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
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
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