Disorders of Motor Function
Inability
to move a part of the body is referred to as paralysis. This can be
produced by interruption of motor pathways anywhere between the motor area of
the cerebral cortex and the muscles themselves. We have seen that the pathway
from cortex to muscle involves at least two neurons. The first of these is
located in the cerebral cortex. Its axon terminates in the spinal cord or in motor
cranial nerve nuclei in the brainstem. From a physiological and clinical point
of view this neuron is referred to as the upper motor neuron. The second
neuron is located in the anterior grey column of the spinal cord(or in motor
nuclei of the brainstem) and sends out an axon that travels through a
peripheral nerve to innervate muscle. This neuron is referred to as the lower
motor neuron. (For the present purpose we may ignore interneurons
present in the pathway). When lower motor neurons are destroyed, or their
continuity interrupted, the muscles supplied by them lose their tone (i.e.,
they become flaccid); and in course of time the muscles undergo atrophy.
Changes in electrical responses of the muscles also take place. These
alterations constitute the reaction of degeneration. In
addition, because of interruption of the efferent part of reflex pathways
tendon reflexes are abolished.
Destruction
or interruption of the upper motor neuron is not followed by any of these
changes. On the other hand it is usually accompanied by an increase in muscle
tone, and exaggeration of tendon reflexes. It is, therefore, possible to
distinguish between an upper motor neuron paralysis (often called spastic
paralysis) and a lower motor neuron (or flaccid) paralysis.
Paralysis may be confined to one limb (monoplegia) or to both limbs on one
side of the body (hemiplegia). Paralysis of both lower limbs is called paraplegia,
and that of all four limbs is called quadriplegia. Paralysis of muscles
supplied by one or more cranial nerves may occur in isolation, or in
combination with hemiplegia. Destruction of a particular region often destroys
lower motor neurons situated at that level resulting in a localised flaccid
paralysis of muscles supplied from that level. This lesion may at the same time
interrupt descending tracts (representing upper motor neurons), resulting in a
spastic paralysis below the level of the lesion. The presence of a localised
flaccid paralysis can thus serve as apointer to the level of lesion.
It is
also important to remember that the fibres of upper motor neurons meant for the
limbs cross the midline in the lower part of the medulla (in the decussation of
the pyramids); and those for the cranial nerves cross just above the level of
their termination. A lesion above the level of crossing produces a paralysis in
the opposite half of the body, and a lesion below this level produces paralysis
on the same side.
Keeping
the considerations discussed above in mind we may now consider the effects of
lesions of the motor pathways at various levels.
1.
Because of the large extent of the motor areas of the cerebral
cortex lesions here produce a relatively localised paralysis e.g., a
monoplegia.
2.
A lesion in the internal capsule is capable of
producing widespread paralysis on the opposite half of the body (hemiplegia)
which may also involve the lower part of the face and the tongue. (The cranial
nerves having a bilateral corticonuclear supply are spared). A lesion in the
internal capsule is most likely to result from thrombosis or rupture of one of
the arteries supplying the capsule. The artery most often involved is Charcot’s
artery of cerebral haemorrhage.
3.
Lesions of corticospinal fibres at various levels in the brainstem,
above the level of the pyramidal decussation, can produce contralateral
hemiplegia. If the lesion crosses the midline symptoms can be bilateral.
Involvement of motor cranial nerve nuclei (or of fibres arising from them) may
result in various combinations. For example, a lesion in the upper part of the
midbrain can produce a paralysis of muscles supplied by the oculomotor nerve on
the side of lesion, along with a hemiplegia on the opposite side (Weber’s
syndrome). A similar lesion in the pons, results in a paralysis of the
lateral rectus muscle (abducent nerve) on the side of lesion with hemiplegia on
the opposite side (Raymond’ssyndrome). Alternatively, facial paralysis of one side
can be combined with contralateral hemiplegia(Millard Gubler syndrome).
Various such combinations may result depending on the level of lesion.
4.
Lesions affecting the lateral corticospinal tract in the
spinal cord produce an upper motor neuron paralysis of muscles on the same side
of the body. Lesions above the fifth cervical segment result in paralysis of
both upper and lower extremities; while lesions below the first thoracic
segment affect only the lower limbs. As in the brainstem, lesions in the spinal
cord may be bilateral. Involvement of lower motor neurons at the level of
lesion produces a flaccid paralysis of muscles supplied from that level, along
with spastic paralysis below the level of injury. A knowledge of muscles
supplied by individual spinal segments can thus help in locating the level of a
lesion in the spinal cord.
In
some diseases (e.g., poliomyelitis) damage may be confined
to lower motor neurons, and the resulting paralysis may be purely flaccid. Such
lesions are accompanied by muscular wasting , muscle twitchings (fasciculation)
and contracture of opposing muscles.
Lesions
of extrapyramidal tracts are marked by great rgidity (of the clasp knife type).
The rigidity leads to greatly reduced mobility. Tendon reflexes are
exaggerated.
In
the above description we have (for sake of simplicity) considered involvement
of motor neurons in isolation. However, disease at any level can involve other
structures resulting in sensory and other disturbances.
Rigidity
Muscle
tone has been described. When tone is excessive the body becomes rigid.
Distinctive types of rigidity are recognised. (a) If we try to flex a rigid
limb, and there is sudden loss of resistance, the rigidity is described as
clasp knife rigidity. (b) When the resistance is uniform over the range of
movement the condition is called lead pipe rigidity. When resistance is
intermittent the rigidity is said to be of the cog wheel type.
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