SOME OBSERVATIONS ON THE ELECTRICAL ACTIVITY IN THE CARDIAC MUSCLE FIBER OF HORSESHOE CRAB

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Abstract
On the excised heart of, Tachypleus tridentatus, the intracellular recording of periodical depolarizations of the cardiac muscle fiber was made simultane-ously with the developed tension and also with the discharges of the dorsal nerve cord. Spontaneous periodical depolarizations appeared at a rate of about 10/sec corresponding to the bursts of nervous discharges. Each depolarization lasted about 3 sec and consisted of the initial depo-larization and the subsequent plateau and repolarization phase on which numerous wavelets were superimposed. The resting potential was 30-50 mV. It was not observed that the depolarization exceeded the zero potential. Depolarization to a single indirect shock could be recorded in the muscle fiber, and the potential summated to form a sustained depolarization when the repetitive stimulation was applied. It could be observed that the depolarization during activity had a certain equilibrium potential which seemed to be lower than the zero potential. From the results it could be concluded that the potential was a synaptic potential at the neuromuscullar junction of the cardiac muscle and that the fiber was multiply innervated. The junctions would be distributed over the surface of the muscle fiber and the junctional potential developed the tension normally without the pro-pagated action potential. The potential was extremely enhanced by TEA ion.

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