Some spinal mechanisms of negative motor phenomena in humans.

A sudden pause in motoneuron firing can occur (1) because many motoneurons have just discharged, (2) because motoneurons have been directly inhibited, or (3) because there has been a pause in the tonic drive responsible for their activity (Fig. 9) If there is a preceding burst of EMG activity, the silent period may be simply a consequence of inhibitory mechanisms activated by the motoneuron discharge-as from the AHP, Renshaw cell activity, or inhibitory effects from afferents excited by the muscle contraction. If there is no preceding burst of EMG activity, the silent period may be due to direct inhibition of motoneurons. If so, the H-reflex will be inhibited at the time of the silent period with the subject at rest. This inhibition could result from one of the many spinal inhibitory systems. Inhibition from Ia reciprocal inhibitory interneurons may be distinguished from inhibition from interneurons in Ib, group II, or cutaneous afferent pathways by the distribution of their actions to various motoneuron pools. If there is no preceding burst of EMG activity and the H-reflex is not inhibited at the time of the silent period with the subject at rest, then the silent period results from a pause in the tonic drive to motoneurons.