IDENTIFYING THE SOURCE OF RADIAL NERVE AFFERENTS THAT INHIBIT BICEPS BRACHII. Michael A. Pascoe, Benjamin K. Barry, Zachary A. Riley and Roger M. Enoka. Dept. of Integrative Physiology, University of Colorado, Boulder, CO USA. Electrical stimulation of the brachioradialis branch of the radial nerve inhibits the discharge of motor units in biceps brachii. Because the distal portion of brachialis is also innervated by a branch of the radial nerve, however, there is uncertainty over the origin of the inhibitory projection to the motor units in biceps brachii. PURPOSE: To determine the extent to which inhibition of the biceps brachii motor neuron pool with radial nerve stimulation is mediated by afferent projections from the brachioradialis or brachialis. METHODS: Stimulating electrode location was optimized for both the brachioradialis and brachialis branches of the radial nerve by observing the electrically evoked motor responses from the interference electromyogram of both muscles. Six male subjects produced low elbow flexion forces (2.8 ± 1.6 %MVC) to maintain the discharge of a biceps brachii motor unit at 11.1 ± 0.8 pps. Stimulation was applied every 2 – 3 s with a 30-ms delay after the discharge of the motor unit. Stimulus intensity was 0.9x motor threshold (MT). Pre- and post-stimulus time histograms were constructed from the 108 ± 12 stimuli that occurred during the discharge of the motor units. The influence of the two stimulus locations was assessed on the same biceps brachii motor unit. Inhibition was quantified as the percent change from the pre- to post-stimulus interspike intervals. RESULTS: Seven motor units were recorded for each stimulation location. To determine the selectivity of stimulation, motor thresholds were identified for both muscles at each stimulation site. There was a limited capacity to selectively stimulate either nerve branch; the stimulus intensity (0.9x MT) for each muscle corresponded to ~0.73x MT for the other muscle. Despite this limited selectivity, the magnitude of inhibition on biceps brachii motor units was significantly less (P < 0.05) when stimulation was optimized for the brachialis branch of the radial nerve (3.4 %) than the brachioradialis branch (6.5 %). CONCLUSION: These data suggest that either there are fewer inhibitory afferent projections from radial nerve-innervated brachialis than from brachioradialis or that brachialis afferents do not inhibit biceps brachii motor units and the observed inhibition was due to activation of brachioradialis afferents at a lower stimulus intensity. Inhibition of biceps brachii motor units with radial nerve stimulation appears to be primarily mediated by afferents from the brachioradialis muscle. Supported by NINDS R01 NS43275 to RME.