structure in the inner ear where vibrations caused by sound are transduced into neural impulses.
cognition
A general term referring to higher order brain functions such as language, emotion, memory, and consciousness; the ability of the central nervous system to attend, identify, and act on complex stimuli.
cognitive neuroscience
The field of neuroscience devoted to studying and understanding cognitive functions.
collapsin
A molecule that causes collapse of growth cones; a member of the semaphorin family of signaling molecules.
colliculi
The two paired hillocks that characterize the dorsal surface of the midbrain; the superior colliculi concern vision, the inferior colliculi audition.
commissures
Axon tracts that cross the midline of the brain. See also anterior commissure.
competitive interaction
The struggle among nerve cells, or nerve cell processes, for limited resources essential to survival or growth.
computational mapping
Central process of assessing and integrating multiple stimulus attributes into an orderly representation that facilitates the extraction and processing of essential information (e.g., the number and configuration of odorant molecules in order to determine the source and nature of a smell).
concha
A component of the external ear.
conduction aphasia
Difficulty producing speech as a result of damage to the connection between Wernicke's and Broca's language areas.
conduction velocity
The speed at which an action potential is propagated along an axon.
conductive hearing loss
Diminished sense of hearing due to the reduced ability of sounds to be mechanically transmitted to the inner ear. Common causes include occlusion of the ear canal, perforation of the tympanic membrane, and arthritic degeneration of the middle ear ossicles. Contrast with sensorineural hearing loss.
cone opsins
The three distinct photopigments found in cones; the basis for color vision.
cones
Photoreceptor cells specialized for high visual acuity and the perception of color.
congenital adrenal hyperplasia (CAH)
Genetic deficiency that leads to overproduction of androgens and a resultant masculinization of external genitalia in genotypic females.
conjugate eye movements
The paired movements of the two eyes in the same direction, as occurs in the vestibulo-ocular reflex. See also vergence movements and vestibulo-ocular reflex.
connexons
Precisely aligned, paired transmembrane channels that form gap junctions between cells. They are formed from connexins, members of a specialized family of channel proteins.
contralateral
On the other side.
contralateral neglect syndrome
Neurological condition in which the patient does not acknowledge or attend to the left visual hemifield or the left half of the body. The syndrome typically results from lesions of the right parietal cortex.
contrast
The difference, usually expressed in terms of a percentage in luminance, between two territories in the visual field (can also apply to color when specified as spectral contrast).
convergence
Innervation of a target cell by axons from more than one neuron. In vision refers specifically to the convergence of both rod and cone photoreceptor cells onto retinal ganglion cells.
cornea
The transparent surface of the eyeball in front of the lens; the major refractive element in the optical pathway.
coronal (frontal) plane
Any sectional plane through the brain that runs parallel to the face, dividing into anterior (front) and posterior (rear) segments. See also horizontal, sagittal.
corpus callosum
The large midline fiber bundle that connects the cortices of the two cerebral hemispheres.
corpus striatum
General term applied to the caudate and putamen; name derives from the striated appearance of these basal ganglia nuclei in sections of fresh material.
cortex
The superficial mantle of gray matter (a sheet-like array of nerve cells) covering the cerebral hemispheres and cerebellum, where most of the neurons in the brain are located.
corticobulbar tract
Pathway carrying motor information from the primary and secondary motor cortices to the brainstem.
corticocortical connections
Connections made between cortical areas in the same hemisphere, or between corresponding areas in the two hemispheres via the cerebral commissures.
corticospinal tract
Pathway carrying motor information from the primary and secondary motor cortices to the spinal cord in humans. Essential for the performance of discrete voluntary movements, especially of the hands and feet.
co-transmitters
Two or more types of neurotransmitters within a single synapse; may be packaged into separate populations of synaptic vesicles or co-localized within the same synaptic vesicles.
co-transporters
Active transporters that use the energy from ionic gradients to carry multiple ions across the membrane in the same direction. See also ion exchangers.
cranial nerve ganglia
The sensory ganglia associated with the cranial nerves; these correspond to the dorsal root ganglia of the segmental nerves of the spinal cord.
cranial nerve nuclei
Nuclei in the brainstem that contain the neurons related to cranial nerves III–XII.
cranial nerves
The 12 pairs of nerves arising from the brainstem that carry sensory information toward (and sometimes motor information away from) the central nervous system.
CREB
See cAMP response element binding protein.
crista
The hair cell–containing sensory epithelium of the semicircular canals.
critical period
A restricted developmental period during which the nervous system is particularly sensitive to the effects of experience.
cuneate nuclei
Sensory relay nuclei that lie in the lower medulla; they contain the second-order sensory neurons that relay mechanosensory information from peripheral receptors in the upper body to the thalamus.
cupulae
Gelatinous structures in the semicircular canals in which the hair cell bundles are embedded.
cytoarchitectonic areas
Distinct regions of the neocortical mantle identified by differences in cell size, packing density, and laminar arrangement (layering). Most prominent in humans is the 6-layered neocortex. The evolutionary older archicortex (or hippocampal cortex) has 3–4 layers, and the ancient paleocortex has 3 layers.

D

decerebrate rigidity
Excessive tone in extensor muscles as a result of damage to descending motor pathways at the level of the brainstem.
declarative memory
Memories available to consciousness that can be expressed by language.
decussation
A crossing of fiber tracts in the midline.
deep cerebellar nuclei
The nuclei at the base of the cerebellum that relay information from the cerebellar cortex to the thalamus.
deep tendon reflex
See myotatic reflex.
delayed response task
A behavioral paradigm used to test cognition and memory.
delta waves
Slow (<4 Hz) electroencephalographic waves that characterize stage IV (slow-wave) sleep.
dendrite
A neuronal process arising from the nerve cell body that receives synaptic input.
denervation
Removal of the innervation to a target.
dentate gyrus
A region of the hippocampus; so named because it is shaped like a tooth.
depolarization
Displacement of a cell's membrane potential toward a less negative value.
dermatome
The area of skin supplied by the sensory axons of a single spinal nerve.
determination
Commitment of a developing cell or cell group to a particular fate.
diencephalon
Portion of the brain that lies just rostral to the midbrain; comprises the thalamus and hypothalamus. The diencephalon and telencephalon compose the prosencephalon.
differentiation
The progressive specialization of developing cells.
dihydrotestosterone
A more potent form of testosterone; masculinizes the external genitalia.
disinhibition
Arrangement of inhibitory and excitatory cells in a circuit that generates excitation by the transient inhibition of a tonically active inhibitory neuron.
disjunctive eye movements
Movements of the two eyes in opposite directions. See also vergence movements.
distal
Farther away from a point of reference (the opposite of proximal).
divergence
The branching of a single axon to innervate multiple target cells.
dopamine
A catecholamine neurotransmitter.
dorsal
Referring to the back.
dorsal column nuclei
Second-order sensory neurons in the lower medulla that relay mechanosensory information from the spinal cord to the thalamus; comprises the cuneate and gracile nuclei.
dorsal columns
Major ascending tracts in the spinal cord that carry mechanosensory information from the first-order sensory neurons in dorsal root ganglia to the dorsal column nuclei; also called the posterior funiculi.
dorsal horn
The dorsal portion of the spinal cord gray matter; contains neurons that process sensory information.
dorsal root ganglia (DRG)
The segmental sensory ganglia of the spinal cord; they contain the first-order neurons of the dorsal column/medial lemniscus and spinothalamic pathways.
dorsal roots
The bundle of axons that runs from the dorsal root ganglia to the dorsal horn of the spinal cord, carrying sensory information from the periphery.
dura mater
The thick external covering of the brain and spinal cord; one of the three components of the meninges, the other two being the pia mater and arachnoid mater.
dysarthria
Difficulty producing speech as a result of damage to the primary motor centers that govern the muscles of articulation; distinguished from aphasia, which results from cortical damage.
dysdiadochokinesia
Difficulty performing rapid alternating movements.
dysmetria
Inaccurate movements due to faulty judgment of distance, especially over- or underreaching. Characteristic of cerebellar pathology.
dystonia
Lack of muscle tone.

