A-l region Brodmann’s areas 41 and 42 that receive projections from the medial geniculate nucleus (geniculotemporal fibers or auditory radiations).
abducens nucleus (nucleus of cranial nerve VI) and nerve General somatic efferent nucleus of the lower pons whose axons innervate the lateral rectus muscle.
absence seizures Brief seizures (approximately 3-10 seconds long) in which the patient is unresponsive and commonly described as "daydreaming"; characterized by a lack of postictal period and electroencephalogram findings of a 3-per-second spike and wave complex.
absolute refractory period Time at the end of the falling phase of the action potential during which voltage-gated sodium channels are in inactivated state and an action potential cannot be generated.
abulia Lack of motor activity.
acalculia Inability to perform calculations; symptom of Gerst-mann syndrome.
accessory cuneate nucleus Relay nucleus situated immediately lateral to the cuneate nucleus; mediates signals from muscle spindles and Golgi tendon organs from the ipsilateral upper limb to the cerebellum
accommodation The change in refractive power of the lens of the eye.
accommodation reflex (reaction) Reflex response associated with both somatomotor and parasympathetic components of cranial nerve III, resulting in constriction of the pupils, bulging of the lens (to allow for focusing at a near object), and medial deviation of the eyes, the overall effect of which is to allow an individual to correctly focus on an object when it is moved from a distant to a near position, acetylcholine (Ach) A small molecule neurotransmitter synthesized from choline in the presence of choline acetyltrans-ferase. Ach serves as a neurotransmitter at the terminals of preganglionic neurons in the sympathetic as well as parasympathetic ganglia, at the terminals of parasympathetic postganglionic neurons, and at the neuromuscular junction. In the ganglia, its release results in the stimulation of postganglionic neurons; at the terminals of parasympathetic nerve terminals, it causes parasympathetic effects (e.g, decrease in heart rate, peristalsis in the gastrointestinal tract); and at the neuromuscular junction, its release results in contraction of the skeletal muscle, acetylcholinesterase An enzyme, which is present at the synapses where acetylcholine (Ach) is released as a neurotransmitter, that inactivates released Ach.
acoustic agnosia Inability to recognize the sounds of animals, speech of individuals, and mechanical sounds, such as horns and bells.
action potential Brief all-or-nothing reversal in membrane potential that is brought about by rapid changes in membrane permeability of Na+ (sodium) and K+ (potassium). During the rising phase of the action potential, there is a rapid depolarization of the membrane due to increased permeability of Na+; the part of the action potential where the inside of the neuron is positive relative to the outside is called the overshoot. During the falling phase of the action potential, the neuron is repolarized by the opening of voltage-gated K+ channels, which allow increased efflux of K+. Hyperpolarization at the end of the falling phase is called after-hyperpolarization or undershoot. The period during which the voltage-gated Na+ channels are in an inactivated state and an action potential cannot be generated is called the absolute refractory period. The relative refractory period follows immediately after the absolute refractory period; during this period, an action potential can be generated, but more depolarizing current is needed to shift the membrane potential to threshold level. See also axon hillock. active transport The passage of ions or molecules across a cell membrane that is mediated by specific carrier proteins and requires coupling of the carrier protein to a source of metabolic energy (e.g., hydrolysis of adenosine triphosphate), active zone Membrane associated with a pyramid-like structure consisting of proteins arising from the intracellular side of the presynaptic terminal membrane and projecting into the cytoplasm of the presynaptic terminal; possesses specialized release sites in the presynaptic terminal. Vesicles containing the neurotransmitter are aggregated near the active zones.
adaptation Diminshing of the intensity of a sensation in the somatic nervous system when the stimulus is continuous for an extended period of time.
adenosine A purinergic neurotransmitter (a neurotransmitter containing a purine ring).
adenosine receptors Metabotropic receptors for adenosine, with at least three subtypes (A1-A3).
