INHIBITION AT THE NERVE MUSCLE JUNCTION IN CRUSTACEA

Abstract

The effect of inhibitory nerve impulses on the potential changes and on the subsequent contractions of crustacean muscle was investigated. The action potentials of inhibitory axons do not differ from those of other nerve fibres. Inhibitory impulses alone have no detectable electrical effect on the muscle. These findings confirm those of other investigators. There are two separate actions of inhibitory impulses on the motor response. (a) An electrical effect, the a-action, reduces the junctional potential (e.p.p.) to an extent depending upon the relative times of arrival of inhibitory and motor impulses at the junction. Due to the reduction of the e.p.p. (1) the setting up of propagated muscle impulses is prevented and (2) local contractions at the junctional region are diminished or abolished. At 17[degree]C. the a-action lasts about 20-25 msec. and declines to half in about 6 msec. The time course of the e.p.p. is not appreciably altered by inhibition. (b) The inhibitory impulse has direct action on the contractile process at the junction, the [beta]-action At low frequencies complete mechanical inhibition can be obtained without reduction of the e.p.p. This confirms observations by Marmont and Wiersma (9). The [beta]-effect can sum at intervals of 0.1 sec. Facilitation responsible for the growth of successive e.p.p.''s is not affected by inhibition at 50/sec. The first or second e.p.p., after cessation of inhibitory impulses, reaches the amplitude to which it would have grown in the absence of inhibition. It is suggested that the [alpha]-effect is due to action of the inhibitor on the junctional receptors of the muscle membrane, and that the release of the transmitter is unaffected.