Neurobiology - Glossary A-E, Blackwell Science

NEUROBIOLOGY Molecules, Cells and Systems
Gary G. Matthews

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Glossary

A band: A darker band at the middle of a sarcomere of a striated muscle fiber, corresponding to the length of the thick filaments.
absolute refractory period: The period of time after an action potential during which a second action potential cannot be triggered. This period corresponds to the time when sodium channel inactivation gates remain closed after repolarization.
acetylcholine (ACh): A neurotransmitter in both the central and peripheral nervous system, formed by the combination of acetate and choline, a reaction catalyzed by the synthetic enzyme choline acetyltransferase.
acetylcholinesterase: The degradative enzyme for acetylcholine, which cleaves the ester bond and releases acetate and choline.
ACh receptor: The receptor molecule that detects the presence of acetylcholine in the extracellular space and initiates the postsynaptic response to the neurotransmitter. See muscarinic ACh receptor and nicotinic ACh receptor.
actin: A motor protein. Actin forms the backbone of the thin filaments of striated muscle cells and is also involved in other cellular motions, such as the movement of growth cones.
action potential: The long-distance signal of the nervous system. Action potentials are triggered in excitable cells in response to depolarization that exceeds the threshold potential.
active zone: The subregion of a presynaptic terminal where synaptic vesicles fuse with the plasma membrane, releasing their neurotransmitter content into the synaptic cleft.
adenohypophysis: The anterior portion of the pituitary gland, containing secretory cells that release 7 different hormones into the blood stream, under control of release factors secreted by hypothalamic neurons.
adenylyl cyclase: The synthetic enzyme that converts ATP into cyclic AMP.
ADP - Adenosine diphosphate: ADP results from hydrolysis of ATP by ATPase or kinase enzymes, with the concomitant release of the terminal phosphate group of ATP.
adrenaline: An alternative term for epinephrine.
afferent pathway: A pathway carrying incoming sensory information into the nervous system. afterhyperpolarization. A period after an action potential during which the membrane potential is more negative than usual.
agnosia: A collective term for the inability to identify objects based on sensory information, even though the primary sensory cortex is intact.
alpha motor neurons: Large motor neurons that innervate the extrafusal muscle fibers of skeletal muscles.
Alzheimer's disease: The most common type of age-related, progressive dementia, produced by death of neurons in a variety of brain regions. The disease is characterized by two pathological features in the brain: neuritic plaques and neurofibrillary tangles.
amacrine cell: A neuron type found in the retina. Amacrine cells are lateral interneurons that receive input from bipolar cells and make synapses onto the synaptic terminals of bipolar cells and onto the dendrites of ganglion cells.
AMPA receptor: A subtype of glutamate receptor. AMPA receptors are ligand-gated ion channels that open when glutamate binds and allow cations to enter the postsynaptic cell.
angiotensin: A neuroactive hormone involved in the regulation of blood osmolarity and water balance, and in drinking behavior.
angular gyrus: A gyrus located at the border between the temporal, parietal, and occipital lobes of the cerebral cortex. Lesions in the angular gyrus of the language-dominant hemisphere interfere with reading and writing.
anion: A negatively charged ion.
anterolateral system: The sensory projection system carrying pain and temperature information through the lateral sensory tract of the spinal cord to the brainstem, thalamus, and somatosensory cortex.
aphagia: Absence of feeding.
aphasia: A disorder of understanding or production of spoken language.
apraxia: A deficit in programming complex series of movements, without paralysis, sensory defects, or difficulty performing simple movements.
archicortex: A simple form of cortex having only a single layer of cells, as opposed to the multiple cell layers found in the neocortex. The hippocampus is an example of archicortex.
arcuate fasciculus: A fiber tract in the cortical white matter containing axons that interconnect Wernicke's and Broca's areas. Damage in this fiber tract produces a form of aphasia called conduction aphasia.
area MT: A portion of the middle temporal cortex (MT) involved in the detection of visual motion. Also called area V5.
area V1: Another name for the primary visual cortex, or striate cortex, located in the occipital lobe.
ATP - Adenosine triphosphate: The high-energy phosphate compound that is the primary source of energy for a variety of energy-requiring cellular processes. ATP also provides the donor phosphate group for phosphorylation of protein molecules by kinase enzymes. The second messenger, cyclic AMP, is synthesized from ATP by the enzyme adenylyl cyclase.
atria (singular: atrium): The upper chambers of the mammalian heart that receive blood returning from the venous circulation (right atrium) or from the lungs (left atrium).
atrioventricular (AV) node: A specialized group of muscle cells located near the center of the heart, between the atria and ventricles. Action potentials spread from the atria to the ventricles through the cells of the AV node, which have a slow conduction speed. Contraction of the ventricles is therefore delayed with respect to the contraction of the atria.
autonomic nervous system: One of the major subdivisions of the nervous system, containing neurons that provide involuntary control of heart muscle, smooth muscle, and glands. The autonomic nervous system consists of the sympathetic and parasympathetic divisions.
axon: The neurite that carries action potentials leaving the cell body of a neuron and distributes the signal to postsynaptic target cells.