Adie’s pupil Disorder in which there is a prolonged and sluggish constriction of the pupil to light; after pupillary constriction, dilatation of the pupil is delayed. Patients with this disorder have pathological changes in the ciliary ganglion, adrenal medulla Site of conversion of amino acid (tyrosine) into adrenalin, norepinephrine, and dopamine; composed of chrommaffin cells.
adrenergic receptors Divided into two major classes: a- and ^-adrenergic receptors; receptors for norepinephrine and epinephrine.
adrenocorticotropic hormone (ACTH) Controls the release of steroid hormones, such as cortisol, in the adrenal cortex; controlled by corticotropin-releasing hormone.
after-hyperpolarization See action potential.
ageusia Loss of taste sensation; total ageusia means loss of all taste sensation, partial ageusia refers to loss of a particular taste sensation, and hypogeusia means decreased sensation of taste.
agnosia Difficulty or inability to recognize objects.
agraphia Inability to write; symptom of Gerstmann syndrome.
agrin A protein that is transported down the axon during development where it binds to the postsynaptic receptor and, in particular, the neuromuscular junction.
agoraphobia Fear of leaving a familiar place.
akinesia Lack of movement occurring in diseases of the basal ganglia.
alar plate Region of the developing nervous system situated dorsal to the sulcus limitans that will become sensory in function.
alexia Reading deficiency; symptom of Gerstmann syndrome.
allosteric Capable of influencing the activity of an enzyme or the conformation of a protein.
alpha carbon Carbon atom that is covalently bonded to a hydrogen atom in an amino acid.
alpha motor neuron Large motor neurons of the brainstem and spinal cord that innervate extrafusal muscle fibers of skeletal muscles and initiate their contraction. See for comparison gamma motor neurons.
alpha rhythm electroencephalogram (EEG) Rhythm of 6-12 Hz associated with quiet and sometimes drowsy states.
alpha-adrenergic receptors One of two classes of G protein-coupled receptors that are catecholamine targets; further subdivided into alpha1-adrenergic receptors (located on the membranes of postsynaptic cells) and alpha2-adrenergic receptors (present on the presynaptic membranes of adrenergic nerve terminals; responsible for autoinhibition).
alveus (of hippocampus) Fiber layer of the hippocampus adjacent to the inferior horn of the lateral ventricle.
Alzheimer’s disease Disorder of severe memory loss in aging individuals correlated with the presence of neurofibrillary tangles, extracellular deposition of the abnormal amyloid protein, and P-amyloid plaques in the cerebral cortex, coupled with cell loss in the basal nucleus of Meynert as well as reduced cholinergic content of cortical tissue.
amnesia Pathological form of forgetting, occurring most often as a result of head injury or a cerebrovascular accident, most typically involving the temporal lobe, particularly the hippocampus, or medial aspects of the thalamus. AMPA a-amino-3-hydroxy-5-methyl-4-isoxazole-propionate;an agonist for AMPA/quisqualate receptor.
AMPA/quisqualate receptor A non-N-methyl-D-aspartic acid ionotropic glutamate receptor.
amphiphilic Substances that have a hydrophilic and a hydro-phobic end.
ampulla Dilation of the end of each semicircular canal of the ear.
amygdala Deep cortical structure situated in the medial temporal lobe just rostral to the hippocampal formation that is associated with regulation of emotional behavior and other hypothalamic functions. Also called amygdaloid complex.
amyotrophic lateral sclerosis (ALS) Also known as Lou Gehrig’s disease, this condition is characterized by degeneration of the motor neurons in the anterior horn of the spinal cord, brain-stem, and cerebral cortex. Symptoms include widespread muscle fasciculations, flaccid weakness, and atrophy of the muscles.
anencephaly Partial or complete absence of the brain with associated defects of the cranial vault and scalp occurring during fetal development.
angiography (arteriography) Technique involving introduction of a specially designed catheter into a femoral artery containing radiopaque contrast material (e.g., an iodinated dye), and x-ray pictures are then taken in rapid sequence as the contrast material flows through the artery (arterial phase), capillaries (capillary phase), and veins (venous phase).
angiotensin II Peptide localized within the supraoptic and para-ventricular hypothalamic neurons; plays an important role in the regulation of blood pressure and drinking behavior.
