Glossary (Neuroscience) Part 12

sexually dimorphic nucleus Nucleus in preoptic region of rodents which is larger in males than in females.

short circumferential arteries Pontine arteries that travel a short distance around the pons and supply substantia nigra and lateral portions of the midbrain tegmentum.

short-term memory Retention of an event for several minutes after it has occurred.

short-term memory disorder A disorder of memory that selectively affects memory of recent events (i.e., those occurring within seconds to minutes of the time of testing).

sigmoid sinus Empties into the internal jugular vein.

simple cell Cell type located near the region of sublayer 4CP of the visual cortex; has circular receptive field and receives inputs from the lateral geniculate nucleus. The cell responds to a bar of light presented in a specific position (i.e., in a given receptive field).

simple diffusion Process whereby a substance passes through a cell membrane without the aid of an intermediary such as an integral membrane.

simple partial seizures Form of seizure that does not cause a change in consciousness, but the patient experiences various sensory and motor symptoms, usually not lasting for more than 1 to 2 minutes. Also called aura.

Sinemet A combination of L-DOPA and carbidopa used for the treatment of Parkinson’s disease. See also carbidopa.


single-photon emission computed tomography (SPECT) Imaging technique used to determine regional cerebral blood flow after the injection of a specific radionuclide; useful in diagnosis of many neurological conditions where there may be alterations in blood flow to specific regions of the brain, such as stroke.

sinoatrial (SA) node Region of the heart where pacemaker cells regulating heart rate are located.

sleep apnea Condition characterized by an interruption of breathing in which there is a decline in the oxygen content of the blood; interruption of breathing leads to arousal from sleep or results in lighter stages of non-REM (non-rapid eye movement) sleep so that breathing can be maintained.

sleep-onset REM Rapid eye movement sleep that occurs in the first hour of nighttime sleep or anytime during daytime multiple sleep latency tests.

slow anterograde transport Transport of materials (e.g., neurofilaments, microtubules, soluble proteins) from the cell body to the terminals at a slow rate (0.25-5 mm/day). See also fast anterograde transport.

smooth pursuit movements Eye movements corresponding to fixation on an object within a person’s visual field; involve primary, secondary, and tertiary regions of the visual cortex, which have connections with the frontal lobe, including the frontal eye fields.

somatic nervous system Component of the nervous system that innervates musculoskeletal structures and the sense organs of skin.

somatosensory Somatic sensation involving touch, pressure, temperature, and pain.

somatostatin Neuroactive peptide synthesized in hypothalamic paraventricular neurons (and other regions of the forebrain).

somatotopic Refers to different parts of a given region associated with distinct parts of the body, both functionally and anatomically.

somesthetic Pertains to kinesthetic and tactile information from the periphery.

somites Mesodermal cells located alongside the neural tube that will develop into skeletal muscle, vertebrae, and dermal layer of the skin.

Sommer’s sector Region of CA1 field of the hippocampus highly susceptible to anoxia, especially during temporal lobe epilepsy.

somnambulism A form of insomnia occurring during non-REM sleep that is characterized by unusual autonomic and/or motor responses; typically takes place during the first few hours of sleep.

sonic hedgehog A protein which induces differentiation in floor place cells, ventral motor neurons, and interneurons. Interactions with the receptor complex in cells help to regulate several protein kinases which, in turn, activate transcription factors that affect induction of various types of neurons.

spasticity Muscle resistance to movement in a single direction.

spatial summation Superimposition (summation) of multiple signals that arrive at the trigger zone (axon hillock in efferent neurons) simultaneously.

special sensory afferent (SSA) neurons Afferent components of cranial nerves associated with the transmission of visual, auditory, and vestibular impulses.

special visceral afferent (SVA) neurons Components of afferent fibers of cranial nerves that are associated with transmission of chemical sensation.

special visceral efferent (SVE) neurons Cell groups that are derived from mesenchyme of the branchial arches.

specific thalamic nuclei Thalamic nuclei that relay specific sensory information originating in the periphery to specific regions of the neocortex.

