Society For Neuroscience Abstract Zar | 2006

MODULATION OF AFFERENT PATHWAYS IS TASK SPECIFIC BUT NOT MUSCLE SPECIFIC BETWEEN THE ELBOW FLEXORS. ZA Riley, BK Barry, MA Pascoe, and RM Enoka. Dept. of Integrative Physiology, University of Colorado, Boulder, CO USA. Single motor units in the biceps brachii were recorded in 8 subjects for two separate experiments while subjects performed elbow flexion contractions at 90°. A spiketriggered stimulation technique was used to assess the effect of an inhibitory reflex pathway across biceps muscles and in different forearm postures. Radial nerve afferents innervating the brachioradialis were stimulated at an intensity below motor threshold (0.9 x MT, 0.5s duration) and the delay in the subsequent motor unit discharge was measured. 38 motor units (19 long head, 19 short head) were recorded in experiment 1 with light flexion forces (long head 3.13 ± 2.4%, short head 4.47 ± 2.4%, P = 0.09) and the forearm in the neutral position. 18 motor units (9 long head, 9 short head) were recorded in experiment 2 in three different positions: pronation (4.07 ± 1.5%), neutral (3.75 ± 1.9%), and supination (4.31 ± 1.9%) (P = 0.65). In both experiments stimulation was applied every 2-3 seconds at a delay of 30ms following the previous motor unit discharge. Inhibition was quantified as the delay (ms) from the post-pre (averaged 3 ISIs) stimulus interspike intervals and from the χ² statistic computed from control and stimulation histograms. There was a significant delay in motor unit discharge following stimulation in experiment 1 (5.92 ms, P = 0.002). 27/38 histograms displayed a difference as well. There was no difference in the amount of inhibition between the biceps long and short heads (P = 0.19). Experiment 2 displayed a similar effect with stimulation (P < 0.001), as well as a posture-dependent effect of stimulation (P = 0.018). Pronation had the greatest inhibition (7.69 ± 8.17 ms), followed by neutral (5.88 ± 8.11 ms), and finally supination (2.93 ± 3.89 ms). However, on an individual subject basis the inhibition was greatest from pronation-supination-neutral in most cases as a result of the large variability across positions. Mean discharge rates (P = 0.41) and the brachioradialis M-wave amplitude stayed the same across the three postures (P > 0.96). The consistency of inhibition was confirmed in two motor units following a repeated neutral task at the end of a session. These results provide evidence that radial nerve projections from the brachioradialis inhibit biceps brachii motor units in both the long and short heads similarly, though the total inhibition can change with forearm posture. Supported by NIH NS43275 to RME. Key words: inhibitory reflex, biceps brachii, forearm posture, motor unit