Effects of short-term denervation and subsequent reinnervation on motor endplates and the soleus muscle in the rat

The rat sciatic nerve was locally frozen, and changes in the nerve, motor e ndplates, and the soleus muscle were examined for up to 6 weeks by light an d electron microscopy, The wet weights of denervated soleus muscles compare d with contralateral values progressively declined to a minimum at 2 weeks after injury (60.7+/-2.5%) and began to reverse following 3 weeks, The scia tic nerve thoroughly degenerated after freezing. However, numerous regenera ted myelinated and thin nerve fibers were observed at 3 weeks. They were co nsiderably enlarged but still smaller than normal counterparts at 6 weeks p ostoperatively. Nerve terminals containing synaptic vesicles of endplates d isappeared at day 1 and mostly reappeared at 3 weeks (about 70% of the endp lates), All endplates examined were reinnervated at 4, 5, and 6 weeks. On t he other hand, postsynaptic folds of muscle fibers seemed to be only slight ly influenced by denervation or reinnervation. Ultrastructural alterations of myofibrils, in particular the loss of register, immediately appeared aft er denervation, spread progressively, peaked at 2 weeks, ameliorated follow ing reinnervation, and became significantly normalized at 6 weeks after fre ezing. The proportion of type II fibers in the soleus muscle similary showe d an increase and a decrease with a short delay in response to denervation and reinnervation, respectively. This study clearly demonstrated that the n erve supply affects the ultrastructural integrity of skeletal muscles. In a ddition, changes in the endplates and the soleus muscle evaluated in this s tudy after short-term denervation are largely reversible following reinnerv ation.