The effects of denervation location on fiber type mix in self-reinnervated mouse soleus muscles

Mouse soleus muscles were denervated by crushing the soleus nerve where it enters the muscle to determine if denervation followed by self-reinnervation can permanently alter the mix of fiber types in a muscle. Reinnervated and contralateral control muscles were sectioned at 2 and 7 months postdenervation and histochemically stained for myosin ATPase to determine the percentages of fast and slow twitch fibers in the muscles. It was found that, at both 2 and 7 months postdenervation, reinnervated muscles had a significantly higher percentage of slow twitch fibers than did contralateral control muscles (86.7 versus 67.8% at 2 months and 90.0 versus 69.3% at 7 months). Soleus muscles were also denervated by crushing the soleus nerve where it exists the gastrocnemius muscle (approximately 4 mm proximal to where the nerve enters the soleus muscle) to ascertain if the location of the nerve lesions plays a role in the ultimate outcome of the process of self-reinnervation. Reinnervated muscles and their contralateral muscles were sectioned at 2 months postdenervation and histochemically stained for myosin ATPase as before. It was found that, in contrast to muscles denervated at the point of nerve entry, muscles denervated 4 mm more proximal exhibited only a small increase in their percentage of slow twitch fibers which was not statistically significant (71.4 versus 68.4%). These results suggest that denervation followed by self-reinnervation can cause a permanent change in a muscle's fibers type mix and that the location of the nerve lesion strongly influences the final outcome of the reinnervation process.