Adaptive changes in motor activity associated with functional recovery following muscle denervation in walking cats

In this investigation we examined the changes in the pattern of activity in the medial gastrocnemius (MG) muscle in walking cats following transection of the nerves innervating synergist muscles (lateral gastrocnemius, soleus , and plantaris). Immediately following the nerve transections, there was a large increase in ankle flexion during early stance (from similar to 10 to similar to 30 degrees) and a marked increase in the magnitude of the MG bu rsts during stance. We attribute this increase in the magnitude of the MG b ursts to an increase in afferent feedback from the abnormally stretched MG muscle. During the week after the nerve transections, there was a progressi ve decrease in ankle yield. This improvement in ankle function was correlat ed with an increase in magnitude of two components of the MG bursts; the in itial component starting during late swing and ending similar to 40 ms afte r ground contact, and a late component associated with stance. The time cou rses of the increases in the initial and late components of the MG bursts w ere different. Large and significant increases in the late component occurr ed the day after the nerve transections, whereas increases in the initial c omponent occurred more gradually. This difference in time course was reflec ted in the kinematics of ankle movement. Over the first few days after the nerve transections, improvement in ankle movement occurred primarily late i n the stance phase, and there was little change in ankle yield during early stance. At 1 wk, however, there was a significant reduction in ankle yield during early stance. This decreased yield was most likely due to an increa se in stiffness of the MG muscle at the lime of ground contact resulting fr om the increase in magnitude of the initial component of the MG bursts. The increases in the magnitude of the initial and late components of the MG bu rsts, as well as the improvement in ankle function, depended on use of the leg. All these changes were delayed by immobilizing the leg for 6 days in a n extended position. We discuss possible mechanisms underlying the increase in the magnitude of the MG bursts and propose that proprioceptive signals from the stretched MG muscles provide an error signal for rescaling the mag nitude of the centrally generated initial component. Our data support the c oncept that proprioceptive feedback functions to scale the magnitude of fee d-forward motor commands to ensure they are appropriate for the biomechanic al properties of the musculoskeletal system.