Evidence for strong synaptic coupling between single tactile afferents andmotoneurones supplying the human hand

1. Electrical stimulation of digital nerves elicits short-latency excitator y and inhibitory spinal reflex responses in ongoing EMG in muscles acting o n the fingers and thumb. Similar responses are elicited by stimulating a po pulation of muscle spindles but not when a single muscle spindle is activat ed. The current study investigated whether short-latency EMG responses coul d be evoked from the discharge of a single cutaneous afferent. 2. Thirty-three tactile afferents were recorded via tungsten microelectrode s in the median nerve of awake humans. Spike-triggered averaging revealed E MG events time-locked to the afferent discharge. The afferents were activat ed by an external probe and the EMG was elicited by a weak voluntary contra ction. 3. Eleven cutaneous afferents (33%) showed a short-latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven af ferents (onset latency 20.0-41.1 ms). For four slowly adapting (XA) type II afferents, EMG showed a periodicity that was correlated to the afferent in terspike interval (r = 0.99). 4. The EMG associated with two rapidly adapting (FA) type I afferents (29%) showed a short-latency excitation while five showed neither excitation nor inhibition. Seven XA II afferents (39 %) showed excitation and 11 no respo nse; and none of the six XA I afferents showed any response. 5. We conclude that, unlike muscle spindle afferents, the input from a sing le cutaneous afferent is strong enough to drive, via interneurones, motoneu rones supplying muscles acting on the digits. The potent short-latency resp onse we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.