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.
McNulty
P A
PA
Prince of Wales Medical Research Institute, Randwick, Sydney, NSW 2031, Australia. p.mcnulty@unsw.edu.au
Türker
K S
KS
Macefield
V G
VG
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*