E

early inward current
The initial electrical current, measured in voltage clamp experiments, that results from the voltage-dependent entry of a cation such as Na+ or Ca2+; produces the rising phase of the action potential.
ectoderm
The most superficial of the three embryonic germ layers; gives rise to the nervous system and epidermis.
Edinger–Westphal nucleus
Midbrain nucleus containing the autonomic neurons that constitute the efferent limb of the pupillary light reflex.
efferent
A neuron or axon that conducts information away from the central nervous system toward the periphery.
electrical synapses
Synapses that transmit information via the direct flow of electrical current at gap junctions.
electrochemical equilibrium
The condition in which no net ionic flux occurs across a membrane because ion concentration gradients and opposing transmembrane potentials are in exact balance.
electrogenic
Capable of generating an electrical current; usually applied to membrane transporters that create electrical currents while translocating ions.
electrophysiological recording
Measure of the electrical activity across the membrane of a nerve cell by use of electrodes. Extracellular recording places the electrode outside but nearby the cell of interest; intracellular recording places the electrode is placed inside the cell of interest.
embryo
The developing organism before birth or hatching.
embryonic stem (ES) cells
Cells derived from pre-gastrula embryos that have the potential for infinite self-renewal and can give rise to all tissue and cell types of the organism. See also glial stem cells; neural stem cells.
endocannabinoids
A family of endogenous signals that participate in several forms of synaptic transmission, interacting cannabinoid receptors. These receptors are the molecular targets of the psychoactive component of the marijuana plant, Cannabis.
endocrine
Referring to the release of signaling molecules whose effects are made widespread by distribution in the general circulation.
endocytosis
A budding off of vesicles from the plasma membrane, which allows uptake of materials in the extracellular medium.
endoderm
The innermost of the three embryonic germ layers. Gives rise to the digestive and respiratory tracts and the structures associated with them.
endogenous opioids
Peptide neuritransmitters in the central nervous system that have the same pharmacological effects as morphine and other derivatives of opium, being agonists at opioid receptors, virtually all of which contain the sequence Tyr-Gly-Gly-Phe. There are three classes: dynorphins, endorphins, and enkephalins.
endolymph
The potassium-rich fluid filling both the cochlear duct and the membranous labyrinth; bathes the apical end of the hair cells.
end plate
The complex postsynaptic specialization at the site of nerve contact on skeletal muscle fibers.
end plate current (EPC)
A macroscopic postsynaptic current resulting from the summed opening of many ion channels; produced by neurotransmitter release and binding at the motor end plate.
end plate potential (EPP)
Depolarization of the membrane potential of skeletal muscle fiber, caused by the action of the transmitter acetylcholine at the neuromuscular synapse.
engram
A term used to describe the physical basis of a stored memory.
enteric system
A subsystem of the visceral motor system, made up of small ganglia and individual neurons scattered throughout the wall of the gut; influences gastric motility and secretion.
ependyma
The epithelial lining of the canal of the spinal cord and the ventricles.
ependymal cells
Epithelial cells that line the ventricular system.
epidermis
The outermost layer of the skin; derived from the embryonic ectoderm.
epigenetic
Referring to influences on development that arise from factors other than genetic instructions.
epinephrine (adrenaline)
Catecholamine hormone and neurotransmitter that binds to adrenergic G-protein-coupled receptors.
epineurium
The connective tissue surrounding axon fascicles of a peripheral nerve.
epithelium
Any continuous layer of cells that covers a surface or lines a cavity.
equilibrium potential
The membrane potential at which a given ion is in electrochemical equilibrium.
estrogens
A group of steroid hormones (including estradiol) that affect sexual differentiation during development and reproductive function and behavior in mature adults.
excitatory amino acid transporters (EAATs)
Family of Na+-dependent glutamate co-transporters involved in synaptic transmission. See also glutamate–glutamine cycle.
excitatory postsynaptic potential (EPSP)
Neurotransmitter-induced postsynaptic potential change that depolarizes the cell, and hence increases the likelihood of initiating an action potential.
excitotoxicity
Destruction of neurons by prolonged excitatory synaptic transmission brought on by abnormally high levels of glutamate or related compounds in the synaptic cleft, usually as a result of local trauma.
exocytosis
A form of cell secretion resulting from the fusion of the membrane of a storage organelle, such as a synaptic vesicle, with the plasma membrane.
explant
A piece of tissue maintained in culture medium.
external segment
A subdivision of the globus pallidus.
extracellular matrix
A matrix composed of collagen, laminin, and fibronectin that surrounds most cells. See also basal lamina.
extrafusal muscle fibers
Fibers of skeletal muscles; a term that distinguishes ordinary muscle fibers from the specialized intrafusal fibers associated with muscle spindles.

F

face cells
Neurons in the temporal cortex of rhesus monkeys that respond specifically to faces.
facilitation
The increased transmitter release produced by an action potential that follows closely upon a preceding action potential.
fasciculation
The aggregation of neuronal processes to form a nerve bundle; also refers to the spontaneous discharge of motor units after muscle denervation.
α-fetoprotein
A protein that actively sequesters circulating estrogens, preventing maternal estrogen from affecting the sexual differentiation of the fetus.
fetus
The term used to refer to a human embryo once its limbs and other body parts become recognizable (around 6 weeks gestation).
fibrillation
Spontaneous contractile activity of denervated muscle fibers.
fibroblast growth factor (FGF)
A peptide growth factor, originally defined by its mitogenic effects on fibroblasts; also acts as an inducer during early brain development.
fibronectin
A large cell adhesion molecule that binds integrins.
filopodium
Slender protoplasmic projection, arising from the growth cone of an axon or a dendrite, that explores the local environment.
fissure
A deep cleft in the brain; distinguished from sulci, which are shallower cortical infoldings.
flexion reflex
Polysynaptic reflex mediating withdrawal from a painful stimulus.
floorplate
Transient structure in the ventral portion of the neural tube; important in the guidance and crossing of growing axons. See also roofplate.
folia
The gyral formations of the cerebellum.
forebrain
The anterior portion of the brain that includes the diencephalon and cerebral hemispheres.
fornix
An axon tract, best seen from the medial surface of the divided brain, that interconnects the hypothalamus and hippocampus.
fourth ventricle
The ventricular space that lies between the pons and the cerebellum.
fovea
Area of the retina specialized for high acuity in the center of the macula; contains a high density of cones and few rods.
foveola
Capillary-free and rod-free zone in the center of the fovea.
free nerve endings
Afferent fibers that lack specialized receptor cells; they are especially important in the sensation of pain.
frontal eye field
A region of the frontal lobe that lies in a rostral portion of the premotor cortex and that contains cells that respond to visual and motor stimuli.
frontal lobe
One of the four lobes of the brain; includes all the cortex that lies anterior to the central sulcus and superior to the lateral fissure.
frontal plane
See coronal plane.

G

G-protein-coupled receptors
A large family of neurotransmitter or hormone receptors, characterized by seven transmembrane domains; the binding of these receptors by agonists leads to the activation of intracellular G-proteins. See also metabotropic receptors.
G-proteins
Proteins that are activated by exchanging bound GDP for bound GTP (and thus also known as GTP-binding proteins).
gamma (γ) motor neurons
Class of spinal motor neurons specifically concerned with the regulation of muscle spindle length; these neurons innervate the intrafusal muscle fibers of the spindle.
ganglion (ganglia)
Collection of hundreds to thousands of neurons found outside the brain and spinal cord along the course of peripheral nerves.
ganglion cell
A neuron located in a ganglion.
gap junction
A specialized intercellular contact formed by channels that directly connect the cytoplasm of two cells.
gastrula
The early embryo during the period when the three embryonic germ layers are formed; follows the blastula stage.
gastrulation
The cell movements (invagination and spreading) that transform the embryonic blastula into the gastrula.
gender
The self-perception of one's alignment with the traits associated with phenotypic females or males in a given culture.
gene
Hereditary unit located on the chromosomes; genetic information is carried by linear sequences of nucleotides in DNA that code for corresponding sequences of amino acids.
generator potential
See receptor potential.
genome
The complete set of an organism's genes.
genomics
Scientific field focusing on the analysis of DNA sequences, including both protein-coding DNA (genes) and non-coding DNA.
genotypic sex
Primary sex determination by the complement of sex chromosomes. In humans, XX is a genotypic female, XY a genotypic male.
germ cell
The egg or sperm (or the precursors of these cells).
germ layers
The three layers‚ ectoderm, mesoderm, and endo-derm‚ of the developing embryo from which all adult tissues arise. Neural cells and structures arise from the ectoderm and from the mesodermally-generated notochord.
glaucoma
Condition in which the eye's aqueous humor is not adequately drained, resulting in increased intraocular pressure, reduced blood supply to the eye, and eventual damage to the retina.
glial cells (glia)
The support cells associated with neurons (astrocytes, oligodendrocytes, and microglial cells in the central nervous system; Schwann cells in peripheral nerves; and satellite cells in ganglia).
glial scarring
Local proliferation of glial precursors and extensive growth of processes from existing glia within or around the site of a brain injury.
glial stem cells
Neural precursor cells in the adult brain that retain the capacity to proliferate and generate both additional precursor cells and differentiated glial cells (and, in some cases, differentiated neurons).
globus pallidus
One of the three major nuclei that make up the basal ganglia in the cerebral hemispheres; relays information from the caudate and putamen to the thalamus.
glomeruli
Characteristic collections of neuropil in the olfactory bulb; formed by dendrites of mitral cells and terminals of olfactory receptor cells, as well as processes from local interneurons.
glutamate–glutamine cycle
A metabolic cycle of glutamate release and resynthesis involving both neuronal and glial cells.
Golf
A G-protein found uniquely in olfactory receptor neurons.
Golgi tendon organs (GTO)
Receptors at the interface of muscle and tendon that provide mechanosensory information to the central nervous system about muscle tension.
Golgi stain
See silver stain.
gracile nuclei
Sensory nuclei in the lower medulla; these second-order sensory neurons relay mechanosensory information from the lower body to the thalamus.
gradient
A systematic variation of the concentration of a molecule (or some other agent) that influences cell behavior.
granule cell layer
The layer of the cerebellar cortex where granule cell bodies are found. Also used to refer to cell-rich layers in neocortex and hippocampus.
gray matter
General term that describes regions of the central nervous system rich in neuronal cell bodies and neuropil; includes the cerebral and cerebellar cortices, the nuclei of the brain, and the central portion of the spinal cord.
green fluorescent protein (GFP)
A reporter or marker protein that generates green fluorescent light. GFP is widely used as a genetic tag in fluorescence microscopy. Originally discovered in a light-emitting jellyfish, It allows observation of specific neurons and some of their components over time in living cells, or even in whole organisms.
growth cone
The specialized end of a growing axon (or dendrite) that generates the motive force for elongation.
gyrus (pl. gyri)
The ridges of the infolded cerebral cortex (the valleys between these ridges are called sulci).

