Evidence for strong synaptic coupling between single tactile afferents and motoneurones supplying the human hand.
1. Electrical stimulation of digital nerves elicits short-latency excitatory and inhibitory spinal reflex responses in ongoing EMG in muscles acting on the fingers and thumb. Similar responses are elicited by stimulating a population of muscle spindles but not when a single muscle spindle is activated. The current study investigated whether short-latency EMG responses could be evoked from the discharge of a single cutaneous afferent. 2. Thirty-three tactile afferents were recorded via tungsten microelectrodes in the median nerve of awake humans. Spike-triggered averaging revealed EMG events time-locked to the afferent discharge. The afferents were activated by an external probe and the EMG was elicited by a weak voluntary contraction. 3. Eleven cutaneous afferents (33 %) showed a short-latency response in the ongoing EMG. Overt increases or decreases in EMG were observed for seven afferents (onset latency 20.0-41.1 ms). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike 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 SA II afferents (39 %) showed excitation and 11 no response; and none of the six SA I afferents showed any response. 5. We conclude that, unlike muscle spindle afferents, the input from a single cutaneous afferent is strong enough to drive, via interneurones, motoneurones supplying muscles acting on the digits. The potent short-latency response we found supports the important role of cutaneous mechanoreceptors in fine motor control of the human hand.
Prince of Wales Medical Research Institute, Randwick, Sydney, NSW 2031, Australia. p.mcnulty@unsw.edu.au
P A
PA
McNulty
K S
KS
Türker
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VG
Macefield
D000293:Adolescent; D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D005385:Fingers / Q000294:innervation; D006225:Hand / Q000294:innervation* / Q000502:physiology; D006801:Humans; D008297:Male; D008839:Microelectrodes; D009046:Motor Neurons / Q000502:physiology*; D009470:Muscle Spindles / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology*; D012023:Reflex, Monosynaptic / Q000502:physiology; D013569:Synapses / Q000502:physiology*; D014110:Touch / Q000502:physiology*