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U N I T 1 0
Nervous System
contractions of all the muscle fibers in a motor unit due
to irritation and hyperexcitability of the cell body and
its motor neuron, and suggest LMN disease.
Spinal Reflex Activity
Testing of deep tendon reflexes (see Understanding
the Stretch Reflex and Muscle Tone) can provide
important information about the status of the CNS in
controlling muscle function. Hyperactive reflexes are
suggestive of a UMN disorder.
Clonus
is the rhythmic
contraction and alternate relaxation of a limb that
is caused by suddenly stretching a muscle and gently
maintaining it in the stretched position. It is seen in the
hypertonia of spasticity associated with UMN lesions,
such as spinal cord injury.
Hyporeflexia
or
areflexia
suggests the presence of an LMN lesion. The distribu-
tion of abnormality in the reflexes is also helpful in
determining the location of the lesion. For example,
hyperreflexia in both lower extremities would suggest
a lesion in the spinal cord, whereas hyperreflexia on
one side of the body would suggest a lesion in the
UMN along the motor pathway (e.g., in the motor
cortex or internal capsule).
Coordination of Movement
Coordination of muscle movement requires that four
areas of the nervous system function in an integrated
manner—the motor system for muscle strength, the
cerebellar system for rhythmic movement and steady
posture, the vestibular system for posture and balance,
and the sensory system for position sense.
In cerebellar disease, one movement cannot be fol-
lowed quickly by its opposite movement. Movements
are slow, irregular, clumsy, unsteady, and inap-
propriately varying in their speed, force, and direc-
tion.
Dysdiadochokinesia
is the failure to accurately
perform rapid alternating movements.
Ataxia
is a
term used to describe a wide-based, unsteady gait.
Dysmetria
is a term used to describe inaccuracies of
movements leading to a failure to reach a specified
target. This can be tested by having the person touch
the examiner’s finger and then alternately touch his
or her own finger. These movements are normally
smooth and accurate. Asking the person to touch the
examiner’s finger with an outstretched arm and finger,
first with the eyes open and then closed, provides a
test for position sense. Repetitive and consistent devi-
ation to one side (referred to as past pointing), which
is worse with the eyes closed, suggests cerebellar or
vestibular disease.
Chorea
(abnormal writhing movements),
dystonia
(abnormal simultaneous contractions of agonist and
antagonist muscles, leading to abnormal postures),
tremor
(rhythmic movements of a particular body part),
bradykinesia
(slowness of movements), and
myoclonus
(involuntary jerking movement) indicate abnormalities
in the basal ganglia, although the exact localization may
be difficult to determine.
Disorders of the Motor Unit
Most of the diseases of the motor unit cause weakness
and wasting of skeletal muscles. The distinguishing fea-
tures of these diseases vary depending on which of the
components of the motor unit is primarily affected—the
cell body of the motor neuron, its axon, the neuromus-
cular junction, or the muscle fibers.
7–9
Disorders affect-
ing the nerve cell body are often referred to as
LMN
disorders,
those affecting the nerve axon as
peripheral
neuropathies,
and primary disorders affecting the mus-
cle fibers as
myopathies.
SUMMARY CONCEPTS
■■
Motor function, whether it involves walking,
running, or precise finger movements, requires
functioning neural pathways consisting of upper
motor neurons (UMNs) that project from the
motor cortex to the brain stem or spinal cord,
where they innervate the lower motor neurons
(LMNs) of the contracting muscles.
■■
Proper control of muscle function requires
not only excitation of the muscle by the LMNs
located in the spinal cord, but also the function of
reflex circuitry that monitors the functional status
of the muscle fibers on a moment-by-moment
basis.The muscle spindles of the stretch reflex
function to monitor and correct for changes in
muscle length when extrafusal fibers are either
shortened (by contraction) or lengthened (by
stretch).
■■
Assessments of muscle strength and muscle
bulk, muscle tone and motor reflexes, and
patterns of motor movement and posture provide
the means for determining the location of
disorders of motor function. Paresis (weakness)
and paralysis (loss of muscle movement) reflect
a loss of muscle strength. UMN lesions tend
to produce spastic paralysis, and LMN lesions
flaccid paralysis. Changes in muscle bulk are
characterized by a loss of muscle mass (atrophy)
or an increase in muscle mass (hypertrophy).
Hypotonia is a condition of less-than-normal
muscle tone, and hypertonia or spasticity is
a condition of excessive tone. Abnormal and
uncoordinated movements and postures are
suggestive of a cerebellar or basal ganglia
pathologic process.