sphenoparietal sinuses Located below the sphenoid bone; drain into the cavernous sinus.

sphincter vesicae Sphincter located at the neck of the bladder; activation of sympathetic nervous system contracts this sphincter during ejaculation and prevents seminal fluid from entering the bladder.

spina bifida (myeloschisis) Failure of the closure of the posterior neuropore during development of the nervous system.

spina bifida aperta A disorder involving the protrusion of either the meninges alone or spinal cord and the meninges.

spina bifida occulta A disorder of mesodermal origin in which one or more vertebrae fail to close.

spinal accessory nerve (cranial nerve [CN] XI) Motor (special visceral efferent) cranial nerve arising from ventral horn cells of the first 5 to 6 cervical segments that supply the trapezius and sternomastoid muscles and that mediate contralateral turning and lifting of the head. spinal cord Thin, cylinder-like structure with five regions that extend from its attachment to the brain downward, including the cervical cord, thoracic cord, lumbar cord, sacral cord, and coccygeal region.

spinal medullary and radicular arteries Arteries that arise from segmental arteries and communicate with the posterior and anterior spinal arteries; they provide blood supply to the thoracic, lumbar, and sacral regions of the spinal cord. spinal nerve Consists of a dorsal root (containing afferent fibers) and a ventral root (containing efferent fibers).

spinal nucleus of the trigeminal nerve Receives inputs from the descending tract of cranial nerve V and mediates second-order sensory signals from the head region to the contralateral ventral posteromedial thalamic nucleus; present throughout the entire length of the medulla and extends into the lower pons.

spinal reflex Involves discharge of an efferent motor neuron in response to afferent stimulation of sufficient intensity. spinal segmentation The system of classification corresponding to the sites (segments—8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) where 31 pairs of spinal nerves emerge from the spinal cord.

spinal veins On the ventral side of the spinal cord, the antero-median spinal vein is located in the midline, and two anterolat-eral spinal veins are located along the line of attachment of the ventral roots. On the dorsal side, the posteromedian spinal vein is located in the midline, and two posterolateral spinal veins are located along the line of attachment of the dorsal roots. The posteromedian and posterolateral veins are drained by posterior spinal medullary and radicular veins. The anterior median and anterolateral spinal veins are drained by anterior spinal medullary and radicular veins.

spinocerebellar pathway Pathway from spinal cord to cerebellum mediating unconscious proprioception.

spino-olivary fibers Fibers originating in spinal cord that project to the inferior olivary nucleus. spinothalamic pathway Pathway from the spinal cord to the thal-amus that mediates pain, temperature, and tactile sensation.

spiral (cochlear) ganglion Located within the spiral canal of the bony modiolus; contains bipolar neurons whose peripheral processes innervate the hair cells and whose central processes form the cochlear division of cranial nerve VIII. splanchnic nerves Axons of the sympathetic preganglionic neurons located in the intermediolateral cell column at thoracic level 5 (T5)-T9, T10-T11, and T12; they pass through the sympathetic chain without synapsing and emerge as greater, lesser, and least splanchnic nerves, respectively. These axons synapse on neurons in the celiac ganglion; the postganglionic fibers emerging from the celiac ganglion innervate smooth muscle and glands of the stomach and small intestine. split brain Cerebral commissures that have been divided in the midline (the practice is carried out in patients to stop the spread of seizures).

stapedectomy Microsurgical procedure to remove the stapes of the middle ear and substitute it with a prosthetic device (usually a nonmagnetic metallic wire of appropriate thickness and length) to correct conduction deafness.

stapedius Small muscle of the middle ear; inserts on the stapes ossicle and is innervated by a branch of the facial nerve. stapes One of the three small bones of the middle ear, resembling a stirrup.

status epilepticus Prolonged seizure activity or the generation of two or more tonic-clonic seizures in a row without regaining consciousness; neurologic emergency with a high morbidity and mortality.

stellate ganglion Fusion of the last two cervical and first two thoracic ganglia to form this star-shaped ganglion; sympathetic postganglionic fibers arising from this ganglion (when it is present) innervate all regions of the heart (sinoatrial node, atria, atrioventricular node, and ventricles).

