| solitary tract | A slender, compact fibre bundle extending longitudinally through the dorsolateral region of the medullary tegmentum, surrounded by the nucleus of the solitary tract, below the obex decussating over the central canal, and descending over some distance into the upper cervical segments of the spinal cord. It is composed of primary sensory fibres that enter with the vagus, glossopharyngeal, and facial nerves, and in part convey information from stretch receptors and chemoreceptors in the walls of the cardiovascular, respiratory, and intestinal tracts; in rostral parts of the tract impulses are generated by the receptor cells of the taste buds in the mucosa of the tongue. Its fibres are distributed to the nucleus of the solitary tract. Synonym: tractus solitarius, fasciculus rotundus, fasciculus solitarius, funiculus solitarius, Gierke's respiratory bundle, Krause's respiratory bundle, round fasciculus, solitary bundle, solitary fasciculus. (05 Mar 2000) |
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| sphincteroid tract of ileum | The thickening of the circular muscular coat at the base of the ileal papilla at the terminal ileum. Synonym: sphincteroid tract of ileum. (05 Mar 2000) |
| spinal tract | Any one of a multitude of fibre bundles ascending or descending in the spinal cord. (05 Mar 2000) |
| spinal tract of trigeminal nerve | <anatomy, nerve> A compact fibre bundle, comma-shaped on transverse section, composed of primary sensory fibres of the portio major of the trigeminal nerve, descending from the level of the entrance of the trigeminus in the upper pons down through the dorsolateral region of the rhombencephalic tegmentum along the lateral side of the descending or spinal nucleus of the trigeminus, emerging on the dorsolateral surface of the lower medulla oblongata as the tuberculum cinereum, and continuing as far as the second cervical segment of the spinal cord. Its fibres are distributed to the descending or spinal nucleus of the trigeminus. Synonym: tractus spinalis nervi trigemini, descending tract of trigeminal nerve, tractus descendens nervi trigemini. (05 Mar 2000) |
| spinocervicothalamic tract | A tract composed of axons that originate from laminae III-V, ascend ipsilaterally to the lateral cervical nucleus (LCN) where they synapse, LCN neurons project to the contralateral thalamus via the medial lemniscus. (05 Mar 2000) |
| spino-olivary tract | Multiple spinal tracts terminating in the accessory olivary nuclei. See: olivospinal tract. (05 Mar 2000) |
| spinoreticular tract | Nerve fibres originating from the spinal cord and terminating in the reticular formation of the brainstem. Synonym: spinoreticular tract. (05 Mar 2000) |
| spinotectal tract | The relatively small component of the spinothalamic tractus that terminates in the intermediate and deep layers of the superior colliculus and in parts of the periaqueductal gray. Synonym: tractus spinotectalis. (05 Mar 2000) |
| spinothalamic tract | A large ascending fibre bundle in the ventral half of the lateral funiculus of the spinal cord, arising from cells in the posterior horn at all levels of the cord, which cross within their segments of origin in the white commissure. In their contralateral ascent, the bundle is intermingled with numerous intersegmental fibres. The spinothalamic tract continues from the spinal cord into the brainstem, occupying a ventrolateral position and issuing numerous fibres to the rhombencephalic and mesencephalic reticular formation, to the lateral part of the central gray substance of the mesencephalon, and to the deep and intermediate layers of the superior colliculus; the relatively few fibres (10 to 20%) that remain form the true spinothalamic tract which enters the diencephalon and ends in the nucleus ventralis posterior (caudal part) and intralaminar nuclei of the thalamus. In its ascent in the spinal cord the tract is composed of a dorsal part, the lateral spinothalamic tract, which conveys impulses associated with pain and temperature sensation, and a more ventral part, the anterior spinsothalamic tract, involved in tactile sensation. Synonym: lemniscus spinalis, spinal lemniscus, tractus spinothalamicus. (05 Mar 2000) |
| spiral foraminous tract | Openings in the cochlear area of the bottom of the internal acoustic meatus through which the fibres of the cochlear nerve leave the bony labyrinth to enter the cranial cavity. Synonym: tractus spiralis foraminosus. (05 Mar 2000) |
| Spitzka's marginal tract | A longitudinal bundle of thin, unmyelinated and poorly myelinated fibres capping the apex of the posterior horn of the spinal gray matter, composed of posterior root fibres and short association fibres that interconnect neighboring segments of the posterior horn. Synonym: fasciculus dorsolateralis, tractus dorsolateralis, dorsolateral tract, fasciculus marginalis, Lissauer's bundle, Lissauer's column, Lissauer's fasciculus, Lissauer's marginal zone, Lissauer's tract, marginal fasciculus, Spitzka's marginal tract, Spitzka's marginal zone, Waldeyer's tract, Waldeyer's zonal layer. (05 Mar 2000) |
| nerve tract | A bundle or group of nerve fibres in the brain or spinal cord. (05 Mar 2000) |
| sulcomarginal tract | Collective term for those fibre tract's which descend in the anterior funiculus of the spinal cord along the wall of the anterior median fissure: tectospinal tract, medial longitudinal fasciculus, and anterior pyramidal tract. (05 Mar 2000) |
| supraopticohypophysial tract | A bundle of unmyelinated fibres originating from all cells of the supraoptic nucleus and an estimated 20% of those of the paraventricular nucleus of the hypothalamus, which extend through the infundibulum and pituitary stalk to their endings in the posterior lobe of the hypophysis; the fibres convey neurosecretory substances, vasopressin and oxytocin, which are stored in (and can be released into the circulating blood from) their terminals. See: hypophysis, neurosecretion. Synonym: tractus supraopticohypophysialis, hypothalamohypophysial tract. (05 Mar 2000) |
| deiterospinal tract | A somatopically organised fibre bundle originating from the lateral vestibular nucleus (nucleus of Deiters) which descends uncrossed into the anterior funiculus of the spinal cord lateral to the anterior median fissure; the tractus extends throughout the length of the cord, distributing fibres at all levels to the medial part of the anterior horn. Excitatory impulses conveyed by the vestibulospinal tract increase extensor muscle tone. Synonym: tractus vestibulospinalis, deiterospinal tract. (05 Mar 2000) |