Control of Body Movement
305
grasping. These neurons play an important role in matching
motor signals concerning the pattern of hand action with sig-
nals from the visual system concerning the three-dimensional
features of the objects to be grasped.
During activation of the cortical areas involved in motor
control, subcortical mechanisms also become active. It is to
these areas of the motor control system that we now turn.
Subcortical and Brainstem Nuclei
Numerous highly interconnected structures lie in the brain-
stem and within the cerebrum beneath the cortex, where they
interact with the cortex to control movements. Their infl uence
Begin
To brain
To contralateral
extensor muscle
Contralateral
extensor muscle
contracts
To contralateral
flexor muscle
Contralateral
flexor muscle
relaxes
Motor neuron
to flexor muscles
Ipsilateral
flexor muscle
contracts
Nociceptor
Afferent nerve
fiber from
nociceptor
Ipsilateral
extensor
muscle
relaxes
Motor neuron
to extensor
muscles
Afferent nerve
fiber from
nociceptor
Excitatory
synapse
Inhibitory
synapse
Excitatory
neuromuscular
junction
Neurons ending with:
Figure 10–9
In response to pain detected by nociceptors (Chapter 7), the
ipsilateral fl exor muscle’s motor neuron is stimulated (withdrawal
refl ex). In the case illustrated, the opposite limb is extended
(crossed-extensor refl ex) to support the body’s weight. Arrows
indicate direction of action potential propagation.
is transmitted indirectly to the motor neurons both by path-
ways that ascend to the cerebral cortex and by pathways that
descend from some of the brainstem nuclei.
It is not known to what extent, if any, these structures
initiate movements, but they defi nitely play a prominent role
in planning and monitoring them. Their role is to establish
the programs that determine the specifi c sequence of move-
ments needed to accomplish a desired action. Subcortical
and brainstem nuclei are also important in learning skilled
movements.
Prominent among the subcortical nuclei are the paired
basal nuclei
(see Figure 10–2b), which consist of a closely
related group of separate nuclei. (These structures are often
referred to as
basal ganglia,
but their presence within the
central nervous system makes the term
nuclei
more techni-
cally correct.) They form a link in some of the looping parallel
circuits through which activity in the motor system is trans-
mitted from a specifi c region of sensorimotor cortex to the
basal nuclei, from there to the thalamus, and then back to the
cortical area where the circuit started (review Figure 10–1).
Some of these circuits facilitate movements and others sup-
press them. The importance of the basal nuclei is particularly
apparent in certain disease states, as we discuss next.
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