Nonpyramidal Motor Systems and Functional Recovery After Damage to the Central Nervous System

Abstract

A number of clinical observations indicate that residual motor function can be retained even after extensive damage to neocortical motor systems in humans: (a) Most patients with hemiplegia regain the capability to stand and to walk with a circumductive gait. Similarly, after unilateral damage to the lateral corticospinal tract, paresis of the leg often resolves and the patient recovers the capability to walk. This recovery occurs, in part, on the basis of bilateral innervation of motor neurons by the contralateral (undamaged) lateral corticospinal tract. (b) In the pseudobulbar state, after bilateral damage to corticobulbar pathways, facial musculature cannot be voluntarily activated. Nevertheless, these facial muscles can be sequentially activated when facial expression is elicited by nonverbal stimuli. This residual response is mediated by alternative pathways. (c) Although patients with apraxia or Wernicke's aphasia owing to cortical lesions are unable to correctly respond to appendicular commands, they retain the capability to respond appropriately to commands involving axial musculature. This dissociation of axial from appendicular motor control demonstrates the retention, in these patients, of an axial motor system that plays a role in orientation and the maintenance of posture. (d) Apraxic motor responses often include fragments of correct responses that involve the activation of proximal limb musculature in a nearly correct manner. Despite the loss of ability to properly activate distal (pyramidal-innervated) muscles in these patients, the ability to utilize proximal (nonpyramidal-innervated) musculature is retained. These observations provide evidence, in humans, for a hierarchical organization of motor control involving (a) a neocortical (pyramidal) motor system mediating high-resolution, fractionated movement of distal parts of the extremities, superimposed on (b) a nonpyramidal motor system innervating proximal musculature bilaterally. This nonpyramidal motor system mediates postural, body steering, and orienting movements. These tiered motor systems can be damaged separately, so that nonpyramidal motor mechanisms can be observed to operate, in isolation from the pyramidal system, after injury to the central nervous system. Residual nonpyramidal movement, after damage to the motor cortex, may have considerable value in rehabilitation from neurological disease.

Keywords

Nonpyramidal motor system —Central nervous system injury —Rehabilitation.

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