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U N I T 1 0
Nervous System
medial surface of the hemisphere in the premotor region
(areas 6 and 8). It is intimately involved in the perfor-
mance of complex, skillful movements that involve both
sides of the body.
The Cerebellum and Basal Ganglia
In addition to the brain stem and cerebral cortex, the
cerebellum and basal ganglia assist in the planning and
execution of motor movements. They provide feedback
circuits that regulate cortical and brain stem motor
areas, and they receive indirect input from various areas
of the motor cortex.
2
The cerebellum and basal ganglia
do not send significant output directly to the spinal cord,
but act on motor neurons in the brain stem.
Although the precise contributions of the cerebellum
and basal ganglia are still unclear, both are necessary for
smooth movement and posture. The basal ganglia pro-
vide the gracefulness of performance as well as the sup-
portive posture for highly skilled movements. Cerebellar
circuits are involved with the timing and coordination
of movements that are in progress and with learning
of motor skills. Damage to the cerebellum by vascular
lesions of certain familial degenerative disorders pro-
duces cerebellar ataxia, a characteristic loss of coordi-
nation and accuracy of limb movement.
The Motor Unit
The motor neurons in the spinal cord represent the final
pathway for integration of motor function.
2,3
Without the
special neuronal circuits in the spinal cord, even the most
complex motor control systems in the brain could not
cause any purposeful muscle movement. Located in each
segment of the anterior horn of the spinal cord are sev-
eral thousand motor neurons called
lower motor neurons
(LMNs). Upper motor neurons (UMNs) control LMNs.
They project from the motor strip in the cerebral cortex
to the anterior horn of spinal cord and are fully contained
in the central nervous system (CNS) (Fig. 36-3).
Motor neurons segregate into two major categories:
alpha
and
gamma.
The alpha motor neurons are large
nerve fibers that innervate the extrafusal muscle fibers
responsible for muscle contraction and force generation.
The much-smaller gamma motor neurons innervate the
intrafusal muscle fibers of the muscle spindles, which
help control basic muscle tone (to be discussed under
spinal reflexes). Each alpha motor neuron undergoes
multiple branchings, making it possible for a single
alpha motor neuron to innervate a few to thousands of
muscle fibers. In general, large muscles—those contain-
ing hundreds or thousands of muscle fibers and provid-
ing gross motor movement—have large motor units.
This contrasts sharply with those that control the hand,
tongue, and eye movements, for which the motor units
are small and permit very precise control.
The alpha motor neuron and the group of muscle
fibers it innervates is called a
motor unit.
When an
alpha
motor neuron develops an action potential, all of the
muscle fibers in the motor unit it innervates develop
action potentials, causing them to contract simultane-
ously. Thus, a motor neuron and the muscle fibers it
innervates function as a single unit—the basic unit of
motor control.
Spinal Reflexes
The spinal cord contains neural reflexes or coordinated,
involuntary motor responses that are initiated by a stim-
ulus applied to peripheral receptors.
3,5
Some reflexes,
such as the flexor-withdrawal reflex, initiate movements
to avoid hazardous situations, whereas others, such as
the stretch reflex or crossed-extensor reflex, serve to
integrate motor movements so they function in a coor-
dinated manner. The anatomic basis of a reflex consists
of an afferent neuron that synapses either directly with
an effector neuron that innervates a muscle or with
an interneuron that synapses with an effector neuron.
Reflexes are essentially “wired into” the CNS so that
they are always ready to function. With training, most
reflexes can be modified to become part of more com-
plicated movements. A reflex may involve neurons in a
single cord segment (i.e., segmental reflexes), several or
many segments (i.e., intersegmental reflexes), or struc-
tures in the brain (i.e., suprasegmental reflexes).
In most cases, reflex activities go on without our con-
scious awareness. There is a significant amount of reflex
circuitry in the spinal cord for the coordinated control
of movements, particularly stereotyped movements con-
cerned with locomotion. Many of these reflexes work
equally well in decerebrate animals (those in which the
brain has been destroyed) as long as the spinal cord is
intact. Other spinal reflexes require the activity of the
brain for their successful completion.
Toes
Ankle
Knee
Hip
Trunk
Shoulder
Elbow
Wrist
Hand
Little
Ring
Middle
Index
Thumb
Neck
Brow
Eyelid and eyeball
Face
Lips
Jaw
Tongue
Swallowing
M
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FIGURE 36-2.
Representation of the relative extent of motor
cortical area 4 devoted to muscles of the various body regions.
Medial surface is at the left, lateral fissure is at the right, with
pharyngeal and laryngeal muscle representation extending
toward the insula. (From Penfield E, RasmussenT. The Cerebral
Cortex in Man: A Clinical Study of Localization of Function.
NewYork: Macmillan; 1968.)
