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Chapter 10
motor control hierarchy. There the motor neurons to the mus-
cles exit the brainstem or spinal cord. The local level of the
hierarchy includes the afferent neurons, motor neurons, and
the interneurons whose function is related to them. Local-
level neurons determine exactly which motor neurons will
be activated to achieve the desired action and when this will
happen. Note in Figure 10–1 that the descending pathways to
the local level arise only in the sensorimotor cortex and brain-
stem. Other brain areas, notably the basal nuclei (also referred
to as the basal ganglia), thalamus, and cerebellum, exert their
effects on the local level only indirectly, via the descending
pathways from the cerebral cortex and brainstem.
The motor programs are continuously adjusted dur-
ing the course of most movements. As the initial motor pro-
gram begins and the action gets underway, brain regions at
the middle level of the hierarchy continue to receive a constant
stream of updated afferent information about the movements
taking place. Afferent information about the position of the
body and its parts in space is called
proprioception.
Say, for
example, that the sweater you are picking up is wet and heavier
than you expected so that the initially determined strength of
muscle contraction is not suffi cient to lift it. Any discrepan-
cies between the intended and actual movements are detected,
program corrections are determined, and the corrections are
relayed to the local level of the hierarchy and the motor neu-
rons. Refl ex circuits acting entirely at the local level are also
important in refi ning ongoing movements. Thus, many pro-
prioceptive inputs are processed, and infl uence ongoing move-
ments, without ever reaching the level of conscious perception.
Thalamus
Basal nuclei
Sensorimotor cortex
Muscle fibers
Afferent
neurons
Motor neurons
(final common
pathway)
Brainstem and
spinal cord
Brainstem
Higher centers
Cerebellum
Receptors
Motor control
hierarchy
Highest level
Middle level
Local level
Figure 10–1
The conceptual hierarchical organization of the neural systems
controlling body movement. Motor neurons control all the skeletal
muscles of the body. Sensorimotor cortex includes those parts of the
cerebral cortex that act together to control skeletal muscle activity.
The middle level of the hierarchy also receives input from the
vestibular apparatus and eyes (not shown in the fi
gure).
Sensorimotor cortex
Cerebellum
Cerebral cortex
Thalamus
Basal
nuclei
Brainstem
(a)
(b)
Figure 10–2
(a) Side view of the brain showing three of the fi ve components of the middle level of the motor control hierarchy. Figure 10–10 shows details
of the sensorimotor cortex. (b) Cross section of the brain showing the thalamus and basal nuclei.
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