angular artery A branch of the middle cerebral artery that supplies the angular gyrus.
angular gyrus Situated in the inferior parietal lobule immediately posterior to the supramarginal gyrus and often associated with the posterior extent of the superior temporal sulcus.
anhidrosis Absence of sweating.
anions Negatively charged ions.
annulospiral endings Primary afferents arising from intrafusal fibers; located on the central part of the nuclear bag and nuclear chain fibers.
anode Positive terminal of a battery; negatively charged ions move toward an anode.
anosmia Loss of olfactory function; may result from damage to the olfactory mucosa caused by infections.
ansa lenticularis Major efferent pathway of the basal ganglia; exits from the ventral aspect of the medial pallidal segment and supplies ventrolateral and ventral anterior thalamic nuclei.
anterior Above the midbrain, anterior means "toward the front of the brain." Below the midbrain, anterior means "toward the ventral surface of the body."
anterior cerebral artery Supplies blood to the medial aspect of the cerebral hemisphere, including parts of the frontal and parietal lobes, and the postcentral and precentral gyri.
anterior cerebral artery syndrome Interruption of blood flow in the trunk of one of the anterior cerebral arteries damages the precentral gyrus and results in contralateral paralysis (contralateral hemiplegia), mainly of the leg (cclusion of both anterior cerebral arteries produces bilateral paralysis, which is pronounced in the lower limbs).
anterior chamber Space between the lens and the cornea of the eye.
anterior choroidal artery Arises near the optic chiasm and supplies the choroid plexus located in the inferior horn of the lateral ventricle, the optic tract, parts of the internal capsule, hippocampal formation, globus pallidus, and lateral portions of the thalamus.
anterior commissure Fiber bundles connecting left and right olfactory and temporal lobes.
anterior communicating artery Connects the left and right anterior cerebral arteries.
anterior (ventral) funiculus Located between the anterior (ventral) median fissure and the site where the ventral roots exit.
anterior hypothalamus Region of the hypothalamus containing preoptic, suprachiasmatic, supraoptic, anterior, and paraven-tricular nuclei. Functions include temperature regulation, release of hormones into posterior pituitary, control of blood pressure, and related autonomic functions and associated emotional behavior. (See each type of nucleus individually.)
anterior inferior cerebellar artery (AICA) Supplies the ventral and inferior surface of the cerebellum and parts of the pons.
anterior lobe One of the three lobes of the cerebellum situated in a rostral position; associated in part with the control of muscle tone.
anterior neuropore Temporary opening of front end of the neural tube of the early embryo.
anterior nucleus of thalamus Nucleus of thalamus situated in the dorsomedial aspect of the rostral thalamus; it projects its axons to cingulate gyrus and receives inputs from hippocampal formation and mammillary bodies.
anterior olfactory nucleus Located in the posterior aspect of each olfactory bulb; receives sensory signals from mitral and tufted cells and relays them to the contralateral olfactory bulb via the anterior commissure.
anterior parietal arteries Supply different regions of the parietal lobe.
anterior spinal artery (ASA) Courses along the midline of the ventral surface of the spinal cord and medulla.
anterior temporal artery Supplies different regions of the temporal lobe.
anterior (ventral) white commissure Located ventral to the gray commissure; consists of decussating axons.
anterograde amnesia Difficulty in generating new memories.
anterograde tracing techniques Involve microinjection of a marker (e.g., a fluorescent dye) at desired site in the central nervous system; marker is taken up by the neuronal cell bodies and transported anterogradely to their axon terminals; marker is then visualized under a microscope to ascertain the projections of the neuron.
anterograde transneuronal degeneration Damage to a neuron resulting in the degeneration of another postsynaptic neuron closely associated with the same function..
antipyrogenic region Region of anterior hypothalamus that responds to actions of pyrogens (i.e., region contains vasopres-sin, which is capable of counteracting the actions of pyrogens, which are substances that increase body temperature).
anxiety Psychological response to a real or perceived dangerous situation.
aphagia Inability or refusal to swallow.
aphasia Speech disorder in which the patient has difficulty in naming objects and repetition of words is impaired, while comprehension remains intact.
