Control of Body Movement
299
If a complex movement is repeated often, learning takes
place and the movement becomes skilled. Then, the initial
information from the middle hierarchical level is more accu-
rate, and fewer corrections need to be made. Movements per-
formed at high speed without concern for fi ne control are
made solely according to the initial motor program.
Table 10–1
summarizes the structures and functions of
the motor control hierarchy.
Voluntary and Involuntary Actions
Given such a highly interconnected and complicated neuroana-
tomical basis for the motor system, it is diffi
cult to use the
phrase
voluntary movement
with any real precision. We will
use it, however, to refer to actions that have the following char-
acteristics: (1) the movement is accompanied by a conscious
awareness of what we are doing and why we are doing it, and
(2) our attention is directed toward the action or its purpose.
The term
involuntary,
on the other hand, describes actions
that do not have these characteristics.
Unconscious, automatic,
and
refl
ex
often serve as synonyms for
involuntary,
although in
the motor system the term
refl
ex
has a more precise meaning.
Despite our attempts to distinguish between voluntary
and involuntary actions, almost all motor behavior involves
both components, and we cannot easily make a distinction
between the two. For example, even such a highly conscious
act as walking involves many refl exive components, as the pat-
tern of contraction of leg muscles is subconsciously varied to
adapt to obstacles or uneven terrain.
Thus, most motor behavior is neither purely voluntary
nor purely involuntary, but falls somewhere between these
two. Moreover, actions shift along this continuum accord-
ing to the frequency with which they are performed. When a
person fi rst learns to drive a car with a manual transmission,
for example, shifting gears requires a great deal of conscious
attention. With practice, those same actions become auto-
matic. On the other hand, refl ex behaviors, which are all the
way at the involuntary end of the spectrum, can with special
effort sometimes be voluntarily modifi ed or even prevented.
We now turn to an analysis of the individual components
of the motor control system. We will begin with local control
mechanisms because their activity serves as a base upon which
the descending pathways exert their infl uence. Keep in mind
throughout these descriptions that motor neurons always
form the fi nal common pathway to the muscles.
Local Control of Motor Neurons
The local control systems are the relay points for instructions to
the motor neurons from centers higher in the motor control hier-
archy. In addition, the local control systems play a major role in
adjusting motor unit activity to unexpected obstacles to move-
ment and to painful stimuli in the surrounding environment.
To carry out these adjustments, the local control systems
use information carried by afferent fi bers from sensory recep-
tors in the muscles, tendons, joints, and skin of the body parts
to be moved. As noted earlier, the afferent fi bers also transmit
information to higher levels of the hierarchy.
Interneurons
Most of the synaptic input to motor neurons from the descend-
ing pathways and afferent neurons does not go directly to
motor neurons, but rather to interneurons that synapse with
the motor neurons. Interneurons comprise 90 percent of spinal
cord neurons, and they are of several types. Some are near the
motor neuron they synapse upon and thus are called local inter-
neurons. Others have processes that extend up or down short
distances in the spinal cord and brainstem, or even throughout
much of the length of the central nervous system. The inter-
neurons with longer processes are important for integrating
complex movements such as stepping forward with your left
foot as you throw a baseball with your right arm.
The interneurons are important elements of the local
level of the motor control hierarchy, integrating inputs not
only from higher centers and peripheral receptors but from
other interneurons as well (
Figure 10–3
). They are crucial
in determining which muscles are activated and when. This
is especially important in coordinating repetitive, rhyth-
mic activities like walking or running, for which spinal cord
interneurons encode pattern generator circuits responsible for
activating and inhibiting limb movements in an alternating
sequence. Moreover, interneurons can act as “switches” that
enable a movement to be turned on or off under the com-
mand of higher motor centers. For example, if you pick up a
Table 10–1
Conceptual Motor Control Hierarchy
for Voluntary Movements
I. Higher centers
a. Function: form complex plans according to
individual’s intention and communicates with the
middle level via command neurons.
b. Structures: areas involved with memory and
emotions, supplementary motor area, and association
cortex. All these structures receive and correlate input
from many other brain structures.
II. The middle level
a. Function: converts plans received from the highest
level to a number of smaller motor programs that
determine the pattern of neural activation required
to perform the movement. These programs are
broken down into subprograms that determine the
movements of individual joints. The programs and
subprograms are transmitted through descending
pathways to the local control level.
b. Structures: sensorimotor cortex, cerebellum, parts of
basal nuclei, some brainstem nuclei.
III. The local level
a. Function: specifi es tension of particular muscles and
angle of specifi c joints at specifi c times necessary to
carry out the programs and subprograms transmitted
from the middle control levels.
b. Structures: levels of brainstem or spinal cord from
which motor neurons exit.
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