H

hair cells
The sensory cells in the inner ear that transduce mech-anical displacement into neural impulses.
Hebb's postulate
The idea that when pre- and postsynaptic neurons fire action potentials at the same time, the synaptic association between those cells strengthens. Sometimes catchily phrased as "cells that fire together wire together," the postulate provides one explanation for the formation of certain neural networks.
helicotrema
The opening at the apex of the cochlea that joins the perilymph-filled cavities of the scala vestibuli and scala tympani.
hemiballismus
A basal ganglia syndrome resulting from damage to the subthalamic nucleus and characterized by violent involuntary movements of the limbs.
heterotrimeric G-proteins
A large group of proteins consisting of three subunits (α, β, and γ) that can be activated by exchanging bound GDP for GTP, resulting in the liberation of two signaling molecules‚ αGTP and the βγ dimer.
higher order neurons
Neurons that are relatively remote from peripheral targets.
hindbrain
See rhombencephalon.
hippocampus
A cortical structure in the medial portion of the temporal lobe; in humans, concerned with short-term declarative memory, among many other functions.
histamine
A biogenic amine neurotransmitter derived from the amino acid histidine.
holoprosencephaly
Disrupted regional differentiation of the forebrain during development resulting in severe brain malformations.
homeotic genes
Genes that determine the developmental fate of an entire segment of an animal. Mutations in these genes drastically alter the characteristics of the body segment (as when wings grow from a fly body segment that should have produced legs).
homologous
Technically, refers to structures in different species that share the same evolutionary history; more generally, can refer to structures or organs that have the same general anatomy and perform the same function.
horizontal cells
Retinal neurons that mediate lateral interactions between photoreceptor terminals and the dendrites of bipolar cells.
horizontal (axial) plane
Any sectional plane that runs parallel to the floor, dividing the brain or body into superior (above) and inferior (below) sections. See also coronal, sagittal.
horseradish peroxidase (HRP)
A plant enzyme widely used to stain nerve cells (after injection into a neuron, it generates a visible precipitate by one of several histochemical reactions).
Hox genes
A group of conserved genes characterized by a specific DNA sequence‚ "the homeobox‚" and that specify body axis (particularly the anterior–posterior axis) and regional identity in the developing vertebrate embryo.
Huntington's disease
An autosomal dominant genetic disorder in which a single gene mutation results in personality changes, progressive loss of control of voluntary movement, and eventually death. Primary target is the basal ganglia.
hydrocephalus
Enlarged cranium as a result of increased cerebrospinal fluid pressure (typically due to a mechanical outflow blockage).
hyperalgesia
Increased perception of pain.
hyperkinesia
Excessive movement.
hyperpolarization
The displacement of a cell's membrane potential toward a more negative value.
hypokinesia
A paucity of movement.
hypothalamus
A collection of small but critical nuclei in the diencephalon that lies just inferior to the thalamus; governs reproductive, homeostatic, and circadian functions.
hypoxia
Lack of oxygen in the brain. Can be local, usually created by diminished blood flow (ischemia) due to local vascular occlusions; or global deprivation of oxygen after an event such as drowning or cardiac arrest.

I

imprinting
A rapid and permanent form of learning that occurs in response to early experience.
inactivation
The time-dependent closing of ion channels in response to a stimulus, typically membrane depolarization.
inducers
Chemical signals originating from one set of cells that influence the differentiation of other cells.
induction
The ability of a cell or tissue to influence the fate of nearby cells or tissues during development by chemical signals.
inferior colliculi (singular, colliculus)
Paired hillocks on the dorsal surface of the midbrain; concerned with auditory processing.
inferior olive
Prominent nucleus in the medulla; a major source of input to the cerebellum. Also called inferior olivary nucleus.
infundibulum
The connection between the hypothalamus and the pituitary gland; also known as the pituitary stalk.
inhibitory postsynaptic potential (IPSP)
Neurotransmitter-induced postsynaptic potential change that tends to decrease the likelihood of a postsynaptic action potential.
innervate
Establish synaptic contact with a target.
innervation
Referring to all the synaptic contacts of a target.
input
The innervation of a target cell by a particular axon; more loosely, the innervation of a target.
input elimination
See synapse elimination.
insula
The portion of the cerebral cortex that is buried within the depths of the lateral fissure. Also called insular cortex.
integral membrane proteins
Proteins that possess hydrophobic domains that are inserted into membranes.
integration
The summation of excitatory and inhibitory synaptic conductance changes by postsynaptic cells.
integrins
A family of receptor molecules found on growth cones that bind to cell adhesion molecules such as laminin and fibronection.
intention tremor
Tremor that occurs while performing a voluntary motor act. Characteristic of cerebellar pathology. Also called action tremor.
interhemispheric connections
The corpus callosum and anterior commissure, together. Mediate corticocortical connections between cortical regions in the opposite hemispheres.
internal arcuate tract
Mechanosensory pathway in the brainstem that runs from the dorsal column nuclei to form the medial lemniscus.
internal capsule
Large white matter tract that lies between the diencephalon and the basal ganglia; contains, among others, sensory axons that run from the thalamus to the cortex and motor axons that run from the cortex to the brainstem and spinal cord.
internal segment
A subdivision of the globus pallidus.
interneuron
Technically, a neuron in the pathway between primary sensory and primary effector neurons; more generally, a neuron whose relatively short axons branch locally to innervate other neurons. Also known as local circuit neuron.
interstitial nuclei of the anterior hypothalamus (INAH)
Four cell groups located slightly lateral to the third ventricle in the anterior hypothalamus of primates; thought to play a role in sexual behavior.
intrafusal muscle fibers
Specialized muscle fibers found in muscle spindles.
ion channels
Integral membrane proteins possessing pores that allow only certain ions to diffuse across cell membranes, thereby conferring selective ionic permeability.
ion exchangers
Membrane transporters that exchange intracellular and extracellular ions against their concentration gradient by using the electrochemical gradient of other ions as an energy source. See also co-transporters.
ionotropic glutamate receptors
Glutamate-gated cation channels that allow the passage of Na+ and K+. They include the AMPA receptors, NMDA receptors, and kainate receptors, all named after the agonists that activate receptor activation always produces excitatory postsynaptic responses.
ionotropic receptors
Receptors in which the ligand binding site is an integral part of the receptor molecule.
ion pumps
See active transporters.
ipsilateral
On the same side of the body.
iris
Circular pigmented membrane behind the cornea of the eye; perforated by the pupil.
ischemia
Insufficient blood supply.

J

joint receptors
Mechanoreceptors found in and around joints; they appear to be most important for judging position of the fingers.

K

kainate receptors
See ionotropic glutamate receptors.
kinocilium
A true ciliary structure which, along with the stereocilia, comprises the hair bundle of vestibular and fetal cochlear hair cells in mammals (it is not present in the adult mammalian cochlear hair cell).
knee-jerk reaction
See myotatic reflex.
Korsakoff's syndrome
An amnesic syndrome seen in chronic alcoholics.

L

labyrinth
A set of interconnected chambers in the internal ear, comprising the cochlea, vestibular apparatus, and the bony canals in which these structures are housed.
lamellipodium
A sheetlike extension, rich in actin filaments, on the leading edge of a motile cell or growth cone.
laminae (singular, lamina)
Cell layers that characterize the neocortex, hippocampus, and cerebellar cortex. The gray matter of the spinal cord is also arranged in laminae.
laminins
Large cell adhesion molecules that bind integrins. Laminin is a major component of the extracellular matrix.
late outward current
The delayed electrical current, measured in voltage clamp experiments, that results from the voltage-dependent efflux of a cation such as K+. Produces the repolarizing phase of the action potential.
lateral columns
The lateral regions of spinal cord white matter that convey motor information from the brain to the spinal cord.
lateral (Sylvian) fissure
The cleft on the lateral surface of the brain that separates the temporal and frontal lobes.
lateral geniculate nucleus (LGN)
A nucleus in the thalamus that receives the axonal projections of retinal ganglion cells in the primary visual pathway.
lateral olfactory tract
The projection from the olfactory bulb to higher olfactory centers.
lateral posterior nucleus
A thalamic nucleus that receives its major input from sensory and association cortices and projects in turn to association cortices, particularly in the parietal and temporal lobes.
lateral superior olive (LSO)
The auditory brainstem structure that processes interaural intensity differences and, in humans, mediates sound localization for stimuli greater than 3 kHz.
learning
The acquisition of novel behavior through experience.
lens
Transparent structure in the eye whose thickening or flattening in response to visceral motor control allows light rays to be focused on the retina.
lexical
The quality of associating a symbol (e.g., a word) with a particular object, emotion, or idea.
lexicon
Dictionary. Sometimes used to indicate region of brain that stores the meanings of words.
ligand-gated ion channels
Ion channels that respond to chemical signals rather than to the changes in membrane potential generated by ionic gradients. The term covers a large group of neurotransmitter receptors that combine receptor and ion channel functions into a single molecule.
limb bud
The limb rudiment of vertebrate embryos.
limbic lobe
Cortex that lies superior to the corpus callosum on the medial aspect of the cerebral hemispheres; forms the cortical component of the limbic system.
limbic system
Term that refers to those cortical and subcortical structures concerned with the emotions; the most prominent components are the cingulate gyrus, the hippocampus, and the amygdala.
lobes
The four major divisions of the cerebral hemispheres (frontal, parietal, occipital, and temporal).
local circuit neuron
General term referring to a neuron whose activity mediates interactions between sensory systems and motor systems. Interneuron is often used as a synonym.
locus coeruleus
A small brainstem nucleus with widespread adrenergic cortical and descending connections; important in the governance of sleep and waking.
long-term
Lasting days, weeks, months, or longer.
long-term depression (LTD)
A persistent weakening of synapses based on recent patterns of activity.
long-term memory
Memories that last days, weeks, months, years, or a lifetime.
long-term potentiation (LTP)
A persistent strengthening of synapses based on recent patterns of activity.
lower motor neuron
Spinal motor neuron; directly innervates muscle (also referred to as α or primary motor neuron).
lower motor neuron syndrome
Signs and symptoms arising from damage to α motor neurons; these include paralysis or paresis, muscle atrophy, areflexia, and fibrillations.

