The effect of afferent stimuli on steadily discharging single motor units in man.

The effect of afferent stimuli on the firing pattern of single motor units has been studied in the human first dorsal interosseous muscle of the hand. A reproducible series of changes in the probability of motor unit firing was seen. There was evidence for an excitatory spinal reflex involving median nerve muscle afferent fibres and ulnar nerve motor fibres. Cutaneous and muscle afferent pathways expected to be important in the control of fine movement of the first digit produced an excitatory reflex response at about 70 ms. All sites stimulated in the ipsilateral limb gave rise to a profound reduction in motor unit firing from about 70-130 ms.