Functioning of the Inhibitory Conditioning Axon Innervating Insect Muscles*

Abstract

ABSTRACT The jumping muscle of locusts is supplied by three motor axons (Hoyle, 1955 a), one of which gives rise to large twitch contractions when stimulated with a single shock and is termed the fast (F) axon, whilst another gives rise to slow contractions only, when repetitively stimulated, and is termed the slow (S) axon (Hoyle, 1955 b). The third axon was found not to give a contraction when stimulated alone but it did cause a hyperpolarization in some of the muscle fibres Thus, by comparison with peripheral inhibitory action at crustacean neuromusclar junctions (Fatt & Katz, 1953), amphibian heart (Hutter & Trautwein, 1956) and cat spinal cord (Eccles, 1953), its function could be expected to be inhibitory; but Hoyle (1955b) found that stimulation of this axon at the same time as stimulation of either of the two excitatory ones did not lead to mechanical inhibition. Instead, the mechanical responses were enhanced, though the effect was slight so functional significance could not be ascribed to it with confidence. Since this phenomenon could not be fully understood it has remained in the background awaiting further investigation.