basal ganglia: An interconnected group of three forebrain nuclei involved in motor control. The three nuclei are the caudate nucleus, the putamen, and the globus pallidus.
basilar membrane: A membrane within the cochlea that vibrates in response to sound stimuli. The sensory structure of the ear (the organ of Corti) rides on top of the basilar membrane.
beta-adrenergic receptor: A G protein-coupled receptor activated by the neurotransmitter norepinephrine. For example, the sympathetic input to the heart is mediated by beta-adrenergic receptors.
beta-amyloid protein (A?): A peptide fragment that forms the principal component of neuritic plaques in the brains of patients with Alzheimer's disease. Beta-amyloid protein is produced by proteolysis of a normal neuronal membrane protein, amyloid precursor protein.
bipolar cell: A neuron type found in the retina. Bipolar cells are second-order neurons that receive synaptic input from photoreceptors and make synapses on amacrine cells and ganglion cells.
blobs: Groups of neurons within the primary visual cortex, containing color-sensitive cells. The cells within the blobs have high numbers of mitochondria, which are revealed by staining for the mitochondrial enzyme, cytochrome oxidase.
brainstem: A collective term for the midbrain (mesencephalon) and hindbrain (rhombencephalon).
Broca's area: Region of the frontal lobe of the dominant cortical hemisphere for language, where damage produces deficits in production of speech (expressive aphasia). Broca's area is located just anterior to the portion of the primary motor cortex that controls the muscles of the mouth and tongue.

calcium-induced calcium release channel: The ion channel of the sarcoplasmic reticulum that allows calcium ions to flow out of the sarcoplasmic reticulum to trigger contraction of skeletal muscle cells.
cation: A positively charged ion.
caudate nucleus: One of the basal ganglia of the forebrain. Together with the putamen, the caudate nucleus is part of the striatum.
cellular adhesion molecules: Membrane proteins on the external surface of cells that allow cells to adhere to one another or to the extracellular matrix.
center-surround receptive field: A receptive field shape in which a circular central region and a ring-shaped surrounding region have opposing effects on the cell's activity.
central sulcus: The large infolding, or groove, in the dorsal surface of the cerebral cortex, approximately midway between the anterior and posterior poles of the brain.
cerebellum: A major part of the brainstem in mammalian brains, involved in integration of sensory information and motor commands.
cerebral cortex: The outer surface of the cerebrum, containing most of the neurons in the cerebrum.
cerebral ventricles: The fluid-filled canal at the center of the brain.
cerebrospinal fluid (CSF): The fluid filling the cerebral ventricles and the spinal canal.
cerebrum: A major subdivision of the forebrain. Together, the cerebrum and basal ganglia make up the telencephalon.
C-fibers: Small-diameter, unmyelinated axons of peripheral nerves. C-fibers carry pain and temperature information.
chemoreceptors: Sensory neurons that detect chemical sensory stimuli.
chemotropic molecule: Diffusible extracellular signal molecule that can guide the movements of nearby growth cones. Chemotropic molecules may either repel or attract growth cones.
choline acetyltransferase: The enzyme that synthesizes the neurotransmitter acetylcholine.
cholinergic neuron: A neuron that uses the neurotransmitter acetylcholine.
chromaffin cell: A secretory cell of the adrenal gland that releases norepinephrine and epinephrine into the blood stream.
cingulate gyrus: Part of the limbic system, this outfolding of the cerebral cortex runs front to back, hidden from view within the interhemispheric fissure that separates the two cerebral hemispheres.
cingulum: An axon bundle that contains axons projecting from the cingulate gyrus to the entorhinal cortex in the limbic system.
circadian rhythm: A cyclical change in a physiological or biochemical process having a period of approximately 24 hours.
cochlea: Part of the inner ear that contains the sensory hair cells of the sense of hearing.
cochlear nucleus: A nucleus in the brainstem that receives synaptic input from the spiral ganglion neurons of the cochlea.
collagen: A structural protein that forms the backbone of the extracellular matrix in many tissues.
cones: One of the two major classes of photoreceptors, the other being rods. Cones are less sensitive to light than rods and mediate color vision.
corpus callosum: The large fiber bundle containing axons interconnecting neurons of the two cerebral hemispheres.
corticobulbar system: The motor pathway carrying descending motor commands from the motor cortex to the brainstem.
corticospinal system: The motor pathway carrying axons from the motor cortex to the spinal cord, bypassing the brainstem motor centers.
cranial nerves: Nerves that directly connect the brain to peripheral targets. The human nervous system includes twelve distinct cranial nerves.
cross bridge: A structure visible through the electron microscope, connecting adjacent thick and thin filaments of a myofibril. The cross bridges are formed by myosin molecules that make up the thick filament.
cuneate nucleus: One of the dorsal column nuclei in the brainstem. The cuneate nucleus receives somatosensory inputs from the lateral portions of the dorsal columns of the spinal cord.
cyclic AMP: An intracellular second messenger synthesized by the enzyme adenylyl cyclase from ATP.
cyclic GMP: An intracellular second messenger synthesized by the enzyme guanylyl cyclase from GTP.

