Neural projection from the brachioradialis to the biceps brachii motoneurones in human was studied using the method of post-stimulus time histogram. Electrical stimulation to the radial branch innervating the brachioradialis produced inhibition in 11 out of 21 biceps motor units. The central delays of the inhibition were 0.7-1.2 ms longer than those of the homonymous facilitation. The inhibition was evoked with the intensity below the motor threshold. Pure cutaneous stimulation provoked no effects on the motor-unit firing. These findings suggest that group I afferents from the brachioradials mediate an oligosynaptic inhibition of the biceps brachii motoneurones.

Neural projections from the pronator teres (PT) muscle to biceps brachii (BB) motoneurones were studied in three healthy human subjects using a post-stimulus time histogram method. In 25 BB motor units, electrical stimulation to the PT nerve with intramuscular needle electrodes induced inhibition in nine units (36%), whereas facilitation was produced in 18 units (72%) by stimulation to the median nerve trunk with surface electrodes at the distal end of the intermuscular septum of the arm or in the cubital fossa. Six motor units (24%) received both inhibition (PT nerve stimulation) and facilitation (median nerve trunk stimulation). In the six, the latency of the inhibition was, on average, 1.2 ms longer than that of the facilitation. The stimulation site for the inhibition was, on average, 4.8 cm distal to that for the facilitation. The inhibition was evoked with an intensity well below the motor threshold. These findings suggest that BB motoneurones receive oligosynaptic inhibition of group I afferents from PT in human.

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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. median nerve; brachioradialis; monosynaptic facilitation; oligosynaptic inhibition; humans