Inhibitory projections from pronator teres to biceps brachii motoneurones in human.

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.