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

• Published in: The Journal of physiology 1999-08, Vol.518 (3), p.883-893
• Main Authors: McNulty, P A, Türker, K S, Macefield, V G
• Format: Article
• Language: English
• Subjects: Adolescent; Adult; Electric Stimulation; Electromyography; Female; Fingers - innervation; Hand - innervation; Hand - physiology; Humans; Male; Microelectrodes; Motor Neurons - physiology; Muscle Spindles - physiology; Neurons, Afferent - physiology; Original; Reflex, Monosynaptic - physiology; Synapses - physiology; Touch - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1111/j.1469-7793.1999.0883p.x

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. 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. 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 11hms1h). For four slowly adapting (SA) type II afferents, EMG showed a periodicity that was correlated to the afferent interspike interval ( r = 0.99). 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. 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.