stem cells Undifferentiated cells from which neurons (as well as other types of cells) may be formed.

stereocilia Hair-like structures at the apical borders of hair cells in the inner ear that grow progressively longer as they approach the kinocilium.

stereognosis Appreciation of shape, size, or texture of objects by means of touch.

stimulants Substances that activate central nervous system functions, which may result in heightened alertness, agitation, mild improvement of mood, increased heart rate, loss of appetite, and aggression (e.g., caffeine, amphetamines, cocaine, and nicotine).

stimulus transduction Process by which a sensory receptor converts a stimulus into neural activity in the somatic nervous system.

strabismic amblyopia A disorder present in some infants; the visual axes of the two eyes are not parallel.

straight sinus Runs caudally and empties into the caudal end of the superior sagittal sinus at the level of the confluence of sinuses.

stratum oriens Layer of hippocampus containing basal dendrites and basket cells.

stratum radiatum-lacunosum-moleculare Two layers of the hippocampus that contain the apical dendrites of pyramidal cells and hippocampal afferents from the entorhinal cortex. stretch reflex See myotatic reflex.

stria (striae) Refers to fiber bundle in the central nervous system.

stria medullaris Pathway passing along the roof of the thalamus conveying afferent and efferent signals in relation to the habe-nular nuclei.

stria terminalis Major efferent pathway of the amygdala, the primary projections of which supply the medial hypothalamus and bed nucleus of the stria terminalis.

striosomes Inputs to the neostriatum that end in patches surrounded by a larger compartment referred to as a matrix; contain acetylcholinesterase, somatostatin, substance P, and enkephalin; receive inputs mainly from limbic regions of cortex, and neurons from this region preferentially project to the substantia nigra.

stroke Characterized by an acute onset of a neurological deficit lasting for at least 24 hours; can be caused by blockage of blood vessels (occlusive stroke) or by bleeding from vessels (hemor-rhagic stroke).

strychnine Antagonist at strychnine-sensitive glycine sites.

strychnine-insensitive glycine site The glycine binding site of the N-methyl-D-aspartic acid receptor where strychnine does not act as an antagonist.

strychnine-sensitive glycine sites Synapses where glycine acts as a neurotransmitter, and strychnine acts as an antagonist.

subarachnoid space Space between the arachnoid and pial membranes filled with cerebrospinal fluid.

subarachnoid hematoma Extravasation of blood into the suba-rachnoid space caused by rupture of an intracranial aneurysm (local dilatation of an artery or vein resulting in a bulge and weakening of the vessel wall).

subcommisural organ One of the seven circumventricular organs, which have fenestrated capillaries, capillary loops, and large perivascular spaces that permit the passage of larger circulating molecules into the adjacent brain tissue rather than a blood-brain barrier.

subdural hematoma Space between the dura and subachnoid caused by extravasation of blood, typically due to head injury.

subfornical organ A small neuronal structure in the third ventricle consisting of fenestrated capillaries that lack a blood-brain barrier and allow passage of blood-borne molecules.

subicular cortex Structure that constitutes a transitional region between the entorhinal cortex and hippocampus. sublenticular component of internal capsule Components of internal capsule fibers that originate in the temporal lobe or posterior parts of the thalamus.

submucosal (Meissner’s) plexus Neurons in the gut that control water and ion movement across the intestinal epithelium.

substance abuse Abnormal use of a substance that leads to impairment of physical, social, or occupational aspects of one’s life.

substance P An undecapeptide (composed of 11 amino acids); a neurotransmitter or neuromodulator.

substantia gelatinosa Region of gray matter of the dorsal horn of spinal cord that receives mainly pain and temperature inputs from the periphery.

substantia innominata Region of the basal forebrain situated immediately lateral to the preoptic region; linked anatomically with the amygdala and projects to widespread regions of the cerebral cortex.

