Sylvian sulcus Lateral cerebral sulcus.
symmetrical synapse Synaptic arrangement in which postsyn-aptic and presynaptic membranes are similar in thickness; this type of synapse is usually inhibitory.
sympathetic chains Consist of nerve trunks (one on each side of the vertebral column) connecting paravertebral ganglia (25 pairs); each ganglion represents one of the sites where sympathetic postganglionic neurons are located. Also known as sympathetic trunks.
sympathetic division of autonomic nervous system Division of the autonomic nervous system associated with the thoracolum-bar cord; regulates heart rate, blood pressure, energy metabolism, and digestive functions. Also known as thoracolumbar division.
sympathetic postganglionic neurons Located in the paraverte-bral and prevertebral ganglia.
sympatholytic drugs Agents that attenuate or block the effects of sympathetic nervous system.
synapse A special zone of contact where one neuron communicates with another one.
synapsin Protein involved in endocytosis; brings about tethering of a newly formed uncoated vesicle to the cytoskeleton.
synaptic terminals (boutons) Enlarged terminal ends of axons.
synaptic transmission Mechanism by which neurons communicate with each other. syncope Brief loss of consciousness caused by generalized cerebral ischemia due to insufficient flow of oxygenated blood to the brain.
synergy of movement Association with movements that are properly grouped for the performance of selective responses that require specific adjustments; one function of the cerebellum.
syringo(hydro)myelia A developmental abnormality. It is characterized by an expansion of the central canal of the spinal cord; there is a segmental loss of pain and thermal sensation because of the interruption of crossing fibers of the spinotha-lamic tracts in the same and adjoining segments at the level of the lesion; tactile sensation is preserved.
tabes dorsalis (tertiary syphilis or neurosyphilis) Disorder characterized by loss of vibration sensation, two-point discrimination, and conscious proprioception, and presence of ataxia; represents late consequences of syphilitic infection of the nervous system. Large-diameter central processes of the dorsal root ganglion neurons (primary afferent sensors) degenerate in the lower thoracic and lumbosacral segments.
tachycardia Increase in heart rate.
tachykinins A group of polypeptides including substance P. tactile sensations Sensations of touch, pressure, and vibration.
tanycytes Specialized ependymal cells found in the floor of the third ventricle; their processes extend into the brain tissue, where they are juxtaposed to blood vessels and neurons. They are implicated in the transport of hormones from the cerebro-spinal fluid (CSF) to capillaries of the portal system and from hypothalamic neurons to the CSF.
tardive dyskinesia Characterized by involuntary movements of the tongue and face, such as repetitive chewing movements and irregular movement of the tongue in and out of the mouth; associated with depletion in gamma aminobutyric acid and its synthesizing enzyme, glutamic acid decarboxylase.
taste bud Sensory organ for taste; located in different types of papillae (protrusions on the surface of the tongue); sense salty, sour, sweet, and bitter tastes.
tectorial membrane Covers the organ of Corti; composed of mucopolysaccharides embedded in a collagenous matrix. tectospinal tract Pathway from the superior colliculus to cervical cord, which follows the medial longitudinal fasciculus and mediates postural movements in response to visual stimuli reaching the superior colliculus. tectum Most dorsal part of the midbrain.
tegmentum Dorsal aspect of the pons and midbrain containing the reticular formation and a variety of nuclear groups.
telencephalon Developing cerebral cortex and basal forebrain (derived from prosencephalon).
temperature Sensation from hot and cold stimuli.
temporal hemiretina Component of retina located lateral to the fovea.
temporal lobe One of four lobes of the cerebral cortex that integrate motor, sensory, autonomic, and intellectual processes and are organized along functional lines.
temporal summation Superimposition (summation) of multiple signals that arrive at the trigger zone (axon hillock in neurons) at different times; arriving signals drive the axon hillock to threshold membrane potential so that an action potential is generated; threshold refers to a membrane potential at the trigger zone at which action potentials automatically trigger the adjacent membrane regions into producing an action potential.
tensor tympani Small muscle of the middle ear; inserts on the manubrium of the malleus and innervated by a branch of the trigeminal nerve. tentorial notch Notch through which the midbrain traverses; rostral free edge of the tentorium cerebelli forms the boundary of this notch.