M

macroscopic currents
Ionic currents flowing through large numbers of ion channels distributed over a substantial area of membrane.
macula lutea
The central region of the retina that contains the fovea (the term derives from the yellowish appearance of this region in ophthalmoscopic examination); also, the sensory epithelia of the otolith organs.
magnocellular layer
A component of the primary visual pathway specialized for the perception of motion; so named because of the relatively large ("magno") cells involved.
mammillary bodies
Small prominences on the ventral surface of the diencephalon; functionally, part of the caudal hypothalamus.
map
The ordered projection of axons from one region of the nervous system to another, by which the organization of the body (or some function) is reflected in the organization of the nervous system.
mechanoreceptors
Receptors specialized to sense mechanical forces.
medial
Located nearer to the midsagittal plane of an animal (the opposite of lateral).
medial dorsal nucleus
A thalamic nucleus that receives its major input from sensory and association cortices and projects in turn to association cortices, particularly in the frontal lobe.
medial geniculate complex (MGC)
The major thalamic relay for auditory information.
medial lemniscus
Axon tract in the brainstem that carries mechanosensory information from the dorsal column nuclei to the thalamus.
medial longitudinal fasciculus
Axon tract that carries excitatory projections from the abducens nucleus to the contralateral oculomotor nucleus; important in coordinating conjugate eye movements.
medial superior olive (MSO)
The auditory brainstem structure that processes interaural time differences and serves to compute the horizontal location of a sound source.
medium spiny neuron
The principal projection neuron of the caudate and putamen.
medulla
The caudal portion of the brainstem, extending from the pons to the spinal cord.
medullary pyramids
Longitudinal bulges on the ventral aspect of the medulla that signify the corticobulbar and corticospinal tracts at this level of the neuraxis.
medulloblastoma
Childhood brain tumor associated with mutations of Sonic hedgehog and other genes in the Shh signaling pathway.
Meissner corpuscles
Encapsulated cutaneous mechanosensory receptors specialized for the detection of fine touch and pressure.
membrane conductance
The reciprocal of membrane resistance. Changes in membrane conductance result from, and are used to describe, the opening or closing of ion channels.
meninges
The external covering of the brain; includes the pia, arachnoid, and dura mater.
Merkel cells
Encapsulated cutaneous mechanosensory receptors specialized for the detection of fine touch and pressure.
mesencephalon
See midbrain.
mesoderm
The middle of the three embryonic germ layers; gives rise to muscle, connective tissue, skeleton, and other structures.
mesopic vision
Vision in light levels at which both the rods and cones are active.
metabotropic receptors
Receptors that are indirectly activated by the action of neurotransmitters or other extracellular signals, typically through the aegis of G-protein activation. Also called G-protein-coupled receptors.
Meyer's loop
That part of the optic radiation that runs in the caudal portion of the temporal lobe.
microglial cells
One of the three major classes of glial cells found in the central nervous system; concerned primarily with repairing damage following neural injury.
microscopic currents
Ionic currents flowing through single ion channels.
midbrain
The most rostral portion of the brainstem; identified by the superior and inferior colliculi on its dorsal surface, and the cerebral penduncles on its ventral aspect. Also called the mesencephalon.
miniature end plate potential (MEPP)
Small, spontaneous depolarization of the membrane potential of skeletal muscle cells, caused by the release of a single quantum of acetylcholine.
mitral cells
The major output neurons of the olfactory bulb.
mnemonic
Having to do with memory.
modality
A category of function. For example, vision, hearing, and touch are different sensory modalities.
molecular layer
Layer of the cerebellar cortex containing the apical dendrites of Purkinje cells, parallel fibers from granule cells, a few local circuit neurons, and the synapses between these elements.
morphine
A plant alkaloid that gives opium its analgesic properties.
morphogen
A molecule that influences morphogenesis.
morphogenesis
The generation of animal form.
motor
Pertaining to movement.
motor cortex
The region of the cerebral cortex lying anterior to the central sulcus and concerned with motor behavior. Includes the primary motor cortex in the precentral gyrus and associated cortical areas in the frontal lobe.
motor neuron
By common usage, any nerve cell that innervates skeletal muscle.
motor neuron pool
The collection of motor neurons that innervates a single muscle.
motor system
A broad term used to describe all the central and peripheral structures that support motor behavior.
motor unit
A motor neuron and the skeletal muscle fibers it innervates; more loosely, the collection of skeletal muscle fibers innervated by a single motor neuron.
mucosa
Term referring the mucus membranes lining the nose, mouth, gut, and other epithelial surfaces.
muscarinic ACh receptors (mAChRs)
A group of G-protein-coupled acetylcholine receptors activated by the plant alkaloid muscarine.
muscle spindles
Highly specialized sensory organs found in most skeletal muscles; provide mechanosensory information about muscle length.
muscle tone
The normal, ongoing tension in a muscle; measured by resistance of a muscle to passive stretching.
myelin
The multilaminated wrapping around many axons formed by oligodendrocytes or Schwann cells.
myelination
Process by which glial cells (oligodendrocytes or Schwann cells) wrap around axons to form multiple layers of glial cell membrane, thus insulating the axonal membrane and increasing conduction velocity.
myotatic reflex
A fundamental spinal reflex that is generated by the motor response to afferent sensory information arising from muscle spindles. The knee jerk reaction is a common example. Also called a "stretch" or "deep tendon" reflex.
myotome
The part of each somite that contributes to the development of skeletal muscles.

