The Paradox of Mytilus Muscle. A New Interpretation

Abstract

1. The mechanism of contraction and relaxation in the anterior byssus retractor muscle (A.B.R.M.) of the lamellibranch mollusc Mytilus edulis has been studied with refined stimulating and recording techniques. Two distinct types of response are present: phasic and tonic. The former can be evoked by all kinds of electrical stimulation, but the latter is most readily elicited by continuous direct current. 2. Serotonin (5-hydroxytryptamine) rapidly abolishes the tonic response, whilst leaving the phasic response practically unaffected for many hours. On electrical stimulation, a serotonin-treated muscle may show spontaneous electrical and mechanical activity, usually of a rhythmical nature. As the muscle is unable to destroy this drug the effects produced by it are likely to be of pharmacological interest only. 3. Electrical activity in the form of irregular volleys of muscle action potentials can be recorded from the intact A.B.R.M. and even after the nerves to the muscle have been cut. In such (externally) denervated preparations the spontaneous electrical activity is almost unaffected by a.c. stimulation but greatly increased by d.c. stimulation which evokes prolonged tonic contraction. 4. Stimulation of inhibitory nerves within the muscle quickly terminates a state of tonic contraction and at the same time reduces the electrical activity considerably. 5. An explanation is offered for the occurrence of electrical activity during relaxation of lamellibranch smooth muscles. The normal functioning of the A.B.R.M.is interpreted, and strong support is given to the tetanus hypothesis of tonic contraction.