tentorium cerebelli A septum that forms a tent-like roof over the posterior cranial fossa; occipital lobes lie on the dorsal surface of this tentorium. teratogen An agent that causes an abnormality in development during early pregnancy. tertiary visual cortex (V3 and V5; Brodmann’s area 19) Located adjacent to the primary visual cortex. tetanus Characterized by hyperexcitation of skeletal muscles, which results in prolonged contractions, this disease is caused by a neurotoxin produced by Clostridium tetani. tethered cord A malformation involving the anchoring of the lowest part of the spinal cord to the sacrum; results in sensory and motor deficits of the lower extremities as well as bladder emptying difficulties, back pain, and scoliosis.
thalamic fasciculus (Hj field of Forel) Collective grouping of the ansa lenticularis, lenticular fasciculus, and dentatotha-lamic fibers as they pass into and through the posterior thala-mus to the ventral anterior and ventrolateral thalamic nuclei. thalamic pain syndrome Syndrome resulting from a thalamic infarct, frequently involving its posterior aspect; the patient feels a very painful and unpleasant sensation that persists generally as a burning sensation. Also called Dejerine-Roussy syndrome.
thalamohypothalamic fibers Circuitry that provides an important anatomical substrate by which the prefrontal cortex and anterior cingulate gyrus can regulate functions associated with the lateral hypothalamus.
thalamostriate fibers Projections from the centremedian nucleus of thalamus to the putamen.
thalamus Dorsal and largest part of the diencephalon; consists of varieties of nuclei that project mainly to the cerebral cortex.
theta rhythm A slow electroencephalographic (EEG) wave of 4-7 Hz recorded over the temporal lobes or within the hip-pocampal formation; in humans, this type of EEG typically indicates dysfunction of hippocampal tissue.
third ventricle One of the brain cavities; connected rostrally to the lateral ventricles (via the foramina of Monro) and caudally to the fourth ventricle (via the aqueduct of Sylvius); forms the medial surface of the thalamus and hypothalamus.
third-order neurons (neuron III) Located in the ventral horn of the spinal cord; exit with the spinal nerve to supply the muscle.
thoracic cord The part of the spinal cord situated between the cervical cord rostrally and lumbar cord caudally; contains 12 pairs of spinal nerves.
threshold A membrane potential at the trigger zone at which action potentials become self-propagating (an action potential automatically triggers the adjacent membrane regions into producing an action potential).
thrombus Blood clot.
thyroid-stimulating hormone (TSH) Peptide secreted by the anterior pituitary gland; controlled by thyrotropin-releasing hormone.
thyrotropin-releasing hormone (TRH) Hypothalamic releasing hormone affecting release of the pituitary hormones, thyrotropin, and prolactin.
tic douloureux Trigeminal neuralgia (severe pain of the face).
tight junctions An anatomic arrangement in which adjacent capillary endothelial cells are in close apposition to each other; this arrangement prevents movement of ions and molecules from the capillaries to the surrounding tissue.
tinnitus A condition characterized by a "ringing" noise in the ears.
tissue plasminogen activator (t-PA) Thrombolytic agent (a substance that lyses blood clots).
tolerance Phenomenon in which repeated administration of a drug elicits weaker responses.
tonic-clonic seizure Seizures, often called "convulsions," involving a tonic phase or stiffening, followed by a clonic phase in which the extremities contract the agonistic and antagonistic muscles.
tonotopic Refers to the spatial relationship of the perception of tones, i.e., that tones close to each other in terms of frequency are represented in topologically neighboring neurons in the brain.
torsion tremor Movements producing severe torsion of the neck or shoulder girdle, resulting in rhythmic shaking of the head.
Tourette syndrome Disorder characterized by the presence of motor and vocal tics including brief, sudden involuntary movements of different parts of the body (e.g., shrugging of the shoulder, turning in a circle, and eye-blinking) as well as by repetition of words and obscenities; cursing; and loud, irritating sounds.; it is believed that the disorder involves dysfunctions of the caudate nucleus and its relationships with the prefrontal cortex.
tract Many axons grouped together that typically pass from a given nucleus to a common target region or to several regions; also called fasciculus.
transducin A G protein present in rods in the eye. See also G protein.
transient ischemic attack (TIA) Refers to symptoms of ischemia that last for short periods of time (about 30 minutes).
transverse pontine fibers Fibers that arise from deep pontine nuclei and pass horizontally to the opposite side where they enter the middle cerebellar peduncle.
transverse sinuses Originate on each side of the confluence of sinuses; each travels laterally and rostrally and curves downward to form the sigmoid sinus.
