The Study on Fibrillation (II) : Fibrillation in Cardiac Muscle

Abstract

Automaticity of atrial muscle of the toad in modified Ringer's solution containing BaCl2 and fibrillation of ventricular muscle of the frog in hypertonic Ringer's solution are studied by microscopical observations and records of intracellular action potential. The atrial muscle which has no automaticity in normal solution begins to contract rhythmically when BaCl2 is added to it. In the region where excitation starts, pace-maker potential with slowly increasing depolarisation is observed. The pace-maker region is almost localized and occurrence of the excitation is rhythmical. Formation of pace-maker cell under such condition suggests a possibility of ectopic automaticity and myogenecity of excitation. In the hypertonic solution with two times excess NaCl, the ventricular muscle also con-tracts rhythmically. But microscopically there appears some quiecent fibers immediately adjoining to contracting fibers, the formers of which are also electrically silent. Namely, separation of unit in. movement is observed. Transition in shape of the intracellular action potential after replacing with the hypertonic solution may be divided into following three periods, that is, the first period when the spike-component disappears and duration of the action potential shortens, the second period when the plateau phase diminishes and a triangular wave remains, and the third period when this triangular wave reduces and finally disappears. From these observations, composition of the action potential may be divided into spikecomponent, plateau-component and slow potential component. This slow potential may be considered to correspond to inter-cellular myo-mvo-junction potential of visceral smooth muscle in normal solution and to peculiar slow potential of striated muscle in Ca++-deficient solution. Basing on these facts mentioned, fibrillation of cardiac muscle is thought to start from ectopic focus of cxcitation and through variable multiple pathway of disturbed inter-cellular connections.