Persisting changes in sensory and motor activity of a muscle following its reflex activation.

A study was made to see if contractions induced by reflex action were followed by an elevation in muscle afferent activity comparable to the post-contraction sensory discharge (PCSD) that occurs after stimulation of the ventral roots. In lightly anesthetized cats, extension reflexes of the triceps surae muscle in the otherwise denervated leg were induced by contralateral stimulation of the popliteal nerve or footpad. The reflex response was followed by an elevated afferent discharge, which disappeared after a brief stretch, as is characteristic also of the PCSD. Crossed-extension responses in fusimotor activity unaccompanied by contraction of the gross muscle were also succeeded by an elevation in sensory discharge and an increased sensitivity to a vibratory stimulus applied to the tendon. Pinna reflexes had a similar effect. In the presence of the elevated discharge, alpha-motoneurons demonstrated increased sensitivity to vibration stimuli. It is suggested that the increase in spindle sensitivity may help stabilize the muscle against minor pertubations in length, at least when background fusimotor activity is relatively quiet.