Peripheral reflex inhibitionin the claw of the crab, Carcinus maenas (L.).

Abstract

ABSTRACT Reflex opening of the claw of the crab, Carcinus maenas (or of the crayfish, Astacus pallipes), elicited by mechanical stimulation at the carpo-propodite joint, can be reflexly inhibited by stroking the inside of the claw. By electrical and mechanical recording from the opener muscle (the apodeme or motor axons of the antagonistic closer muscle being cut), and by electrical recording from the motor and inhibitor axons to this muscle, it is shown that this reflex inhibition is effected mainly peripherally, through the inhibitor axon specific to the opener muscle. Central inhibition of the motor axon plays at the most a minor part. The externally recorded action potentials of the opener muscle show strong peripheral inhibition of both the α (‘supplemented’) and β (‘simple’) forms (i.e. with and without attenuation of muscle potentials, respectively) during the reflexinhibitory stimuli, as well as sporadic attenuation at other times. Attenuation of a muscle potential results, in Carcinus claw opener, when an inhibitor impulse precedes the motor impulse by less than 15-20 msec., and is maximal when this interval is less than approximately 5 msec. Thus α inhibition predominates over β inhibition at the higher frequencies of discharge. The frequency of impulses in the opener motor axon commonly ranges from 20 to 80/sec. with brief bursts of 80-200/sec. during reflex contraction of the opener muscle, and does not fall significantly during reflex-inhibitory stimulation. The opener inhibitor, which often manifests an irregular low-frequency ‘background’ discharge during reflex opening, responds to concurrent reflex-inhibitory stimulation with a sharp increase in frequency to 100-200 impulses/sec. initially and falling to 50-100/sec. with continued reflex inhibition. Reflex-inhibitory stimulation alone elicits only inhibitor impulses. Following cessation of stimulation the tension declines ‘passively’. Owing to continuing motor activity, however, this decline is often slower than during active inhibition; it is often also erratic due to the occurrence of inhibitor impulses. When a relatively high ratio of inhibitor/motor impulse frequencies precedes a burst of motor impulses eliciting a rise in tension, the rate of this tension rise is lower than that excepted from the frequency of motor impulses evoking the rise. When there is a relatively high-frequency ‘background’ discharge in the opener inhibitor the tension in the muscle shows rapid fluctuations, whereas with lower inhibitor frequencies fairly steady tensions are sustained. This is in part because the peripheral latency of mechanical inhibition varies inversely with the ratio of inhibitor/ motor frequencies preceding the inhibition. It is suggested that a background of peripheral inhibition, by making possible quicker relaxation, may be an important factor in rapid movement.