Selective reinnervation of skeletal muscle in the newt Triturus cristatus.

1. A study was made of the effectiveness of synapses formed by foreign and original nerves during reinnervation of skeletal muscle of the newt Triturus cristatus. The extensor cranialis nerve (e.c.n.) of the forelimb was implanted into the humeroantebrachialis muscle (biceps) which was denervated by cutting or crushing the forelimb flexor nerve (f.f.n.). 2. Although biceps became innervated by the implanted nerve, neuromuscular transmission was abnormal. The ratio of the tensions developed by biceps during single and repetitive (50 Hz) stimulation of e.c.n. was lower than either that obtained in normal biceps or during stimulation of f.f.n. after it had regenerated. Similarly, the mean quantal content of e.p.p.s evoked in biceps during stimulation of e.c.n. were lower (m = 17.1) than those evoked in normal muscles (m = 74.6) or during stimulation of the regenerated f.f.n. (m = 40.4). 3. Although the implanted e.c.n. had innervated biceps, after 2-3 months a sprout had grown out of the side of the nerve to reinnervate the extensor digitorum communis muscle (e.d.c.) of the forearm. The mean quantal content of e.p.p.s evoked in this muscle by stimulation of e.c.n. (m = 32.2) was higher than that of those e.p.p.s evoked by stimulation of e.c.n. (m = 32.2) was higher than that of those e.p.p.s evoked by stimulation of the same nerve in biceps (m = 17.1). 4. The results suggest that the synapses formed when a muscle is innervated by an inappropriate nerve are less effective than those formed when reinnervation by the correct nerve occurs. This may account for the tendency of the inappropriate synapses to regress following reinnervation by the correct nerve. In addition however, in the newt there seem to exist mechanisms which ensure that regenerating nerves reinnervate their correct muscles.