Responses of constantly firing motor units to afferent stimulation.

Constantly firing motor units of the short abductor muscles of the first finger in the human hand and the abdominal wall muscle in immobilized rats responded to afferent stimulation of the median and sciatic nerves respectively with changes in the nature of spike activity. In the first 250 msec of the post-stimulus period, the frequency of motor unit spike activity became unstable and peri-stimulus histograms were individually quite distinct. This was followed by relative stabilization of motor unit discharge frequencies, and the subsequently (750 msec) determined motor unit spike frequency depended on most cases on the background spike frequency.