Time course of functional recovery during first three months after surgical transection and repair of nerves to feline soleus and lateral gastrocnemius muscles

Locomotion outcomes after peripheral nerve injury and repair in cats have been described in the literature for the period immediately following the injury (muscle denervation period) and then again for an ensuing period of long-term recovery (at three months and longer) resulting in muscle self-reinnervation. Little is known about the changes in muscle activity and walking mechanics during mid-recovery, i.e. the early reinnervation period that takes place between 5 and 10 weeks of recovery. Here, we investigated hindlimb mechanics and EMG activity of ankle extensors in six cats during level and slope walking before and every two weeks thereafter in a 14-week period of recovery after the soleus (SO) and lateral gastrocnemius (LG) muscle nerves in one hindlimb were surgically transected and repaired. We found that the continued increase in SO and LG EMG magnitudes and corresponding changes in hindlimb mechanics coincided with the formation of neuromuscular synapses revealed in muscle biopsies. Throughout the recovery period, EMG magnitude of SO and LG during the stance phase and the duration of the stance-related activity were load-dependent, similar to those in the intact synergistic medial gastrocnemius and plantaris. These results and the fact that EMG activity of ankle extensors and locomotor mechanics during level and upslope walking recovered 14 weeks after nerve transection and repair suggest that loss of the stretch reflex in self-reinnervated muscles may be compensated by the recovered force-dependent feedback in self-reinnervated muscles, by increased central drive and increased gain in intermuscular motion-dependent pathways from intact ankle extensors.

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