trapezoid body Commissure comprising fibers from the cochlear nuclei and superior olivary nucleus that pass to the inferior colliculus via the lateral lemniscus.
tricyclic antidepressant drugs Class of drugs used in the treatment of depression, anxiety disorders, obsessive-compulsive disorder, and other psychological disorders.
tricyclic compounds A class of antidepressant drugs (such as amitriptyline and clomipramine) that increases monoamine concentrations in brain.
trigeminal ganglion Ganglion for cranial nerve V. Also called Gasserian or semilunar ganglion.
trigeminal nuclei and nerve (cranial nerve [CN] V) Includes sensory and motor nuclei associated with CN V. Sensory nuclei, which receive somatosensory inputs from the head, include (1) the spinal nucleus, which is located mainly in medulla and mediates pain and temperature; (2) the main sensory nucleus, which is located in the middle pons and mediates conscious proprioception; and (3) the mesenceph-alic nucleus which is located in the upper pons and mediates signals associated with muscle spindles. The motor nucleus, which is situated in the middle pons, controls muscles of mastication.
trigger zone The site where action potentials are generated; the axon hillock is the trigger zone in neurons.
trochlear nerve (cranial nerve [CN] IV) Motor nerve whose cell bodies lie in the midbrain at the level of the inferior col-liculus and whose axons supply the superior oblique muscle. trophic Relating to nutrition. tuber cinereum See median eminence.
tuberal nuclei Hypothalamic nuclei adjoining the third ventricle; concerned with release of releasing and release-inhibiting hormones transmitted through the portal system to the anterior pituitary gland, the net effect of which is to regulate the release of hormones from this region. tuberculum cuneatus One of two swellings on the dorsal surface of the medulla.
tuberculum gracilis One of two swellings on the dorsal surface of the medulla.
tufted cell Particular excitatory cell type of the olfactory bulb that discharges spontaneously.
two-point discrimination (Weber two-point discrimination) A technique for measuring the minimal distance between two stimuli applied by a two-point caliper to two adjacent areas of skin simultaneously to determine the minimal distance where the two stimuli can be distinguished. The principle is that the closer the distance between the two stimuli that can be perceived, the denser the innervation of the receptors subserving tactile discrimination; used as a test of somatosensory function when the minimal distance for two-point discrimination is compared between the identical regions of the two sides of the body.
tympanic cavity Space between the round window (the secondary tympanic membrane) and the middle ear.
tympanic membrane Membrane located at the end of the auditory canal.
tyrosine hydroxylase Enzyme involved in the synthesis of cate-cholamine neurotransmitters; catalyzes conversion of tyrosine to dihydroxyphenylalanine (rate-limiting step in the synthesis of catecholamine neurotransmitters).
uncinate fasciculus Fibers arising from the fastigial nucleus of cerebellum that lie in a position just dorsal to the superior cer-ebellar peduncle and that project bilaterally to the reticular formation. uncinate fits See cacosmia.
uncus Medial extension of the anterior end of the parahip-pocampal gyrus.
undifferentiated schizophrenia Type of schizophrenia in which the individual displays one or more of the general criteria listed for the other types of schizophrenia but cannot be clearly placed into one of these categories.
unipolar neuron Neuron in which there is only a single neurite (axon), of which different segments serve as receptors or axon terminals; found in invertebrates.
upper motor neuron Neurons of the brain that innervate, either directly or through an interneuron, lower motor neurons of the spinal cord and brainstem.
upper motor neuron syndrome A form of paralysis associated with damage to the motor cortex or its descending pathways; in addition to paralysis, the affected limb(s) display hyperto-nia, spasticity, hyperreflexia, and a positive Babinski sign.
upper quadrantanopia Lesion restricted to the inferior bank of the calcarine sulcus causing loss of vision of one quarter of the visual field of both eyes; if the lesion affects the left side of the brain, a right upper quadrantanopia will result, etc.
