The formation of synapses in regenerating mammalian striated muscle.

1. A histological and electrophysiological study has been made of the formation of synapses in soleus and extensor digitorum longus muscles of adult rats regenerating from minced muscle fragments.2. In the first 8 days of regeneration, myoblast and myotubes formed within the basement membranes of degenerating myofibres.3. Eleven- to fourteen-day regenerates contained fasicles of myotubes and myofibres and both nerves and spontaneous miniature end-plate potentials (m.e.p.p.s) were observed for the first time, although very few synapses show evoked end-plate potentials (e.p.p.s).4. Eighteen- to twenty-six-day regenerates contained. myofibres on which only a single end-plate could be detected; 30% of these synapses had a normal amplitude-frequency distribution of m.e.p.p.s and subthreshold e.p.p.s whilst at the remainder the e.p.p. was either suprathreshold or did not occur.5. Thirty- to sixty-day regenerates contained myofibres and end-plates with the same morphological and functional characteristics as those in normal muscle.