MYOGENIC HEARTBEAT IN THE PRIMITIVE CRUSTACEAN TRIOPS LONGICAUDATUS

Pacemaker mechanisms in the heart of the primitive crustacean Triops l ongicaudatus were examined electrophysiologically. The heart is tubula r and the heart wall consists of a single layer of myocardial cells. N o nerve cells were found in the heart, either with methylene blue vita l staining or by light microscopy of serial sections. The heart beats rhythmically at a frequency of 120 to 240 beats/min, and each beat is associated with a slow membrane potential change in the heart muscle. The amplitude of the slow potential varies widely and no spikes appear on it. The heart muscle cells are electrically copied with each other and generate synchronous slow potentials. No localized portion of the heart exhibited a frequency that always preceded the others. The musc le activity could be phase-shifted by injection of a single brief curr ent pulse and could be entrained to a lower or higher frequency by rep eated brief current pulses injected into the muscle cell. The frequenc y of muscle activity could be changed by the injection of DC current i nto the muscle cell, and the change in frequency was linearly related to the intensity of the current, When the intensity of hyperpolarizing DC current exceeded a certain value, the muscle activity disappeared abruptly, and the heart stopped beating completely, These results show clearly that the heartbeat of Triops is myogenic. The heart is diffus ely myogenic and should be regarded as a single muscle oscillator.