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55-101 (38)
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Motor System II

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Biological Sciences

Outline of Lecture 37 (03-05 B; Shadmehr) Motor System II – Descending Tracts 0. Motor control is a high level action, e.g. it may involve complex joint motions but smooth trajectories I. CORTICOSPINAL TRACT - Origin at premotor cortex, motor cortex (M1), somatosensory cortex (S1) - Axons run through pyramids (medulla) and 90% cross in lower medulla to form lateral corticospinal tract and 10% remain ipsolateral mostly going to ventral corticospinal tract - Split-brain patients (surgical separation of corpus callosum) - Distribution of CST fibers provides good control of contralateral arm/hand, but poor control of ipsolateral and little control of ipso. hand - Language center is on left hemisphere, can only vocally identify items perceived by right side of body (thus left arm is an “alien arm”) - Conduction speed in CST increases early in life due to myelination but remains relatively constant afterwards despite growth/lengthening of pathway II. TECTOSPINAL TRACT - Origin at tectum (midbrain), crosses in caudal midbrain, terminates in cervical spine - Function unclear (don’t worry about this tract) III. RUBROSPINAL TRACT - Origin at red nucleus (midbrain), and receives input from cortex (M1) and cerebellum - Crosses immediately after origin, descends laterally to excite arm and
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