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

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Abstract
Changes in the probability of firing of motor units active during voluntary muscle contraction were studied in human 1st and 2nd dorsal interosseous muscles in response to muscle afferent stimulation. Short latency, possible monoxynaptic, excitatory responses were found in both 1st and 2nd dorsal interosseous muscles following brief mechanical pulses applied to the belly of 1st dorsal interosseous. A 2nd longer latency excitatory response followed the 1st with a mean latency of 25 ms. 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 2 motor neuron pools appears to be weighted in the same way as the more complicated input associated with a gradually increasing voluntary contraction. 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.