Rehabilitation following brain damage: some neurophysiological mechanisms. Physiological correlates of clinically observed changes in posture and tone following lesions of the central nervous system.

Extensive investigation of the role of the fusimotor system in the production of hypotonic and hypertonic disorders of posture and tone has been undertaken in humans and experimental animals. The data from human studies have usually been from indirect assessment of the fusimotor system, and results are often contradictory. Results are now available from animal studies utilizing direct recording of muscle spindle afferent discharge in a number of models of human disorder. Conditions resulting in hypotonia, e.g. cerebellar ablations, medullary pyramidotomy, VL nucleus, thalamotomy, acute spinal cord transection, and acute motor cortex ablation uniformly result in a depression of muscle spindle primary afferent discharge. Conditions resulting in hypertonia, e.g. chronic spinal cord transection and chronic motor cortex ablation, fail to show heightened muscle spindle afferent discharge, however. Rather the spindle afferent discharge returns to control levels in the models. Recovery from the hypotonic to the hypertonic state is, however, associated with significant recovery of spindle afferent function.