Characteristics of end-plates formed in mouse skeletal muscles reinnervated by their own or by foreign nerves

Morphological and electrophysiological techniques were used to study the neuromuscular junctions of soleus and extensor digitorum longus muscles in normal mice and 6 months after reinnervation by either their original or foreign nerves. In muscles reinnervated by foreign nerves, there were increased incidences of morphological abnormalities, including ultra-terminal axonal sprouting, multiaxonal innervation of end-plates, and ectopic synapse formation, as compared with both normal muscles and muscles reinnervated by their correct nerves. In spite of the morphological abnormalities, however, there was no evidence that the effectiveness of synapses (as estimated from the mean quantal content, m, of end-plate potentials) formed between nerves and inappropriate muscles was impaired. As had also been found in normally innervated muscles, the value of m was again higher in extensor digitorum longus than in soleus following reinnervation by inappropriate nerves. These results suggest that in mammals, when muscles are reinnervated by foreign nerves, mechanisms exist to maintain the efficacy of neuromuscular transmission, in contrast to the situation in amphibians (Sayers and Tonge, 1982: J. Physiol. (Lond.), 330:57-68). The abnormalities observed in this study following reinnervation may be a morphological manifestation of these corrective mechanisms.