N

Na+/K+ transporter
A type of ATPase transporter in the plasma membrane of most cells that is responsible for accumulating intracellular K+ and extruding intracellular Na+. Also known as the Na+ pump.
nasal division
Referring to the region of the visual field of each eye in the direction of the nose. See also binocular field.
near reflex triad
Reflexive response induced by changing binocular fixation to a closer target; comprises convergence, accommodation, and pupillary constriction.
neocortex
The six-layered cortex that forms the surface of most of the cerebral hemispheres.
Nernst equation
A mathematical formula that predicts the electrical potential generated ionically across a membrane at electrochemical equilibrium.
nerve
A collection of peripheral axons that are bundled together and travel a common route.
nerve growth factor (NGF)
A neurotrophic protein required for survival and differentiation of sympathetic ganglion cells and certain sensory neurons. Preeminent member of the neurotrophin family of growth factors.
netrins
A family of diffusible molecules that act as attractive or repulsive cues to guide growing axons.
neural cell adhesion molecule (N-CAM)
Molecule that helps bind axons together and is widely distributed in the developing nervous system. Structurally related to immunoglobin.
neural crest
A transient region where the edges of the folded neural plate come together, at the dorsalmost limit of the neural tube. Gives rise to neural crest cells that migrate to become a variety of cells types and structures, including peripheral sensory neurons, enteric neurons, and glial cells.
neural plate
The thickened region of the dorsal ectoderm of a neurula that gives rise to the neural tube.
neural stem cell
A precursor cell type that can give rise to the full complement of cell classes found in neural tissue‚ i.e., neurons, astrocytes, and oligodendroglia, as well as more neural stem cells.
neural tube
The primordium of the brain and spinal cord; derived from the neural ectoderm.
neurexin
An adhesion molecule of the presynaptic membrane in developing synapses. Binds to neuroligin in the postsynaptic membrane, promoting adhesion, and helps localize synaptic vesicles, docking proteins, and fusion molecules.
neurite
A neuronal branch (usually used when the process in question could be either an axon or a dendrite, such as the branches of isolated nerve cells in tissue culture).
neuroblast
A dividing cell, the progeny of which develop into neurons; an immature nerve cell.
neurogenesis
The development of the nervous system.
neuroglia
See glial cells.
neurohormone
A chemical signal produced and released by neurons that subsequently functions as a hormone.
neuroleptics
A group of antipsychotic agents that cause indifference to stimuli by blocking brain dopamine receptors.
neuroligin
Postsynaptic binding partner of the presynaptic adhesion molecule neurexin. Promotes the clustering of receptors and channels of the postsynaptic density as the synapse matures.
neuromere
The repeating units of the neural tube. See also rhombomere.
neuromuscular junction
The synapse made by a motor axon on a skeletal muscle fiber.
neuronal geometry
The spatial arrangement of neuronal branches.
neuron-glia cell adhesion molecule (Ng-CAM)
A cell adhesion molecule, structurally related to immunoglobin molecules, that promotes adhesive interactions between neurons and glia.
neurons
Cells specialized for the conduction and transmission of electrical signals in the nervous system. Also called nerve cells.
neuropathic pain
Any chronic pain caused by damage to the central or peripheral nervous system.
neuropeptides
A general term describing a large number of peptides that function as neurotransmitters or neurohormones.
neuropil
The dense tangle of axonal and dendritic branches, and the synapses between them, that lies between neuronal cell bodies in the gray matter of the brain and spinal cord.
neurotransmitter
Substance released by synaptic terminals for the purpose of transmitting information from one cell (the presynaptic cell) to another (the postsynaptic cell).
neurotrophic factors
Chemical substances, secreted by cells in a target tissue, that promote the growth and survival of neurons.
neurotrophic hypothesis
The idea that developing neurons compete for a limited supply of trophic factors secreted by their targets.
neurotrophins
A family of trophic factor molecules that promote the growth and survival of several different classes of neurons.
neurula
The early vertebrate embryo during the stage when the neural tube forms from the neural plate; follows the gastrula stage.
neurulation
The process by which the neural plate folds to form the neural tube.
NMDA receptors
See ionotropic glutamate receptors.
nociceptors
Cutaneous and subcutaneous receptors (especially free nerve endings) specialized for the detection of harmful (noxious) stimuli.
nodes of Ranvier
Periodic gaps in the myelination of axons where action potentials are generated.
nondeclarative memory
Unconscious memories such as motor skills and associations. Also called procedural memory.
non-rapid eye movement (non-REM) sleep
Collectively, those phases of sleep (stages I–IV) characterized by the absence of rapid eye movements.
norepinephrine
Catecholamine hormone and neurotransmitter that binds to α- and β-adrenergic receptors, both of which are G-protein-coupled receptors. Also known as noradrenaline.
notochord
A transient, cylindrical structure of mesodermal cells underlying the neural plate (and later the neural tube) in vertebrate embryos. Source of important inductive signals for neural development.
nucleus (pl. nuclei)
Collection of nerve cells in the brain that are anatomically discrete, and which typically serve a particular function.
nucleus proprius
Region of the dorsal horn of the spinal cord that receives information from nociceptors.
nystagmus
Literally, nodding. Refers to repetitive movements of the eyes normally elicited by large-scale movements of the visual field (optokinetic nystagmus). Nystagmus in the absence of appropriate stimuli usually indicates brainstem or cerebellar pathology.

O

occipital lobe
The posterior lobe of the cerebral hemisphere; primarily devoted to vision.
ocular dominance columns
The segregated termination patterns of thalamic inputs representing the two eyes in the primary visual cortex of some mammalian species.
odorants
Molecules capable of eliciting responses from receptors in the olfactory mucosa.
olfactory bulb
Olfactory relay station that receives axons from cranial nerve I and transmits this information via the olfactory tract to higher centers.
olfactory epithelium
Pseudostratified epithelium that contains olfactory receptor cells, supporting cells, and mucus-secreting glands.
olfactory receptor neurons (ORNs)
Bipolar neurons in olfactory epithelium that contain receptors for odorants.
olfactory tracts
See lateral olfactory tract.
oligodendrocytes
One of the three major classes of glial cells found in the central nervous system; their major function is to lay down myelin.
Onuf's nucleus
Sexually dimorphic nucleus in the human spinal cord that innervates striated perineal muscles mediating contraction of the bladder in males, and vaginal constriction in females.
opioid
Any natural or synthetic drug that has pharmacological actions similar to those of morphine.
opsins
Proteins in photoreceptors that absorb light (in humans, rhodopsin and the three specialized cone opsins).
optic chiasm
The junction of the two optic nerves on the ventral aspect of the diencephalon, where axons from the nasal divisions of each retina cross the midline.
optic cup
See optic vesicle.
optic disk
The region of the retina where the axons of retinal ganglion cells exit to form the optic nerve and where the ophthalmic artery and vein enter the eye. Also called the optic papilla.
optic nerve
The nerve (cranial nerve II) containing the axons of retinal ganglion cells; extends from the eye to the optic chiasm.
optic radiation
Portion of the internal capsule that comprises the axons of lateral geniculate neurons that carry visual information to the striate cortex.
optic tectum
The first central station in the visual pathway of many non-mammalian vertebrates (analogous to the superior colliculus in mammals).
optic tract
The axons of retinal ganglion cells after they have passed through the region of the optic chiasm en route to the lateral geniculate nucleus of the thalamus.
optic vesicle
The evagination of the forebrain vesicle that generates the retina and induces lens formation in the overlying ectoderm.
optokinetic eye movements
Movements of the eyes that compensate for head movements; the stimulus for optokinetic movements is large-scale motion of the visual field.
optokinetic nystagmus
Repeated reflexive responses of the eyes to ongoing large-scale movements of the visual scene.
orbital prefrontal cortex
Divisions of the prefrontal cortex that lie above the orbits in the most rostral and ventral extension of the sagittal fissure. Important in emotional processing and rational decision-making.
orientation selectivity
A property of many neurons in visual cortex in which they respond to edges presented over a narrow range of orientations.
oscillopsia
Inability, as a result of vestibular damage, to fixate visual targets while the head is moving.
ossicles
The bones of the middle ear.
otoconia
The calcium carbonate crystals that rest on the otolithic membrane overlying the hair cells of the sacculus and utricle.
otolithic membrane
The gelatinous and fibrous membrane on which the otoconia lie and in which the tips of the hair bundles are embedded.
otolith organs
The two organs in the labyrynth of the inner ear‚ the utricle and saccule‚ that respond to linear accelerations of the head and static head position relative to the gravitational axis.
otoliths
Literally, "ear stones." Dense calciferous structures in the inner ear, important in generating the vestibular signals pertinent to balance.
outer segment
Portion of photoreceptors made up of membranous disks that contain the photopigment responsible for initiating phototransduction.
oval window
Site where the middle ear ossicles transfer vibrational energy to the cochlea.
overshoot phase
The peak, positive-going phase of an action potential, caused by high membrane permeability to a cation such as Na+ or Ca2+.
oxytocin
A 9-amino acid neuropeptide that is both a putative neurotransmitter and a neurohormone. Functions in pair bonding and maternal behavior.

