HYSTERESIS IN REPOLARIZATION OF CARDIAC ACTION POTENTIALS EFFECTS OF SPATIAL HETEROGENEITY AND SLOW REPOLARIZATION CURRENTS %D8%B1%D8%B3%D8%A7%D9%84%D8%A9 %D8%AF%D9%83%D8%AA%D9%88%D8%B1%D8%A7%D9%87 %D8%A8%D8%B9%D9%86%D9%88%D8%A7%D9%86