substantia nigra Structure found in the ventral aspect of the midbrain associated with functions of the basal ganglia; one group of cells (pars compacta) produces dopamine and supplies the neostriatum, while a second group (pars reticulata) produces gamma aminobutyric acid and supplies the ventrolateral and ventral anterior thalamic nuclei. subthalamic nucleus Structure found in the ventral aspect of diencephalon associated with functions of the basal ganglia;receives inputs from neostriatum and projects to the globus pallidus.

subthalamus Group of structures in the posterior third of the diencephalon, including the subthalamic nucleus and the zona incerta.

subunit Polypeptide contributing to the formation of a large protein molecule. sudomotor fibers Axons of postganglionic neurons located in the inferior and middle cervical paravertebral ganglia that innervate the sweat glands.

sulcus (sulci) Fissure that separates the lobes of the cerebral cortex.

sulcus limitans Landmark in the developing nervous system that separates sensory from motor regions.

sumatriptan (Imitrex) A 5-hydroxytryptamine1D receptor agonist; a vasoconstrictor found useful in treating migraine headaches.

superficial middle cerebral vein Passes along the lateral sulcus, drains the temporal lobe, and empties into the cavernous sinus.

superior At positions above and below the cephalic flexure, superior means toward the dorsal surface of the cerebral cortex.

superior anastomotic vein of Trolard Largest superficial vein; travels across the parietal lobe and drains into the superior sagittal sinus.

superior cerebellar artery Arises just caudal to the bifurcation of the basilar artery and supplies the rostral level of the pons, caudal part of the midbrain, and superior surface of the cerebellum.

superior cerebellar artery syndrome Symptoms include analgesia and thermoanesthesia on the ipsilateral side of the face (due to damage to the nucleus and spinal tract of the trigemi-nal nerve), ipsilateral Horner’s syndrome (from disruption of the descending sympathetic pathways that supply the eye), contralateral loss of sensations of pain and temperature (due to damage to the spinothalamic tract), and ipsilateral limb and gait ataxia (due to damage to the superior cerebellar peduncle and adjoining cerebellar efferents, respectively).

superior cerebellar peduncle (brachium conjunctivum) One of the three cerebellar peduncles constituting the major output pathway of the cerebellum.

superior colliculus Rostral aspect of the tectum of midbrain associated with visual reflexes and tracking movements of the eyes.

superior medullary velum Roof of the fourth ventricle attached to the superior cerebellar peduncle on each side.

superior oblique muscle An extraocular muscle innervated by the trochlear nerve; stimulation of this nerve results in contraction of the superior oblique muscle, causing the eye to be moved downward when it is located in a medial position.

superior olivary nucleus Auditory relay nucleus of the lower pons. superior sagittal sinus Lies along the superior border of the falxcerebri and empties into the confluence of sinuses.

superior salivatory nucleus of the facial nerve Nucleus of cranial nerve VII; located in the lower pons; provides parasym-pathetic innervation to the lacrimal, sublingual, and subman-dibular salivary glands.

superior temporal gyrus The most superior aspect of the temporal lobe containing transverse gyri of Heschl, which constitute the primary auditory receiving area.

supplemental motor area Region of cortex immediately rostral to the precentral gyrus (area 6 of Brodmann).

suprachiasmatic nucleus Nucleus found in the ventromedial aspect of the anterior hypothalamus. It receives inputs from the retina, which affect circadian rhythms for release of several hormones.

supramarginal gyrus Situated in the inferior parietal lobule just superior to the posterior extent of the lateral sulcus.

supranuclear paralysis Paralysis resulting from a cortical lesion affecting corticobulbar fibers normally directed to motor nuclei of cranial nerves. supraoptic nucleus Hypothalamic nucleus located near the lateral edge of the optic chiasm concerned with the release of vasopressin and oxytocin.

suspensory ligaments of the lens Ligaments attached to the lens of the eye; relaxation or stretching of these ligaments causes the lens to become more convex (suitable for near vision) or less convex (suitable for distant vision), respectively.

sustentacular cell Supporting cell of an epithelial membrane or tissue.

swinging flashlight test Conducted to test the pupillary light reflex in patients with Marcus-Gunn pupil, in which one of the eyes has an optic nerve lesion.

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