Neuromuscular mechanisms of a locust skeletal muscle

Abstract

The physiology of neuromuscular transmission of the three motor nerve fibres supplying the extensor tibialis muscle of the jumping leg of the locust Locusta migratoria migratorioides R. & F. is studied with the aid of intracellular microelectrodes. The largest axon, supplied by the crural nerve trunk, branches repeatedly to innervate every muscle fibre at several points. Single impulses produce large end-plate potentials at all the points of innervation and active membrane responses complete the depolarization, sometimes overshooting the zero potential. A brief, powerful twitch of the muscle results from each impulse. On repetitive stimulation the tension increases up to about 8 $\times $ twitch tension. The tetanus tension is about 20 000 g/g, considerably greater than for mammalian muscle, expressed in the same terms. The almost equally large axon supplied by the third metathoracic nerve trunk produces electrical effects in only about 30% of the muscle fibres. These are of two principal kinds: very small, long-lasting depolarizations which summate following repetitive activity to give a depolarization plateau, and larger potentials resembling end-plate potentials. The latter may or may not show facilitation depending on their initial magnitude, and may give rise to active membrane responses. The third and smaller axon, also supplied by the third nerve, gives rise to small, long-lasting hyperpolarizations in many muscle fibres. Quick contractions used in hopping and jumping are effected by the largest axon. All other activities of the muscle, from very slow, prolonged tonic contraction, to quite rapid shortening involved in walking, are effected by the other large axon. It is suggested that the hyperpolarizations produced by the smaller axon prepare the muscle fibres before quick contraction, by raising the resting potential.