The effect of inhibitory nerve impulses on a crustacean muscle fibre

Synaptic inhibition has recently been studied by Brock, Coombs & Eccles (1952), who found that an inhibitory nerve impulse increases the resting potential of a motor nerve cell, thereby raising its threshold to excitatory impulses. The authors concluded that this was the sole mechanism by which direct inhibition of spinal motoneurones is produced, and it is of interest to inquire to what extent this conclusion may be generalized. A very different preparation for a study of direct synaptic inhibition is the crustacean nervemuscle system (Biedermann, 1887; Hoffmann, 1914; Marmont & Wiersma, 1938; Kuffler & Katz, 1946) which enables one to stimulate single inhibitor and motor nerve axons and record their effects on individual muscle fibres (see Fatt & Katz, 1953 a). Previous work indicated that the inhibitory process interferes with the crustacean muscle response at two stages: (a) in blocking transmission from the nerve to the muscle membrane; and (b) in uncoupling excitation and contraction processes within the muscle fibre. Only the first of these two actions is considered in this paper, which deals particularly with the electrical membrane changes set up in a crustacean muscle fibre by inhibitory nerve impulses. It will be shown that the main effect of inhibitory impulses is to attenuate the 'end-plate potentials', i.e. to diminish the local depolarization produced by motor impulses. Inhibitory impulses do not by themselves change the resting potential of the muscle fibres, unless this has previously been displaced from its normal level. But even though no potential change may be recorded, the inhibitory impulse was found to have a peculiar effect on the electrical properties of the muscle membrane: it always produces a transient increase of membrane conductance (or 'ion permeability') whose nature will be discussed below. METHODS