The central motor system is arranged as a hierarchy of control levels, with the forebrain at the
top and the spinal cord at the bottom (see Table 14.1).
The highest level concerned with strategy – the goal of the movement and the
movement strategy that best achieves that goal
The middle level is concerned with tactics – the sequences of muscle contractions,
arranged in space and time, required to smoothly and accurately achieve the strategic
The lowest level is concerned with execution – activation of the motor neuron and
interneuron pools that generate the goal-directed movement and make any necessary
adjustments of posture
The proper functioning of each level of the motor control hierarchy relies so heavily on sensory
information that the motor system of the brain might be considered a sensorimotor system.
At the highest level, sensory information generates a mental image of the body and its
relationship to the environment
At the middle level, tactical decisions are based on the memory of sensory information
from past movements
At the lowest level, sensory feedback is used to maintain posture, muscle length, and
tension before and after each voluntary movement.
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