304
Chapter 10
(
Figure 10–9
). The same stimulus causes just the opposite
response in the
contralateral
leg (on the opposite side of the
body from the stimulus). Motor neurons to the extensors are
activated while the fl exor muscle motor neurons are inhibited.
This
crossed-extensor refl
ex
enables the contralateral leg to
support the body’s weight as the injured foot is lifted by fl ex-
ion (see Figure 10–9). This concludes our discussion of the
local level of motor control.
The Brain Motor Centers and the
Descending Pathways They Control
We now turn our attention to the motor centers in the brain
and the descending pathways that direct the local control sys-
tem (review Figure 10–1).
Cerebral Cortex
The cerebral cortex plays a critical role in both the plan-
ning and ongoing control of voluntary movements, func-
tioning in both the highest and middle levels of the motor
control hierarchy. The term
sensorimotor cortex
is used
to include all those parts of the cerebral cortex that act
together to control muscle movement. A large number of
nerve fi bers that give rise to descending pathways for motor
control come from two areas of sensorimotor cortex on the
posterior part of the frontal lobe: the
primary motor cor-
tex
(sometimes called simply the
motor cortex
) and the
premotor area
(
Figure 10–10
). The neurons of the motor
cortex that control muscle groups in various parts of the
body are arranged anatomically into a
somatotopic map,
as shown in
Figure 10–11
.
Other areas of sensorimotor cortex include the
supple-
mentary motor cortex,
which lies mostly on the surface on
the frontal lobe where the cortex folds down between the two
hemispheres (see Figure 10–10b), the
somatosensory cor-
tex,
and parts of the
parietal-lobe association cortex
(see
Figures 10–10a and b).
Although these areas are anatomically and function-
ally distinct, they are heavily interconnected, and individual
muscles or movements are represented at multiple sites. Thus,
the cortical neurons that control movement form a neural net-
work, meaning that many neurons participate in each single
movement. In addition, any one neuron may function in more
than one movement. The neural networks can be distributed
across multiple sites in parietal and frontal cortex, including
the sites named in the preceding two paragraphs. The interac-
tion of the neurons within the networks is fl
exible so that the
neurons are capable of responding differently under different
circumstances. This adaptability enhances the possibility of
integrating incoming neural signals from diverse sources and
the fi nal coordination of many parts into a smooth, purpose-
ful movement. It probably also accounts for the remarkable
variety of ways in which we can approach a goal. For example,
you can comb your hair with the right hand or the left, start-
ing at the back of your head or the front. This same adapt-
ability also accounts for some of the learning that occurs in all
aspects of motor behavior.
We have described the various areas of sensorimotor
cortex as giving rise, either directly or indirectly, to pathways
descending to the motor neurons. However, these areas are
not the prime initiators of movement, and other brain areas
are certainly involved. We currently don’t know exactly where
or how intentional movements are initiated.
Association areas of the cerebral cortex also play a role
in motor control. For example, neurons of parietal associa-
tion cortex are important in the visual control of reaching and
Figure 10–8
Neural pathways underlying the Golgi tendon organ component of
the local control system. In this diagram, contraction of the extensor
muscles causes tension in the Golgi tendon organ and increases the
rate of action potential fi ring in the afferent nerve fi ber. By way of
interneurons, this increased activity results in (path A) inhibition of
the motor neuron of the extensor muscle and its synergists and (path
B) excitation of the fl exor muscles’ motor neurons. Arrows indicate
the direction of action potential propagation.
Figure 10–8
physiological
inquiry
Explain how the Golgi tendon organ protects against excessive
force exertion that might tear a muscle or tendon.
Answer can be found at end of chapter.
Begin
Afferent
nerve
fiber from
Golgi tendon
organ
Extensor
muscle
Kneecap
(bone)
Extensor
muscle
tendon with
Golgi
tendon
organ
Flexor muscle
Spinal
cord
A
B
Motor neuron to
extensor muscles
Motor neuron to
flexor muscles
Excitatory
synapse
Inhibitory
synapse
Excitatory
neuromuscular
junction
Neurons ending with:
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