Morphology of motor nerve terminals on rat soleus muscle fibers reinnervated by the original and by a "foreign" nerve.

Abstract

Motor nerve terminals in normal “slow” soleus and “fast” peroneus brevis muscles and reinnervated soleus muscles of rats were stained with methylene blue. Reinnervation was induced by cutting the original tibial nerve innervation, and was established either by the tibial nerve within the original innervation band or by the previously transposed “foreign” superficial peroneal nerve outside the innervation band. Terminals were classified as morphological types A, B, or C according to previously defined criteria, and their sizes were measured. In the normal muscles, type B terminals predominate in the soleus muscle, whereas type A ones predominate in the peroneus brevis muscle. In soleus muscles the size of the terminals increases with increasing animal size and age. In the reinnervated muscles the three types of terminals can be recognized. In the self-reinnervated soleus muscles the new terminals after 16 and 54 weeks conform with those occurring normally in soleus muscles. The first identifiable “foreign” terminals ten days after denervation resemble those in normal soleus muscles (type B). With increasing postdenervation time, however, terminals resembling type A ones increase in number at the expense of type B ones. Thus, 3 and 14 weeks after induced reinnervation, the frequency distribution of types of “foreign” terminals is changed and resembles that in normal peroneus brevis muscle. The development continues, however, and after a year most “foreign” terminals resemble type A ones, rendering the frequency distribution consistently different from that in normal soleus as well as peroneus brevis muscles. The size of the terminals steadily increases during this period, an increase beyond the normal increase with age. The results indicate that morphological features of the new motor nerve terminals formed in the experimental models used mainly depend on the type of motor axons reinnervating a muscle, modified however by the type of muscle fibers being reinnervated.