Recurrent inhibition of alpha-motoneurons in patients with upper motor neuron lesions.

The recurrent inhibition of alpha-motoneurons by Renshaw cells, brought about by a conditioning reflex discharge, was estimated by measuring the amplitude of a test reflex which involved only those soleus motoneurons which had already fired in response to the conditioning volley. The results obtained in 95 patients with upper motor neuron disease and in 31 normal subjects were compared. At rest, there was no evidence for a decrease in the excitability of Renshaw cells: on the contrary, recurrent inhibition, if it changed at all, was increased in 55 out of 95 patients. In most patients the changes in Renshaw cells excitability which occur during postural or voluntary contractions in normal subjects were not found. We conclude that changes in recurrent inhibition are not responsible for the hyperexcitability of the stretch reflex which characterizes human spasticity. On the other hand, the paralysis of the supraspinal control of Renshaw cells, which normally accompanies voluntary movements, could account in part for two difficulties exhibited by spastic patients: in grading the strength of a muscular contraction, and in regulating reciprocal Ia inhibition according to the requirements of a voluntary movement.