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.
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.