Adaptive changes in locomotor control after partial denervation of triceps surae muscles in the cat

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Abstract
This report concerns a test of the hypothesis that gain in the stretch reflex pathway of cat medial gastrocnemius (MG) muscle during locomotion increases after denervation of its synergists, lateral gastrocnemius (LG), soleus (SOL) and plantaris (PL) muscles. In four cats, MG, tibialis anterior (TA) and vastus lateralis (VL) muscles were implanted with electromyogram (EMG) electrodes. The cats walked on a row of elevated pegs, some of which were spring‐loaded and could be triggered to pop up at the moment of foot touchdown, rapidly dorsiflexing the foot. Pre‐stretch EMG activity in MG as well as short‐, medium‐ and long‐latency responses to the dorsiflexions were compared before and after unilateral denervation of synergists. Short‐ and medium‐latency responses of MG to perturbations increased in proportion to the increase in pre‐stretch EMG in the days and weeks after partial denervation. This argues against an adaptive increase in stretch reflex gain independent of centrally generated extensor drive. Local anaesthesia of the skin of the paw did not significantly change the sizes of the stretch responses of MG before or after partial denervation. We conclude that pre‐stretch EMG activity as well as stretch reflexes in MG muscle increased substantially after denervation of synergistic muscles. The data were consistent with an adaptive increase in central locomotor drive, causing more motoneuronal activity, which in turn resulted in an increase in the size of stretch reflexes.