P

p75 receptor
One of two classes of receptors for the neurotrophin family of growth factors; the other is the Trk (tyrosine kinase) receptors.
Pacinian corpuscles
Encapsulated cutaneous mechanosensory receptors specialized for the detection of high-frequency vibrations.
Papez's circuit
System of interconnected brain structures (mainly cingulate gyrus, hippocampus, and hypothalamus) in the medial aspect of the telencephalon and diencephalon described by James Papez. Participates in emotional processing, short-term declarative memory, and autonomic functions.
paracrine
Referring to the secretion of hormone-like agents whose effects are mediated locally rather than by the general circulation.
parallel fibers
The bifurcated axons of cerebellar granule cells that synapse on dendritic spines of Purkinje cells.
paralysis
Complete loss of voluntary motor control.
paramedial pontine reticular formation (PPRF)
Neurons in the reticular formation of the pons that coordinate the actions of motor neurons in the abducens and oculomotor nuclei to generate horizontal movements of the eyes; also called the horizontal gaze center.
parasympathetic nervous system
A division of the visceral motor system in which the effectors are cholinergic ganglion cells located near target organs.
paresis
Partial loss of voluntary motor control; weakness.
parietal lobe
The lobe of the brain that lies between the frontal lobe anteriorly, and the occipital lobe posteriorly.
Parkinson's disease
Neurodegenerative disease of the substantia nigra that results in a characteristic tremor at rest and a general paucity of movement.
parvocellular layer
A component of the primary visual pathway specialized for the detection of detail and color; so named because of the relatively small cells involved.
passive flow
The flow of electrical current across neuronal membranes that does not entail the action potential mechanism.
patch clamp
An extraordinarily sensitive voltage clamp method that permits the measurement of ionic currents flowing through individual ion channels.
periaqueductal gray matter
Region of brainstem gray matter that contains, among others, nuclei associated with the modulation of pain perception.
perilymph
The potassium-poor fluid that bathes the basal end of the cochlear hair cells.
perineurium
The connective tissue that surrounds a nerve fascicle in a peripheral nerve.
peripheral nervous system (PNS)
All nerves and neurons that lie outside the brain and spinal cord.
phasic
Transient firing of action potentials in response to a prolonged stimulus; the opposite of tonic.
phenotype
The visible (or otherwise discernible) characteristics of an animal that arise during development.
phenotypic sex
The visible body characteristics associated with sexual behaviors.
pheromones
Species-specific odorants that play important roles in behavior in some animals, including many mammals.
photopic vision
Vision at high light levels, which is mediated almost entirely by cone cells. Contrast with scotopic vision.
photoreceptors
The specialized neurons in the eye‚ rods and cones‚ that are sensitive to light.
phototransduction
The process by which light is converted in electrical signals in the retina.
pia mater
The innermost of the three layers of the meninges, which is closely applied to the surface of the brain.
pigment epithelium
Pigmented coat underlying the retina important in the normal turnover of photopigment in rods and cones.
pineal gland
Midline neural structure lying on the dorsal surface of the midbrain; important in the control of circadian rhythms (and, incidentally, considered by Descartes to be the seat of the soul).
pinna
A component of the external ear.
pituitary gland
Endocrine structure comprising an anterior lobe made up of many different types of hormone-secreting cells, and a posterior lobe that secretes neuropeptides produced by neurons in the hypothalamus.
pituitary stalk
See infundibulum.
placebo
An inert substance that when administered may, because of the circumstances, have physiological effects (a "placebo effect").
planum temporale
Region on the superior surface of the temporal lobe posterior to Heschl's gyrus; notable because it is larger in the left hemisphere in about two-thirds of humans.
plasticity
Term that refers to structural or functional changes in the nervous system, or the ability to make such changes.
plexus
A complex network of nerves, blood vessels, or lymphatic vessels.
polarity
Referring to a continually graded organization along one of the major axes of an animal.
polymodal
Responding to more than one sensory modality.
polyneuronal innervation
A state in which neurons or muscle fibers receive synaptic inputs from multiple, rather than single, axons.
pons
One of the three components of the brainstem, lying between the midbrain rostrally and the medulla caudally.
pontine-geniculo-occipital (PGO) waves
Characteristic encephalographic waves that signal the onset of rapid eye movement sleep.
pontine nuclei
Collections of neurons in the pons that receive input from the cerebral cortex and send their axons across the midline to the cerebellar cortex via the middle cerebellar peduncle.
pore
Structural feature of an ion channel that allows ions to diffuse through the channel.
pore loop
An extracellular domain of amino acids, found in certain ion channels, that lines the channel pore and allows only certain ions to pass.
postcentral gyrus
The gyrus that lies just posterior to the central sulcus; contains the primary somatic sensory cortex.
posterior
Toward the back; sometimes used as a synonym for caudal or dorsal.
postganglionic axons
Axons that link visceral motor neurons in autonomic ganglia to their targets.
postsynaptic
Referring to the component of a synapse specialized for transmitter reception; downstream at a synapse.
postsynaptic current (PSC)
The current produced in a postsynaptic neuron by the binding of neurotransmitter released from a presynaptic neuron.
postsynaptic density
A cytoskeletal junction in developing synapses that may serve to organize postsynaptic receptors and speed their response to neurotransmitter.
postsynaptic potential (PSP)
The potential change produced in a postsynaptic neuron by the binding of neurotransmitter released from a presynaptic neuron.
post-tetanic potentiation (PTP)
An enhancement of synaptic transmission resulting from high-frequency trains of action potentials.
precentral gyrus
The gyrus that lies just anterior to the central sulcus; contains the primary motor cortex.
prefrontal cortex
Cortical regions in the frontal lobe that are anterior to the primary and association motor cortices; thought to be involved in planning complex cognitive behaviors and in the expression of personality and appropriate social behavior.
preganglionic
Referring to neurons and axons that link visceral motor neurons in the spinal cord and brainstem to autonomic ganglia.
premotor cortex
Motor association areas in the frontal lobe anterior to the primary motor cortex; thought to be involved in planning or programming of voluntary movements.
pre-propeptides
The first protein translation products synthesized in a cell. These polypeptides are usually much larger than the final, mature peptide and often contain signal sequences that target the peptide to the lumen of the endoplasmic reticulum.
presynaptic
Referring to the component of a synapse specialized for transmitter release; upstream at a synapse.
pretectum
A group of nuclei located at the junction of the thalamus and the midbrain; these nuclei are important in the pupillary light reflex, relaying information from the retina to the Edinger–Westphal nucleus.
prevertebral ganglia
Sympathetic ganglia that lie anterior to the spinal column (distinct from the sympathetic chain ganglia).
primary auditory cortex (A1)
The major cortical target of the neurons in the medial geniculate nucleus.
primary motor cortex
A major source of descending projections to motor neurons in the spinal cord and cranial nerve nuclei; located in the precentral gyrus (Brodmann's area 4) and essential for the voluntary control of movement.
primary neuron
A neuron that directly links muscles, glands, and sense organs to the central nervous system.
primary visual cortex (V1)
Brodmann's area 17 in the occipital lobe; major cortical target of the retinal sensory cells. Also called striate cortex because the prominence of layer 4 in myelin-stained sections gives this region a striped (striated) appearance.
primary visual pathway
Pathway from the retina via the lateral geniculate nucleus of the thalamus to the primary visual cortex; carries the information that allows conscious visual perception. Also known as the retinogenticulocortical pathway.
primate
Any mammal in a group that includes lemurs, tarsiers, marmosets, monkeys, apes, and humans.
priming
A change in the processing of a stimulus due to a previous encounter with the same or a related stimulus, with or without conscious awareness of the original encounter.
primitive streak
Axial indentation in the gastrulas of birds and mammals that generates the notochord and defines the embryonic midline. At the thickened anterior end of the streak is an indentation, the primitive pit, which is an important source of neural inductive signals during gastrulation.
procedural memory
See nondeclarative memory.
projection neuron
A neuron with long axons that project to distant targets.
promoter
DNA sequence (usually within 35 nucleotides upstream of the start site of transcription) to which the RNA polymerase and its associated factors bind to initiate transcription.
propeptides
Partially processed forms of proteins containing peptide sequences that play a role in the correct folding of the final protein.
proprioceptors
Sensory receptors (usually limited to mechanosensory receptors) that sense the internal forces acting on the body; muscle spindles and Golgi tendon organs are the preeminent examples.
prosencephalon
The part of the brain that includes the diencephalon and telencephalon (derived from the embryonic forebrain vesicle).
prosody
The normal rhythm, stress, and tonal variation of speech that give it emotional meaning.
prosopagnosia
The inability to recognize faces; usually associated with lesions to the right inferior temporal cortex.
proteoglycan
Molecule consisting of a core protein to which one or more long, linear carbohydrate chains (glycosaminoglycans) are attached.
proximal
Closer to a point of reference (the opposite of distal).
psychotropic
Referring to drugs that alter behavior, mood, and perception.
pulvinar
A thalamic nucleus that receives its major input from sensory and association cortices and projects in turn to association cortices, particularly in the parietal lobe.
pupil
The perforation in the center of the iris that allows light to enter the eye. The pupillary light reflex mediates pupillary constriction in full light and expansion (dilation) in dim light; these responses can also be induced by chemicals and by certain emotional states, and thus can be clinically important.
Purkinje cell
The large principal projection neuron of the cerebellar cortex. Its defining characteristic is an elaborate apical dendrite.
putamen
One of the three major nuclei that make up the basal ganglia (the other two are the caudate and the globus pallidus).
pyramidal tract
White matter tract that lies on the ventral surface of the medulla and contains axons descending from motor cortex to the spinal cord.
pyriform cortex
Component of cerebral cortex in the temporal lobe pertinent to olfaction; so named because of its pearlike shape.

R

radial glia
Glial cells that contact both the luminal and pial surfaces of the neural tube, providing a substrate for neuronal migration.
ramus
Branch; typically applied to the white and gray communicating rami that carry visceral motor axons to the segmental nerves.
Raphe nuclei
A collection of serotonergic nuclei in the brainstem tegmentum important in the governance of sleep and waking.
rapid eye movement (REM) sleep
The phase of sleep characterized by low-voltage, high-frequency electroencephalographic activity accompanied by rapid eye movements.
receptive field
The region of a receptive surface (e.g., the body surface, or a specialized structure such as the retina) within which a specific stimulus elicits the greatest action potential response from sensory cells.
receptor
A protein specialized to bind any one of a large number of chemical signals, including neurotransmitters.
receptor neuron
A neuron specialized for the transduction of energy in the environment into electrical signals.
receptor potential
The membrane potential change elicited in receptor neurons during sensory transduction. Also called generator potential.
red nucleus
A midbrain structure involved in motor coordination, especially in non-human mammals. See also rubrospinal tract.
reflex
A stereotyped (involuntary) motor response elicited by a defined stimulus.
refractory period
The brief period after the generation of an action potential during which a second action potential is difficult or impossible to elicit.
reserpine
An antihypertensive drug that is no longer used due to side effects such as behavioral depression.
resting potential
The inside-negative electrical potential that is normally recorded across all cell membranes.
reticular activating system
Region in the brainstem tegmentum that, when stimulated, causes arousal; involved in modulating sleep and wakefulness.
reticular formation
A network of neurons and axons that occupies the core of the brainstem, giving it a reticulated ("net-like") appearance in myelin-stained material; major functions include control of respiration and heart rate, posture, and state of consciousness.
retina
Laminated neural component of the eye that contains the photoreceptors (rods and cones) and the initial processing machinery for the primary (and other) visual pathways.
retinoic acid (RA)
A derivative of vitamin A that acts as an inducer during early brain development.
retinotectal system
The pathway between ganglion cells in the retina and the optic tectum of vertebrates.
retrograde
Signals or impulses that travel "backward," e.g., from the axon terminal toward the cell body, or from the postsynaptic cell to the presynaptic terminal, or from the periphery to the CNS.
reversal potential
Membrane potential of a postsynaptic neuron (or other target cell) at which the action of a given neurotransmitter causes no net current flow.
rhodopsin
The photopigment found in rods.
rhombencephalon
The part of the brain that includes the pons, cerebellum, and medulla (derived from the embryonic hindbrain vesicle).
rhombomere
Segment of the developing rhombencephalon.
rising phase
The initial, depolarizing, phase of an action potential, caused by the regenerative, voltage-dependent influx of a cation such as Na+ or Ca2+.
rods
Photoreceptor cells specialized for operating at low light levels.
roofplate
A transient structure in the dorsal portion of the neural tube that facilitates differentiation of cells that give rise to sensory relay neurons and related interneurons in the spinal cord and hindbrain (see also floorplate).
rostral
Anterior, or "headward."
rostral interstitial nucleus
Cluster of neurons in the reticular formation that coordinates the actions of neurons in the oculomotor nuclei to generate vertical movements of the eye; also called the vertical gaze center.
rostral migratory stream
A specific migratory route, defined by a distinct subset of glial cells, that facilitates migration of newly generated neurons from the stem cell niche of the anterior subventricular zone to the olfactory bulb.
rubrospinal tract
In non-human mammals, the pathway from the red nucleus of the midbrain to the spinal cord, essential for normal gait and motion. In humans, however, the corticospinal tract serves this function and the rubrospinal tract is vestigial (perhaps even nonexistent).
Ruffini corpuscles
Encapsulated cutaneous mechanosensory receptors specialized for the detection of cutaneous stretching produced by digit or limb movements.

