Electrocardiogram (ECG) illustration showing a fast response cell and slow response cell. Cardiomyocytes whose action potential 0 phase depolarization depends on sodium channels are fast response cells. Their refractory period ends during the action potential. Cardiomyocytes with action potential 0 phase depolarization dependent on calcium channels are slow response cells. The end of their refractory period exceeds the action potential time course.