Control of Body Movement
297
Motor Control Hierarchy
Throughout the nervous system, the neurons involved in con-
trolling skeletal muscles can be thought of as being organized in
a hierarchical fashion, with each level of the hierarchy having a
certain task in motor control (
Figure 10–1
). To begin a move-
ment, a general intention such as “pick up sweater” or “write sig-
nature” or “answer telephone” is generated at the highest level of
the motor control hierarchy. These higher centers include many
regions of the brain, including those involved in memory, emo-
tions, and motivation. Very little is known, however, as to exactly
where intentions for movement form in the brain.
Information is relayed from these higher-center “com-
mand” neurons to parts of the brain that make up the middle
level of the motor control hierarchy. The middle-level struc-
tures specify the individual postures and movements needed
to carry out the intended action. In our example of picking
up a sweater, structures of the middle hierarchical level coor-
dinate the commands that tilt the body and extend the arm
and hand toward the sweater and shift the body’s weight to
maintain balance. The middle-level hierarchical structures are
located in parts of the cerebral cortex as well as in the cerebel-
lum, subcortical nuclei, and brainstem (see Figure
10–1 and
Figure 10–2a and b
). These structures have extensive inter-
connections, as the arrows in Figure 10–1 indicate.
As the neurons in the middle level of the hierarchy
receive input from the command neurons, they simultaneously
receive afferent information from receptors in the muscles,
tendons, joints, and skin, as well as from the vestibular appa-
ratus and eyes. These afferent signals relay information to the
middle-level neurons about the starting positions of the body
parts that are “commanded” to move. They also relay infor-
mation about the nature of the space just outside the body
in which a movement will take place. Neurons of the middle
level of the hierarchy integrate all of this afferent information
with the signals from the command neurons to create a
motor
program
—defi ned as the pattern of neural activity required
to properly perform the desired movement. Impairment of a
person’s sensory pathways also results in slow and uncoordi-
nated voluntary movement.
The information determined by the motor program is
transmitted via
descending pathways
to the local level of the
The building blocks for these movements—as for all movements—are
motor units,
each comprising one
motor neuron together with all the skeletal muscle fi bers that this neuron innervates (Chapter 9). The motor
neurons are the fi nal common pathway out of the central nervous system because all neural infl uences on
skeletal muscle converge on the motor neurons and can only affect skeletal muscle through them.
All the motor neurons that supply a given muscle make up the
motor neuron pool
for the muscle. The
cell bodies of the pool for a given muscle are close to each other either in the ventral horn of the spinal
cord or in the brainstem.
Within the brainstem or spinal cord, the axons of many neurons synapse on a motor neuron to control
its activity. Although no single input to a motor neuron is essential for contraction of the muscle fi bers it
innervates, a balanced input from all sources is necessary to provide the precision and speed of normally
coordinated actions. For example, if inhibitory synaptic input to a given motor neuron is decreased, the
still-normal excitatory input to that neuron will be unopposed and the motor neuron fi ring will increase,
leading to excessive contraction. This is exactly what happens in the disease
tetanus
.
It is important to realize that movements—even simple movements such as fl exing a fi nger—are rarely
achieved by just one muscle. All body movements are achieved by activation, in a precise sequence, of
many motor units in various muscles.
This chapter deals with the interrelated neural inputs that converge upon motor neurons to control
their activity. We fi rst present a general model of how the motor system functions and then describe
each component of the model in detail. Keep in mind throughout this chapter that many of the
contractions that skeletal muscles execute, particularly the muscles involved in postural support, are
isometric (Chapter 9). Even though the muscle is active during these contractions, no movement occurs.
In the following discussions, the general term
muscle movement
includes these isometric contractions.
In addition, remember that all information in the nervous system is transmitted in the form of graded
potentials or action potentials.
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