S

saccades
Ballistic, conjugate eye movements that change the point of foveal fixation.
sacculus
The otolith organ that detects linear accelerations and head tilts in the vertical plane.
sagittal
A vertical plane passing from front to rear and dividing the body or brain into right and left sections. If the plane passes through the midline, it is referred to as midsagittal.
saltatory conduction
Mechanism of action potential propagation in myelinated axons; so named because action potentials "jump" from one node of Ranvier to the next due to generation of action potentials only at these sites.
Scarpa's ganglion
See vestibular nerve ganglion.
Schaffer collaterals
The axons of cells in the CA3 region of hippocampus that form synapses in the CA1 region.
Schwann cells
Glial cells in the peripheral nervous system that lay down myelin (named after the nineteenth-century anatomist and physiologist Theodor Schwann).
sclera
The external connective tissue coat of the eyeball.
scotoma
A small deficit in the visual field resulting from pathological changes in some component of the primary visual pathway.
scotopic vision
Vision in dim light, where the rods are the operative receptors.
second-order neurons
Projection neurons in a sensory pathway that lie between the primary receptor neurons and the third-order neurons.
segment
One of a series of more or less similar anterior-posterior units that make up segmental animals.
segmentation
The anterior–posterior division of animals into roughly similar repeating units.
semaphorins
A family of diffusible, growth-inhibiting molecules. See also collapsin.
semicircular canals
The vestibular end organs in the inner ear that sense rotational accelerations of the head.
sensitization
Increased sensitivity to stimuli in an area surrounding an injury. Also, a generalized aversive response to an otherwise benign stimulus when it is paired with a noxious stimulus.
sensorineural hearing loss
Diminished sense of hearing due to damage of the inner ear or its related central auditory structures. Contrast with conductive hearing loss.
sensory system
Term sometimes used to describe all the components of the central and peripheral nervous system concerned with sensation.
sensory transduction
Process by which the energy of a stimulus is converted into electrical signals by peripheral sensory receptors and then processed by the central nervous system.
serotonin
A biogenic amine neurotransmitter derived from the amino acid tryptophan.
sexually dimorphic
Having two different forms depending on genotypic or phenotypic sex.
short-term memory
See working memory.
silver stain
A classical method for visualizing neurons and their processes by impregnation with silver salts (the best-known technique is the Golgi stain, developed by the Italian anatomist Camillo Golgi in the late nineteenth century).
size principle
The orderly recruitment of motor neurons by size to generate increasing amounts of muscle tension.
sleep spindles
Bursts of electroencephalographic activity, at a frequency about 10–14 Hz and lasting a few seconds; spindles characterize the initial descent into non-REM sleep.
small-molecule neurotransmitters
The non-peptide neurotransmitters such as acetylcholine, the amino acids glutamate, aspartate, GABA, and glycine, as well as the biogenic amines.
smooth pursuit movements
Slow, tracking movements of the eyes designed to keep a moving object aligned with the fovea.
soma
The cell body.
somatic cells
Referring to the cells of an organism's body other than its germ cells.
somatosensory cortex
The region of the cerebral cortex concerned with processing sensory information from the body surface, subcutaneous tissues, muscles, and joints. Located primarily in the posterior bank of the central sulcus and on the postcentral gyrus.
somatic sensory system
Components of the nervous system involved in processing sensory information about the mechanical forces active on both the body surface and on deeper structures such as muscles and joints.
somatic stem cell
A cell that can divide to give rise to more cells like itself, but also can divide to give rise to a new stem cell plus one or more differentiated cells of the relevant tissue type (e.g., a hematopoeitic stem cell can give rise to all types of blood cells; neural stem cells give rise to all neuronal types, and glial stem cells to glia. Contrast with embryonic stem cell.
somatotopic maps
Cortical or subcortical arrangements of sensory pathways that reflect the organization of the body.
somites
Segmentally arranged masses of mesoderm that lie alongside the neural tube and give rise to skeletal muscle, vertebrae, and dermis.
Sonic hedgehog (Shh)
An inductive signaling hormone essential for development of the mammalian nervous system; believed to be particularly important for establishing the identity of neurons in the ventral portion of the developing spinal cord and hindbrain.
specificity
Term applied to neural connections that entail specific choices between neurons and their targets.
spike timing-dependent plasticity (STDP)
Timing-dependent activity, probably the result of Ca2+ signaling in the postsynaptic cell, that is required for the establishment of some forms of synaptic plasticity.
spina bifida
A congenital defect in which the neural tube fails to close at its posterior end.
spinal cord
The portion of the central nervous system that extends from the lower end of the brainstem (the medulla) to the cauda equina.
spinal ganglia
See dorsal root ganglia.
spinal nucleus of the bulbocavernosus (SNB)
Sexually dimorphic collection of neurons in the lumbar region of the rodent spinal cord that innervate striated perineal muscles.
spinal shock
The initial flaccid paralysis that accompanies damage to descending motor pathways.
spinal trigeminal tract
Brainstem tract carrying fibers from the trigeminal nerve to the spinal nucleus of the trigeminal complex (which serves as the relay for painful stimulation of the face).
spinocerebellum
Region of the cerebellar cortex that receives input from the spinal cord, particularly Clarke's nucleus in the thoracic spinal cord.
spinothalamic pathway
See anterolateral pathway.
spinothalamic tract
Ascending white matter tract carrying information about pain and temperature from the spinal cord to the VP nuclear complex in the thalamus; also referred to as the anterolateral tract.
split-brain patients
Individuals who have had the cerebral commissures divided in the midline to control epileptic seizures.
sporadic
Cases of a disease that apparently occur at random in a population; contrasts with familial or inherited.
SRY
Gene on the Y chromosome whose expression triggers a hormone cascade that masculinizes the developing fetus.
stem cell
See embryonic stem (ES) cells, glial stem cells; neural stem cells; somatic stem cell.
stem cell niche
A local environment in regenerating tissue that is conducive to the division and initial differentiation of the somatic stem cells that will reconstitute adult tissue.
stereocilia
The actin-rich processes that, along with the kinocilium, form the hair bundle extending from the apical surface of the hair cell; site of mechanotransduction.
stereopsis
The perception of depth that results from the fact that the two eyes view the world from slightly different angles.
strabismus
Developmental misalignment of the two eyes; may lead to binocular vision being compromised.
stretch reflex
The monosynaptic reflex arc that occurs involuntarily when a muscle is stretched.
stria vascularis
Specialized epithelium lining the cochlear duct that maintains the high potassium concentration of the endolymph.
striate cortex
See primary visual cortex.
striatum
See corpus striatum.
striola
A line found in both the sacculus and utricle that divides the hair cells into two populations with opposing hair bundle polarities.
subarachnoid space
The cerebrospinal fluid-filled space over the surface of the brain that lies between the arachnoid and the pia.
substance P
An 11-amino acid neuropeptide; the first neuropeptide to be discovered.
substantia nigra
Basal ganglionic nucleus at the base of the midbrain that receives input from several cortical and subcortical structures. The cells of the substantia nigra pars compacta send their output to the caudate/putamen, while the cells of the substantia nigra pars reticulata send their output to the thalamus.
subthalamic nucleus
A nucleus in the ventral thalamus that receives input from the caudate/putamen and participates in the modulation of motor behavior.
subventricular zones
Cell-dense regions adjacent to the ventricular spaces of cortical hemispheres.
sulcus (pl. sulci)
Infoldings of the cerebral hemisphere that form the valleys between the gyral ridges; see also gyrus.
summation
The addition in space and time of sequential synaptic potentials to generate a larger than normal postsynaptic response.
superior colliculus
Laminated structure that forms part of the roof of the midbrain; plays an important role in orienting movements of the head and eyes.
suprachiasmatic nucleus (SCN)
Hypothalamic nucleus lying just above the optic chiasm that receives direct input from the retina; involved in light entrainment of circadian rhythms.
Sylvian fissure
See lateral fissure.
sympathetic nervous system
A division of the visceral motor system in vertebrates comprising, for the most part, adrenergic ganglion cells located relatively far from the related end organs.
synapse
Specialized apposition between a neuron and its target cell for transmission of information by release and reception of a chemical neurotransmitter or, less commonly, by an electrical signal.
synapse elimination
The developmental process by which the number of axons innervating some classes of target cells is diminished. Also called input elimination.
synaptic cleft
The space that separates pre- and postsynaptic neurons at chemical synapses.
synaptic depression
A short-term decrease in synaptic strength resulting from the depletion of synaptic vesicles at active synapses.
synaptic facilitation
A rapid increase in synaptic strength that occurs when two or more action potentials invade the presynaptic terminal within a few milliseconds of each other.
synaptic plasticity
Changes in the efficacy and local geometry of synaptic connections and transmission; one basis of learning, memory, and other forms of brain plasticity.
synaptic transmission
The chemical and electrical process by which the information encoded by action potentials is passed from a presynaptic (initiating) cell to a postsynaptic (target) cell.
synaptic vesicles
Spherical, membrane-bound organelles in presynaptic terminals that store neurotransmitter molecules.
synaptic vesicle cycling
Sequence of budding and fusion reactions that occurs in presynaptic terminals to maintain the supply of synaptic vesicles.
syncytium
A group of cells in protoplasmic continuity; a multinucleate cellular mass.

