We investigate an integrate and fire model for two cardiomyocytes interacting with each other. A feature of the model is to incorporate the refractory periods of the cardiomyocytes as well as the influence of firing of adjacent cells. The present model predicts that, if refractory periods of the two cells are nearly equal, the beating rhythms of the two cells always synchronize and their beating rate is tuned to the faster rate between the two cells. On the other hand, if their refractory periods significantly differ, they exhibit various kinds of harmonious beating rhythms. These results successfully explain the well known characteristics of synchronized beating of cultured cardiomyocytes. We also discuss effects of a delay time of cell-to-cell interaction, that gives further complicated phase diagrams for the beating rhythms.
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