Control of Body Movement
309
axons of single corticospinal neurons diverge markedly to syn-
apse with a number of different motor neuron populations
at various levels of the spinal cord, thereby ensuring that the
motor cortex can coordinate many different components of a
movement.
This apparent “blurriness” of control is surprising when
you think of the delicacy with which you can move a fi ngertip,
because it is the corticospinal pathways that control rapid, fi ne
movements of the distal extremities, such as those you make
when you manipulate an object with your fi ngers. After dam-
age occurs to the corticospinal pathways, all movements are
slower and weaker, individual fi nger movements are absent,
and it is diffi cult to release a grip.
Brainstem Pathways
Axons from neurons in the brainstem also form pathways that
descend into the spinal cord to infl uence motor neurons. These
pathways are sometimes referred to as the
extrapyramidal
system,
or indirect pathways, to distinguish them from the
corticospinal (pyramidal) pathways. However, no general term
is widely accepted for these pathways, and for convenience we
will refer to them collectively as the brainstem pathways.
Axons of some of the brainstem pathways cross from
their side of origin in the brainstem to affect muscles on the
opposite side of the body, but most remain uncrossed (see
Figure 10–12). In the spinal cord, the fi bers of the brainstem
pathways descend as distinct clusters, named according to
their sites of origin. For example, the vestibulospinal pathway
descends to the spinal cord from the vestibular nuclei in the
brainstem, whereas the reticulospinal pathway descends from
neurons in the brainstem reticular formation.
The brainstem pathways are especially important in con-
trolling muscles of the trunk for upright posture, balance, and
walking.
Concluding Comments on the Descending Pathways
As stated previously, the corticospinal neurons generally have
their greatest infl uence over motor neurons that control mus-
cles involved in fi ne, isolated movements, particularly those of
the fi ngers and hands. The brainstem descending pathways,
in contrast, are involved more with coordination of the large
muscle groups used in the maintenance of upright posture, in
locomotion, and in head and body movements when turning
toward a specifi c stimulus.
There is, however, much interaction between the descend-
ing pathways. For example, some fi bers of the corticospinal
pathway end on interneurons that play important roles in pos-
ture, whereas fi bers of the brainstem descending pathways
sometimes end directly on the alpha motor neurons to control
discrete muscle movements. Because of this redundancy, one
system may compensate for loss of function resulting from
damage to the other system, although the compensation is
generally not complete.
The distinctions between the corticospinal and brain-
stem descending pathways are not clear-cut. All movements,
whether automatic or voluntary, require the continuous coor-
dinated interaction of both types of pathways.
Muscle Tone
Even when a skeletal muscle is relaxed, there is a slight and
uniform resistance when it is stretched by an external force.
This resistance is known as
muscle tone,
and it can be an
important diagnostic tool for clinicians assessing a patient’s
neuromuscular function.
Muscle tone is due both to the passive elastic properties
of the muscles and joints and to the degree of ongoing alpha
motor neuron activity. When a person is deeply relaxed, the
alpha motor neuron activity probably makes little contribution
to the resistance to stretch. As the person becomes increas-
ingly alert, however, more activation of the alpha motor neu-
rons occurs and muscle tone increases.
Abnormal Muscle Tone
Abnormally high muscle tone, called
hypertonia,
accompa-
nies a number of disease processes and is seen very clearly
when a joint is moved passively at high speeds. The increased
resistance is due to a greater-than-normal level of alpha motor
neuron activity, which keeps a muscle contracted despite the
person’s attempt to relax it. Hypertonia usually occurs with
disorders of the descending pathways that normally inhibit the
motor neurons.
Clinically, the descending pathways and neurons of the
motor cortex are often referred to as the
upper motor neu-
rons
(a confusing misnomer because they are not really motor
neurons at all). Abnormalities due to their dysfunction are
classed, therefore, as
upper motor neuron disorders
.
Thus,
hypertonia indicates an upper motor neuron disorder. In this
clinical classifi cation, the alpha motor neurons—the true
motor neurons—are termed
lower motor neurons.
Spasticity
is a form of hypertonia in which the muscles
do not develop increased tone until they are stretched a bit,
and after a brief increase in tone, the contraction subsides for
a short time. The period of “give” occurring after a time of
resistance is called the
clasp-knife phenomenon
.
(When an
examiner bends the limb of a patient with this condition, it
is like folding a pocket knife—at fi rst, the spring resists the
bending motion, but once bending begins, it closes easily.)
Spasticity may be accompanied by increased responses of
motor refl exes such as the knee jerk, and by decreased coordi-
nation and strength of voluntary actions.
Rigidity
is a form
of hypertonia in which the increased muscle contraction
is continual and the resistance to passive stretch is constant
(as occurs in the disease tetanus, which is described in detail
at the end of this section). Two other forms of hypertonia
that can occur suddenly in individual or multiple muscles are
spasms,
which are brief contractions, and
cramps,
which are
prolonged and painful.
Hypotonia
is a condition of abnormally low muscle tone,
accompanied by weakness, atrophy (a decrease in muscle bulk),
and decreased or absent refl ex responses. Dexterity and coor-
dination are generally preserved unless profound weakness is
present. While hypotonia may develop after cerebellar disease,
it more frequently accompanies disorders of the alpha motor
neurons (lower motor neurons), neuromuscular junctions, or
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