The control of motor function, to which much of the human nervous
system is largely committed, is accomplished through the integrated
action of a vast array of segmental and suprasegmental motor neurons.
As originally conceived by Hughlings Jackson in 1858, purely on
the basis of clinical observations, the motor system is organized
hierarchically in three levels, each higher level controlling the
one below. It was Jackson’s concept that the spinal and
brainstem neurons represent the lowest, simplest, and most closely
organized motor centers; that the motor neurons of the posterior
frontal region represent a more complex and less closely organized
second motor center; and that the prefrontal parts of the cerebrum
are the third and highest motor center. This scheme is still regarded
as being essentially correct, although Jackson failed to recognize
the importance of the parietal lobe and basal ganglia in motor control.
Since Jackson’s time, physiologists have repeatedly
analyzed these three levels of motor organization and have found
their relationships to be remarkably complex. Motor and sensory
systems, although separated for practical clinical purposes, are
not independent entities but are closely integrated. Without sensory
feedback, motor control is ineffective. And at the higher cortical
levels of motor control, motivation, planning, and other frontal
lobe activities that subserve volitional movement are always preceded
and modulated by activity in the parietal sensory cortex.
Motor activities include not only those that alter the position
of a limb or other part of the body (isotonic contraction) but also
those that stabilize posture (isometric contraction). Movements
that are performed slowly are called ramp movements.
Very rapid movements are called ballistic (they
are too fast for sensory control). Another way of classifying movements,
stressed by Hughlings Jackson, is in terms of their automaticity:
reflex movements are the most automatic, willed movements the least
automatic. Physiologic studies, cast in their simplest terms, indicate that
the following parts of the nervous system are engaged primarily
in the control of movement and, in the course of disease, yield
a number of characteristic derangements.
1. The large motor neurons in the anterior horns of the
spinal cord and the motor nuclei of the brainstem. The
axons of these nerve cells comprise the anterior spinal roots, the
spinal nerves, and the cranial nerves, and they innervate the skeletal
muscles. These nerve cells and their axons constitute the primary,
or lower, motor neurons, complete lesions of which result in a loss
of all movement—voluntary, automatic, postural, and reflex.
The lower motor neurons are the final common path by
which all neural impulses are transmitted to muscle.
2. The motor neurons in the frontal cortex adjacent to
the rolandic ...