Urbach-Wiethe disease Rare disorder involving calcification of parts of the anterior aspect of the temporal lobe (which includes the amygdala); patients may have difficulties in recognizing stimuli that one would normally characterize as fearful.
utricle An ovoid sac-like structure in the vestibular labyrinth; detects linear acceleration.
utriculosaccular duct Connects the utricle with the saccule.
uveal tract Iris, ciliary body, and choroid of the eye.
uveitis Inflammation of the structures of the uveal tract, which usually is secondary to an injury or infection; typical treatment consists of administration of atropine to relieve ciliary muscle spasm, which is the cause of pain in this condition.
vagus nerve (cranial nerve [CN] X) Consists of general (baroreceptor) and special visceral afferent (chemoreceptor) fibers, general somatic afferent (from the back of the ear) and general visceral efferent (parasympathetic) fibers innervating thoracic and abdominal viscera, and special visceral efferent fibers innervating the larynx and pharynx.
vasoactive intestinal peptide (VIP) Present in very high concentrations in the cerebral cortex and is also present within the suprachiasmatic nucleus of the hypothalamus, whose functions it appears to modulate; also present in the anterior pituitary, where it may act to modulate the release of anterior pituitary hormones.
vasoconstrictor fibers Axons of postganglionic neurons that innervate the arterioles.
vasodilation Dilation of blood vessels.
vasopressin (antidiuretic hormone) Hormone synthesized in paraventricular and supraoptic hypothalamic neurons; released from the posterior pituitary; causes water retention (decrease in urine production of the kidney).
ventral Above the cephalic flexure, ventral refers to the bottom of the brain, and below the flexure, it refers to the ventral surface of the body.
ventral amygdalofugal pathway Fiber bundle arising chiefly from the basolateral amygdala and parts of the adjoining pyri-form cortex that course medially from the amygdala into the lateral hypothalamus where many fibers terminate.
ventral anterior nucleus (VA) Thalamic nucleus situated at approximately the same level as the anterior nucleus, which receives inputs from the globus pallidus and substantia nigra; projects mainly to the premotor cortex and to widespread regions of other parts of the frontal lobe. ventral (anterior) white commissure Bundle of nerve fibers that cross the midline of the spinal cord just anterior to the gray commissure (Rexed lamina X).
ventral basal complex Region of thalamus consisting of the ventral posterolateral and ventral posteromedial nuclei. ventral funiculus White matter of the ventral aspect of spinal cord; contains the short tracts that connect different spinal segments.
ventral lateral nucleus (VL) Thalamic nucleus situated posterior to the ventral anterior nucleus; receives inputs from the globus pallidus, substantia nigra, and cerebellum and projects to the primary motor cortex.
ventral posterolateral nucleus (VPL) Nucleus situated in the ventrolateral aspect of the posterior thalamus; receives inputs from the medial lemniscus and spinothalamic tract and projects to the postcentral gyrus.
ventral posteromedial nucleus (VPM) Nucleus situated in the ventrolateral aspect of the posterior thalamus just medial to the ventral posterolateral nucleus; receives inputs from secondary trigeminal fibers and projects to the lateral aspect of the postcentral gyrus.
ventral respiratory group (VRG) Group of respiratory neurons located in the ventrolateral medulla.
ventral root Root of the spinal cord formed by axons functionally linked by nerve fibers from the mantle layer.
ventral (anterior) spinocerebellar tract Conveys information about whole limb movements and postural adjustments to the cerebellum.
ventral tegmental area Region of the ventral midbrain adjoining the pars compacta of the substantia nigra; contains dop-amine neurons that supply much of the forebrain except the neostriatum.
ventral tegmental decussation Refers to the crossing of axons of the red nucleus in the ventral midbrain tegmentum.
ventricles Cavities present within the brain that contain cere-brospinal fluid.
ventricular layer Inner layer of the neural tube that is in contact with the cavity of the neural tube.
ventromedial nucleus of the hypothalamus Prominent nucleus of the ventromedial aspect of the hypothalamus; concerned with inhibition of feeding, endocrine regulation, and expression of rage behavior.
vermis Midline region of the cerebellar cortex associated with the control of balance and eye movements.
vertebral artery First branch arising from the subclavian artery in the chest; ascends through foramina in the cervical vertebrae and enters the cranium through the foramen magnum.
vertical gaze center Region of the midbrain that coordinates the up and down movements of the eyes.
vertigo Sensation of turning or rotation in space; usually, debris from the otolithic membrane accumulates at the ampulla of the posterior semicircular canal and adheres to the cupula, making it more sensitive to angular movement.
vestibular membrane One of two membranes binding the scala media; separates the endolymph contained in the scala media from the perilymph contained in the scalae tympani and ves-tibuli. Also known as Reissner’s membrane.
vestibular neuronitis Inflammation of the vestibular labyrinth characterized by vertigo, postural imbalance, nausea, and nystagmus.
