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Group I fibers from muscle spindles and Golgi tendon organs modulate motoneuron excitabilities to coordinate smooth movements. In this study, to elucidate the effects of group I fibers of the median nerve (MN) on the excitabilities of the brachioradialis (BR), we evaluated the changes in the firing probability of a BR motor unit after electrical conditioning stimulation (CS) to MN with a post-stimulus time-histogram technique in six healthy human subjects. We tested 171 motor units: in 72 of them CS to MN at the elbow with the intensity just below the threshold of alpha motor fibers (MT) produced a facilitatory effect (facilitation), while in 43 of them it produced inhibitory one (inhibition). The facilitation and inhibition were not produced by electrical stimulation of the skin overlaying MN. The central synaptic delays of the facilitation and inhibition were on average -0.13 and 0.13 msec, respectively, longer than those of the homonymous facilitation mediated by a monosynaptic path. The thresholds of the facilitation and inhibition were less than 0.7-0.8 and 0.7-0.9 times MT, respectively. CS to MN of hand muscles produced facilitatory effects and that of the pronator teres, palmaris longus, and flexor carpi radialis inhibitory effects. The facilitatory and inhibitory effects were compatible, for latency, with the facilitation and inhibition. These findings suggest that BR motoneurons receive monosynaptic facilitation and oligosynaptic inhibition from MN in humans. Group I fibers of the hand and forearm muscles should mediate the facilitation and inhibition, respectively, to coordinate movements of the hand, forearm, and elbow.

Modulation of Brachioradialis Motoneuron Excitabilities by Group I Fibers of the Median Nerve in Humans