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PHGY 210
Rob Beamish

DESCENDING SPINAL TRACTS How does the brain communicate with the motor neurons of the spinal cord? Axons from the brain descend through the spinal cord along two major groups of pathways (see Fig. 14.2):  One is the lateral column of the spinal cord o The lateral pathways are involved in voluntary movement of the distal musculature and are under direct cortical control  The other is the ventromedial column o The ventromedial pathways are involved in the control of posture and locomotion and are under brain stem control The Lateral Pathways The most important component of this pathway is the corticospinal tract (see Fig. 14.3a). Originating in the neocortex, it is the longest and one of the CNS tracts.  Two-thirds of the axons in the tract originate in areas 4 and 6 and is called the motor cortex.  The remaining axons derive from the somatosensory areas of the parietal lobe to regulate the flow of somatosensory information to the brain. A much smaller component of the lateral pathways is the rubrospinal tract, which orginates in the red nucleus of the midbrain (see Fig. 14.3b). A major source of input to the red nucleus is the region of the front cortex that contributes to the corticospinal tract.  It contributes to motor control in many mammalian species o In humans it appears to be reduced, most of its functions subsumed by the corticospinal tract. The Effects of Lateral Pathway Lesions Experimental lesions in both corticospinal and rubrospinal tracts in monkeys rendered them unable to make fractionated movements of arms and hands (i.e. could not move their shoulders, elbows, wrists, and fingers independently).  Voluntary movements were also slower and less accurate Lesions in the corticospinal tracts alone caused a movement deficit as severe as that observed after lesions in the lateral columns.  However, many functions gradually reappeared  The only permanent deficit was some weakness of the distal flexors and an inability to move the fingers independently  Lesion in the rubrospinal tract completely reversed this recovery o This suggests that the corticorubrospinal pathway was able to partially compensate for the loss of the corticospinal tract input The Ventromedial Pathways It contains four descending tracts that orginate in the brain stem and terminate among the spinal interneurons controlling pr
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