decussation: A cross over of axons from one side of the brain to the other.
dendrite: A neurite that receives synaptic inputs from other neurons.
dendritic spine: A short, hair-like projection from dendrite. Excitatory synapses commonly contact dendrites of CNS neurons at dendritic spines.
dentate nucleus: One of the deep nuclei of the cerebellum, in the brainstem.
depolarization: Movement of the membrane potential in the positive direction, from its normal negative level.
diacylglycerol: An intracellular second messenger, formed by the enzyme phospholipase C from membrane lipid.
diencephalon: A subdivision of the forebrain, comprising the thalamus and hypothalamus.
dorsal column nuclei: Nuclei in the brainstem that receive synaptic inputs from somatosensory neurons whose axons are located in the dorsal columns. The dorsal column nuclei are the gracile nucleus and the cuneate nucleus.
dorsal column: A fiber bundle along the dorsal surface of the spinal cord, primarily consisting of branches of primary somatosensory neurons ascending from the spinal cord to the brainstem.
dorsal root ganglion: A ganglion located just outside the spinal cord at each vertebral segment, containing the cell bodies of sensory neurons whose axons enter the spinal cord at that segment.
dorsal root: The fiber bundle containing incoming (afferent) sensory axons entering the spinal cord at each vertebral segment.

efferent pathway: A pathway carrying outgoing motor information from neurons.
emboliform nucleus: One of the deep nuclei of the cerebellum, in the brainstem.
end-plate potential: The postsynaptic electrical response elicited in a skeletal muscle cell by activation of its motor neuron.
end-plate: The synaptic zone of a skeletal muscle cell, where postsynaptic ACh receptors are clustered at high density across from the synaptic terminal of the motor neuron.
entorhinal cortex: A part of the olfactory system and the limbic system. The entorhinal cortex provides input to the hippocampus, and receives synaptic connections from the cingulate gyrus. The entorhinal cortex is an example of paleocortex, having only two cellular layers.
epinephrine: A hormone secreted by adrenal chromaffin cells into the bloodstream. Also used as a neurotransmitter.
equilibrium potential: The value of membrane potential at which a permeant ion is at equilibrium. The equilibrium potential is calculated from the Nernst equation.
excitation-contraction coupling: The process through which an action potential triggers contraction of a muscle cell.
excitatory postsynaptic potential (e.p.s.p.): A postsynaptic change in membrane potential that promotes firing of an action potential in the postsynaptic cell, by bringing the membrane potential toward the threshold potential.
exteroceptors: Sensory receptors that detect stimuli originating outside the organism. For example, photoreceptors are exteroceptors.
extracellular matrix: An external lattice of proteins and polysaccharides that is secreted by surrounding cells in most tissues.
extrafusal muscle fibers: The muscle cells of a skeletal muscle that generate the contractile force of the muscle and are located outside of muscle spindles.
extraocular muscles: The skeletal muscles that control the movements of the eyes.

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