T

target
The object of innervation, which can be either non-neuronal targets, such as muscles, glands, and sense organs, or other neurons.
taste buds
Onion-shaped structures in the mouth and pharynx that contain taste cells.
tectorial membrane
The fibrous sheet overlying the apical surface of the cochlear hair cells; produces a shearing motion of the stereocilia when the basilar membrane is displaced.
tectum
A general term referring to the dorsal region of the brainstem (tectum means "roof").
tegmentum
A general term that refers to the central gray matter of the brainstem.
telencephalon
The part of the brain derived from the anterior part of the embryonic forebrain vesicle; includes the cerebral hemispheres (cerebrum).
temporal division
Referring to the region of the visual field of each eye in the direction of the temple.
temporal lobe
The hemispheric lobe that lies inferior to the lateral fissure.
terminal
A presynaptic (axonal) ending.
testicular feminization
See androgen insensitivity syndrome (AIS).
tetraethylammonium
A quaternary ammonium compound that selectively blocks voltage-sensitive K+ channels; eliminates the delayed K+ current measured in voltage clamp experiments.
tetrodotoxin (TTX)
An alkaloid neurotoxin, produced by certain puffer fish, tropical frogs, and salamanders, that selectively blocks voltage-sensitive Na+ channels; eliminates the initial Na+ current measured in voltage clamp experiments.
thalamus
A collection of nuclei that forms the major component of the diencephalon. Although its functions are many, a primary role of the thalamus is to relay sensory information from lower centers to the cerebral cortex.
thermoreceptors
Receptors specialized to transduce changes in temperature.
threshold potential
The level of membrane potential at which an action potential is generated.
tight junction
A specialized junction between epithelial cells that seals them together, preventing most molecules from passing across the cell sheet.
tip links
The filamentous structures that link the tips of adjacent stereocilia; thought to mediate the gating of the hair cell's transduction channels.
tonic
Sustained activity in response to an ongoing stimulus; the opposite of phasic.
tonotopy
The topographic mapping of sound frequency across the surface of a structure, which originates in the cochlea and is preserved in ascending auditory structures, including the auditory cortex.
transcription factors
A general term applied to proteins that regulate transcription, including basal transcription factors that interact with the RNA polymerase to initiate transcription, as well as those that bind elsewhere to stimulate or repress transcription.
transducin
G-protein involved in the phototransduction cascade.
transduction
See sensory transduction.
transforming growth factor (TGF)
A class of peptide growth factors that acts as an inducer during early development.
transit amplifying cell
An intermediate class of neural stem cell. Their cell cycle is much faster than that of the stem cells, and they divide asymmetrically to produce one postmitotic neuroblast or glioblast, plus another transit amplifying cell that reenters the cell cycle.
transmitter
See neurotransmitter.
trichromatic
Referring to the presence of three different cone types in the human retina, which generate the initial steps in color vision by differentially absorbing long, medium, and short wavelength light.
tricyclic antidepressants
A class of antidepressant drugs named for their three-ringed molecular structure; thought to act by blocking the reuptake of biogenic amines.
trigeminal nerve
Cranial nerve V, responsible for sensation in the face. Also has certain motor functions in chewing and swallowing.
Trk receptors
Tyrosine kinase receptors; one of two classes of receptors for the neurotrophin family of growth factors (the other is the p75 receptor). Important in the regulation of synaptic strength and plasticity.
trochlear nerve
Cranial nerve IV, an efferent motor nerve that controls the superior oblique muscle of the eye.
trophic
The ability of one tissue or cell to support another; usually applied to long-term interactions between pre- and postsynaptic cells.
trophic factors
Any molecule that mediates trophic interactions.
trophic interactions
Referring to the long-term interdependence between nerve cells and their targets.
tropic
An influence of one cell or tissue on the direction of movement (or outgrowth) of another.
tropic factors
Molecules that influence the direction of growth or movement.
tropism
Orientation of growth in response to an external stimulus.
tuning curve
A threshold function determined by a common physiological test in which the receptive field properties of neurons are gauged against a varying stimulus such that maximum sensitivity or maximum responsiveness can be defined by the peak of the tuning curve.
tympanic membrane
The eardrum.

U

undershoot
The final, hyperpolarizing phase of an action potential, typically caused by the voltage-dependent efflux of a cation such as K+.
upper motor neuron
A neuron that gives rise to a descending projection that controls the activity of lower motor neurons in the brainstem and spinal cord.
upper motor neuron syndrome
Signs and symptoms that result from damage to descending motor pathways; these include paralysis, spasticity, and a positive Babinski sign.
utricle
The otolith organ that senses linear accelerations and head tilts in the horizontal plane.

V

vasopressin
A 9-amino-acid neuropeptide that acts as a neurotransmitter, as well as a neurohormone.
ventral
Referring to the belly; the opposite of dorsal.
ventral horn
The ventral portion of the spinal cord gray matter; contains the primary motor neurons.
ventral posterior complex
Group of thalamic nuclei that receive the somatic sensory projections from the dorsal column nuclei and the trigeminal nuclear complex.
ventral posterior lateral nucleus
Component of the ventral posterior complex of thalamic nuclei that receives brainstem projections carrying somatic sensory information from the body (excluding the face).
ventral posterior medial nucleus
Component of the ventral posterior complex of thalamic nuclei that receives brainstem projections carrying somatic sensory information from the face.
ventral roots
The collection of nerve fibers containing motor axons that exit ventrally from the spinal cord and contribute the motor component of each segmental spinal nerve.
ventricles
The fluid-filled spaces in the vertebrate brain that represent the lumen of the embryonic neural tube.
ventricular zone
The sheet of cells closest to the ventricles in the developing neural tube.
vergence movements
Disjunctive movements of the eyes (convergence or divergence) that align the fovea of each eye with targets located at different distances from the observer.
vesicle
Literally, a small sac. Used to refer to the organelles that store and release transmitter at nerve endings. Also used to refer to any of the three dilations of the anterior end of the neural tube that give rise to the three major subdivisions of the brain.
vestibulocerebellum
The part of the cerebellar cortex that receives direct input from the vestibular nuclei or vestibular nerve.
vestibular nerve ganglion
Contains the bipolar afferent neurons that innervate the semicircular canals and otolith organs of the auditory vestibule. Also called Scarpa's ganglion.
vestibulo-ocular reflex (VOR)
Involuntary movement of the eyes in response to displacement of the head. This reflex allows retinal images to remain stable while the head is moved.
visceral
Referring to the internal organs of the body cavity, particularly the gut.
visceral nervous system
The components of the peripheral and central nervous systems concerned with the regulation of smooth muscle, cardiac muscle, and glands Synonymous with autonomic nervous system; sometimes called the "involuntary" nervous system. Consists of sympathetic and parasympathetic divisions.
visual field
The area in the external world normally seen by one or both eyes (referred to, respectively, as the monocular and binocular fields).
vital dye
A reagent that stains living cells.
voltage clamp
A technique that uses electronic feedback to simultaneously control the membrane potential of a cell and measure the transmembrane currents that result from the opening and closing of ion channels.
voltage-gated
Term used to describe ion channels whose opening and closing is sensitive to membrane potential.

W

Wallerian degeneration
The process by which the distal portion of a damaged axon segment degenerates; named after Augustus Waller, a nineteenth-century physician and neuroanatomist.
Wernicke's aphasia
Difficulty comprehending speech as a result of damage to Wernicke's language area. Also called sensory or receptive aphasia.
Wernicke's area
Region of cortex in the superior and posterior region of the left temporal lobe that helps mediate language comprehension. Named after the nineteenth-century neurologist Carl Wernicke.
white matter
A general term that refers to large axon tracts in the brain and spinal cord; the phrase derives from the fact that axonal tracts have a whitish cast when viewed in the freshly cut material.
working memory
Memories held briefly in mind that enable a particular task to be accomplished (e.g., efficiently searching a room for a lost object). Also called short-term memory.
Go