The contribution of motor unit synergies to the nociceptive withdrawal response in the rat tail
The planning of movement by the brain is complicated by complexity introduced through muscular redundancy, where multiple muscles have similar mechanical actions. Synergies, correlated patterns of muscle activity, provide one potential solution to simplify computational complexity. The rat tail, which is controlled by over 300 extrinsic and intrinsic muscles or muscle fascicles, is an ideal model system to use electromyography (EMG) to explore the role of muscular synergies in a hyper-redundant limb or appendage. Previous studies in our laboratory recorded muscle activity from intrinsic tail muscles; however, their approach did not allow for selective recording from intrinsic tail muscles. Our goal was to develop a method for selectively recording muscle activity from the six intrinsic tail muscles and to use the method to explore synergies within these muscles. Rats were briefly anesthetized to insert into the tail bipolar fine wire electrodes. Electrodes were placed into dorsolateral, lateral and ventral muscles that span coccygeal vertebrae. Either six electrodes were placed in each of the six muscles at the same level to evaluate crosstalk, or within one muscle (right dorsolateral) at seven locations to evaluate the pattern of muscle activity. Under isometric conditions, heat stimuli were delivered to the lateral surface of the tail at multiple locations along the tail and EMG was recorded. Our results demonstrated the intrinsic muscles of the tail can be recorded with minimal cross talk between adjacent muscles at the same level (mean crosstalk = 13.5%). The percent crosstalk decreased with distance from the source muscle (the muscle where the action potential is assumed to originate in). Extensive co-contraction of left and right muscles was observed. Regarding dependence on stimulus location, while some trials suggested a potential trend, there was no significant relationship overall between stimulus location and muscle activity. Principal component analysis was used to identify synergies; one principal component accounted for 80% of the variation in muscle activity. Beyond developing a new, selective technique for recording from intrinsic tail muscles, we have shown that the tail is largely controlled through a stereotyped pattern, or synergy, of muscle activity that incorporates both left and right, and rostral and caudal, muscle activity.