The physiology, pharmacology, and trophic effectiveness of synapses formed by autonomic preganglionic nerves on frog skeletal muscle.

1. Frog sartorius muscles, newly denervated and transplanted to the lymph sac of the back, are reinnervated by implanted cholinergic nerves (spinal somatic motor nerves or the preganglionic sympathetic splanchnic nerve), but not by nerves). 2. Foreign somatic motor nerves (s.m.n.s) form synapses that resemble normal sartorius neuromuscular junctions electrophysiologically. 3. Axons of the sympathetic preganglionic splanchnic nerve (s.p.n.) grow throughout the muscle, but only a small percentage of fibres form synapses. Most e.p.p.s are of low quantal content, generally subthreshold. Long onset latencies and multiple post-synaptic responses indicate that innervation is multiple, multi-terminal, and by unmyelinated axons. 4. Spontaneous miniature e.p.p.s at splanchnic junctions occur at an average rate under 0.1/sec. Their average amplitude and time course are about the same as for control muscles, but the variability of amplitudes is greater than for control muscles. 5. The amount of facilitation shown by s.p.n.-evoked e.p.p.s is the same as by s.m.n. e.p.p.s, but the time course is almost twice as long. 6. S.p.n.-reinnervated fibres show dramatic post-tetanic potentiation preceded by depression, following as few as 20--50 stimuli. 7. As judged by standard physiological and histochemical criteria, AChEsterase is absent at s.p.n. junctions. 8. The pharmacological responses of the s.p.n. junctions are similar to those of normal or foreign s.m.n. innervated neuromuscular junctions in their sensitivity to the cholinergic blocking agents D-tubocurarine and hexamethonium. 9 The s.p.n. is capable of restricting ACh sensitivity to the sites of nerve contacts, although this restriction occurs more slowly and less completely than with s.m.n. reinnervation. The loss of extrajunctional ACh sensitivity can be correlated with effectiveness of innervation; but significant restriction occurs even in s.p.n. reinnervated fibres that probably never contract to nerve stimulation.