THE SMALL-NERVE MOTOR SYSTEM TO SKELETAL MUSCLE

Abstract

The small diam. motor nerve system to frog''s skeletal muscle is widely distributed and has well-defined properties. It has been studied in the intact anaesthetized animal, in isolated muscles, and in dissected nerve-muscle prepns. Small-nerve fibers could be stimulated exclusively after (1) dissection and isolation, (2) blocking larger fibers by pressure or constant current, and (3) by reflex stimulation. In contrast to the well-known twitch response set up by a single nerve impulse in a large-nerve motor fiber, excitation of a small-nerve fiber causes appreciable muscle shortening only on repetitive excitation. This shortening is local but considerable tension can be generated. Stimulation of a small-nerve fiber sets up a local potential change at the neuromuscular junction. It spreads decrementally over several mm. along the muscle fibers and its general electrical properties are analogous to those of the curarized endplate potential of vertebrate muscle and to the local potentials in crustaceans. Since it is set up by small nerve fibers at the junctional region, it will be called small-fiber junction potential. The thresholds of the small-nerve fibers are 3-6 times higher than that of the most excitable large twitch-producing fibers. Their conduction velocities are 4-8 m./sec. at 22[degree]C. They are not of sympathetic origin. Small-nerve fibers divide and innervate numerous fibers in a muscle. A given fiber may be innervated by both small and large diam. nerve fibers. This is not conclusively demonstrated. Depending on the kind of stimulation, the muscle fiber appears to respond with local or propagated responses. The small-nerve fiber system is reflexly active in animals immobilized by light ether anesthesia or decere-bration.