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 endplates, and the soleus muscle were examined for up to 6 weeks by light and electron microscopy. The wet weights of denervated soleus muscles compared 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 sciatic nerve thoroughly degenerated after freezing. However, numerous regenerated myelinated and thin nerve fibers were observed at 3 weeks. They were considerably enlarged but still smaller than normal counterparts at 6 weeks postoperatively. Nerve terminals containing synaptic vesicles of endplates disappeared at day 1 and mostly reappeared at 3 weeks (about 70% of the endplates). All endplates examined were reinnervated at 4, 5, and 6 weeks. On the other hand, postsynaptic folds of muscle fibers seemed to be only slightly influenced by denervation or reinnervation. Ultrastructural alterations of myofibrils, in particular the loss of register, immediately appeared after denervation, spread progressively, peaked at 2 weeks, ameliorated following reinnervation, and became significantly normalized at 6 weeks after freezing. The proportion of type II fibers in the soleus muscle similary showed an increase and a decrease with a short delay in response to denervation and reinnervation, respectively. This study clearly demonstrated that the nerve supply affects the ultrastructural integrity of skeletal muscles. In addition, changes in the endplates and the soleus muscle evaluated in this study after short-term denervation are largely reversible following reinnervation.