| septomarginal tract | See: semilunar fasciculus. (05 Mar 2000) |
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| pyramidal tract | A massive bundle of fibres originating from pyramidal cells of various sizes in the fifth layer of the precentral motor (area 4), the premotor area (area 6), and to a lesser extent from the postcentral gyrus. Cells of origin in area 4 include the gigantopyramidal cells of Betz. Fibres from these cortical regions descend through the internal capsule, the middle third of the crus cerebri, and the ventral part of the pons to emerge on the ventral surface of the medulla oblongata as the pyramis. Continuing caudally, most of the fibres cross to the opposite side in the pyramidal decussation and descend in the dorsal half of the lateral funiculus of the spinal cord as the lateral pyramidal tract, which distributes its fibres throughout the length of the spinal cord to interneurons of the zona intermedia of the spinal gray matter. In the (extremity-related) spinal cord enlargements, fibres also pass directly to motoneuronal groups that innervate distal extremity muscles subserving particular hand-and-finger or foot-and-toe movements. The uncrossed fibres form a small bundle, the anterior pyramidal tract, which descends in the anterior funiculus of the spinal cord and terminates in synaptic contact with interneurons in the medial half of the anterior horn on both sides of the spinal cord. Interruption of the pyramidal tract at or below its cortical origin causes impairment of movement in the opposite body-half, especially severe in the arm and leg; characterised by muscular weakness, spasticity and hyperreflexia, and a loss of discrete finger and hand movements. Babinski's sign is associated with this condition of hemiplegia. Synonym: tractus corticospinalis, tractus pyramidalis, corticospinal tract. (05 Mar 2000) |
| hypothalamohypophysial 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) |
| 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) |
| 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) |