Reinnervation of fast and slow mammalian muscles by a superfluous number of motor axons.

In this study peripheral nerves from flexor digitorum longus, (alien nerve) as well as the deep branch of the muscle's own lateral popliteal nerve were cut and connected to the distal stump of the lateral popliteal nerve. Extensor digitorum longus and tibialis anterior muscles then became reinnervated to a similar extent by either nerve, showing no preference for its own nerve. A significant proportion of the endplates in these muscles remained permanently supplied by more than one axon, and a proportion of the muscle fibres was supplied by both nerves. No ectopic endplates were formed on fast muscle fibres. The same two nerves were also connected to the slow soleus muscle and this muscle became preferentially reinnervated by the nerve to flexor digitorum longus. In contrast to fast muscles, endplates of soleus muscle fibres were only rarely contacted by more than one axon, and ectopic endplates were often found in this muscle. In both types of muscles that had an excess of motor nerves, extensive sprouting persisted for many months. Thus, identical motor nerves induce different patterns of innervation in slow and fast muscles, and muscle fibres do not show a preference for their own nerve.