Single motor units, i.e., anterior horn cells plus the muscle fibers they innervate, have recently been placed under direct observation while responding to a normal reflex stimulus. Details such as the normal rate of discharge, latent period, refractory period, influence of fatigue, etc., have been carefully studied. The work of Denny-Brown, Adrian and Bronk, Eccles and Sherrington have all indicated that the natural rate of discharge of the anterior horn cell is slow, i.e., 5 to 25 per sec., and never more than 80 to 90 per sec. under intense stimulation. A motor unit discharging at 10 per sec. may continue in activity for indefinite periods of time without fatigue. Tonic responses are maintained by such rates of discharge and therefore no special tonic mechanisms need be postulated to explain the absence of fatigue.