apoptosis A genetically determined process of cell death; characterized by shrinkage of the cell, cellular fragmentation, and condensation of the chromatin; involves activation of a latent biochemical pathway in the cells.
apraxia Inability to produce a motor act correctly even though sensory and motor circuits are intact; disorder is associated with damage to the premotor cortex or the posterior parietal cortex.
aqueduct of Sylvius (cerebral aqueduct) Channel present in the midbrain that allows cerebrospinal fluid to flow from the third ventricle to the fourth ventricle.
aqueous humor A watery fluid produced by the epithelial cells of the ciliary processes; fills the anterior chamber. arachnoid granulations Large aggregations of arachnoid villi.
arachnoid mater Membrane covering the brain and spinal cord; lies between the dura and pia mater. arachnoid trabeculae Fine strands of connective tissue that arise from the arachnoid, span the subarachnoid space, and then connect with the pia.
arachnoid villi Small tufts of arachnoidal tissue that project into the superior sagittal and other dural sinuses. They act as oneway valves, allowing flow of cerebrospinal fluid into the sinuses but not in the reverse direction. Large aggregations of arachnoid villi are called arachnoid granulations.
arcuate fasciculus Pathway through which Broca’s area and Wernicke’s area communicate with each other.
arcuate nucleus Located in the extreme ventromedial aspect of the hypothalamus at the level of the median eminence; many of the neurons contain dopamine that may inhibit prolactin release (i.e., prolactin release-inhibiting hormone).
area postrema Rounded eminence located immediately rostral to the obex on each side of the fourth ventricle; contains arte-rioles, sinusoids, and a few unipolar neurons. Considered to be a chemoreceptor zone that triggers vomiting in response to emetic substances (e.g., digitalis glycosides, and apomorphine) circulating in the blood.
areflexia Absence of neurologic reflexes.
Argyll Robertson pupil Disorder in which the pupil reacts to accommodation but not to light.
Arnold-Chiari malformation Disorder in which parts of the medulla and cerebellum are displaced and consequently pulled through the foramen magnum, blocking the flow of cerebrospi-nal fluid that normally passes from the roof of the fourth ventricle to the cisterns and causing hydrocephalus. association areas Areas of the cerebral cortex that do not comprise primary motor or sensory cortices but may be linked to them as secondary or tertiary regions. These areas mediate complex functions of the cerebral cortex. association nuclei of the thalamus Receiving few, if any, afferents from ascending sensory pathways, these nuclei project to specific regions of cortex and, in turn, receive inputs from cerebral cortex, limbic nuclei, basal ganglia, and nonspecific thalamic nuclei. astereognosia Disorder in which the patient cannot identify by touch the shape, size, or texture of an object in the hand con-tralateral to the side of a parietal cortical lesion.
astigmatism Disorder in which the shape of the cornea is oblong, causing the curvature of the cornea in one plane to be less than the curvature in the other plane. Because the light rays coming from an object are bent to a different extent in these two different planes, the light rays do not come to a single focal point, causing visual disturbances.
astrocyte Glial cell that supports neurons in the brain and regulates the chemical and extracellular environments.
asymmetrical synapse Postsynaptic membrane of the synapse is thicker than the presynaptic membrane; synapse is usually excitatory. See also symmetrical synapse.
asynergy Loss of coordination.
ataxia Errors in the range, rate, force, and direction of movement that result in loss of muscle coordination.
athetosis Variant of choreiform movement involving slow, writhing movements of the extremities.
atonic seizures Brief complete loss of muscle tone found mainly in children with neurologic disorders; characterized by a brief burst of polyspike and slow wave complexes.
atrioventricular (AV) node Conductile tissue between the atria and the ventricles of the heart.
atrium One of the upper chambers of the heart.
auditory nuclei Nuclei associated with transmission of signals necessary for hearing.
autoinhibition A normal physiological mechanism in which a released neurotransmitter inhibits its own further release.
autonomic insufficiency Impaired function of the autonomic nervous system due to a defect in the central or peripheral nervous system.
