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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