vestibular nuclei Pertaining to lateral, medial, inferior, and superior vestibular nuclei, which are associated with transmission of signals necessary for balance; receive direct inputs from the vestibular apparatus; many of the neurons project through the medial longitudinal fasciculus to cranial nerve nuclei of extraocular muscles, cerebellum, and spinal cord.
vestibular system Maintains the position of the body in space, which, in turn, is important for coordination of motor responses, eye movements, and posture; consists of the saccule, utricle, and three semicircular canals.
vestibulocochlear nerve (cranial nerve [CN] VIII) Special sensory afferent nerves mediating auditory and vestibular inputs through separate receptors and neural pathways.
vestibulospinal tracts Pathway (medial) arising from the medial vestibular nucleus and descends to cervical levels of the spinal cord to control muscles that produce orientation of the individual in response to forces that cause changes in posture and balance; pathway ( lateral) arising from the lateral vestibular nucleus and projecting to all levels of the spinal cord to play an important role in the maintenance of posture by exciting neurons that innervate extensor (i.e., antigravity) muscles, mainly of the lower limbs.
viscerotopy Topographic association of positional arrangement of neurons in the sympathetic chain ganglia innervating the viscera; e.g., preganglionic neurons innervating the eye are located most rostrally, whereas those innervating the genitals lie most caudally.
visual agnosia Disorder of a secondary visual area in which there is failure to understand the meaning or use of an object.
visual field of an eye Region of space that the eye can see while looking straight ahead without movement of the head.
vitreous body (vitreous humor) Thick, gelatinous material filling the space between the lens and retina; contains phagocytes that remove blood and debris in the eye under normal circumstances.
voltage-gated Ca2+ channel Channel through which Ca2* (calcium) ions enter postsynaptic neurons and activate enzymes; structure similar to voltage-gated Na* (sodium) channel. See also voltage-gated Na* channel.
voltage-gated channel Channel that is opened or closed by a change in the membrane potential.
voltage-gated K+ channel Structure similar to voltage-gated Na* (sodium) channel but with four polypeptides instead of one; presence of pore loop makes the channel more permeable to K* (potassium) than to Na*. See also voltage-gated Na* channel.
voltage-gated Na+ channel Channel made more permeable to Na* (sodium) than to K* (potassium) due to presence of pore loop; formed by a single long polypeptide that has four domains (I-IV).
Wallerian degeneration Changes that occur in the axon distal to the site of axonal damage; the axon swells up and becomes irregular; eventually, the axon and the terminals are broken down into fragments that are phagocytosed by adjacent mac-rophages and Schwann cells.
Weber’s syndrome (superior alternating hemiplegia) Disorder affecting the ventromedial midbrain, damaging oculomotor nerve fibers and axons of the crus cerebri and thus causing an ipsilateral oculomotor paralysis and contralateral upper motor neuron paralysis of the limbs. Also called medial midbrain syndrome.
Weber’s test Used to ascertain if a person’s deafness is due to conduction disturbance or sensorineural loss. The base of a vibrating tuning fork is placed at the top of the head in the midline. If the person’s deafness is due to conduction disturbance in the right ear, the sound from a vibrating tuning fork will be heard louder in the same (right) ear. If the person’s deafness is due to sensorineural loss in the right ear, the sound will be louder in the left ear.
Wernicke’s area Region of the ventral aspect of the angular and supramarginal gyrus, extending onto the adjoining part of the superior temporal gyrus; essential for comprehension of spoken language; lesions of this region produce a form of aphasia characterized by impairment of comprehension and repetition, although speech remains fluent.
white matter Brain and spinal cord tissue that appear white with the naked eye due to the presence of a large number of myeli-nated axons.
working memory Form of short-term memory requiring recall of a sequence of events for a few minutes in order to complete a task.
xanthochromia Process by which red blood cells undergo lysis and release hemoglobin, which is broken down into bilirubin that imparts a yellow color to the cerebrospinal fluid after subarachnoid hemorrhage.
zona incerta Thin band of forebrain cells separating the lenticular fasciculus from the thalamic fasciculus.
zone of Lissauer (dorsolateral tract or fasciculus) Refers to the tract formed by central axons of sensory neurons mediating pain that branch into ascending and descending collaterals.
zonule fibers Radially arranged connective tissue bands that are attached to the ciliary muscle and hold the lens in place in the eye.