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The reflex responses of single motor units in human first dorsal interosseous muscle following cutaneous afferent stimulation.

1. Changes in the probability of firing of motor units active during voluntary muscle contraction have been studied in human first dorsal interosseous muscle in response to cutaneous afferent stimulation. 2. Electrical stimulation of the digital nerves of the index finger produces a reflex response consisting of three phases, early excitation followed by inhibition followed by late excitation. 3. Motor units recruited at low levels of voluntary contraction strength had larger early inhibitory and larger late excitatory responses than those recruited at higher levels of force. 4. Taking together the size of the short latency excitatory and inhibitory responses, units recruited at low levels of voluntary contraction strength and with slow twitch contraction times had predominantly inhibitory responses. In contrast, units in which the short latency excitatory responses predominated had fast twitch contraction times and were recruited at high levels of contraction strength. 5. It is concluded that for the cutaneous reflex pathway there are differences in the balance of excitatory and inhibitory sets of interneurones impinging on first dorsal interosseous motoneurones which are related to the kind of muscle unit innervated.