The reflex responses of single motor units in human hand muscles following muscle afferent stimulation.

1. Changes in the probability of firing of motor units active during voluntary muscle contraction have been studied in human first and second dorsal interosseous muscles in response to muscle afferent stimulation. 2. Short latency, possibly monosynaptic, excitatory responses were found in both first and second dorsal interosseous muscles following brief mechanical pulses applied to the belly of first dorsal interosseous. A second longer latency excitatory response followed the first with a mean latency of 25 msec. 3. All units in both muscles showed qualitatively the same responses but units recruited at low levels of voluntary contraction strength were more responsive to muscle afferent input than units recruited at high contraction strengths. The excitatory effect of muscle afferent input within the two motoneurone pools appears to be weighted in the same way as the more complicated input associated with a gradually increasing voluntary contraction. 4. These findings are discussed in relation to the relative importance of short and long latency reflex effects of muscle afferent input in the control of hand movements.