Action potential of sinoatrial node and ECG (electrocardiogram), illustration. The 4-phase membrane potential of sinoatrial node cells is unstable and can be depolarized spontaneously. After depolarization reaches the threshold potential, it triggers a new action potential. This electrophysiological feature is called automaticity. In the human heart, the sinoatrial node has the strongest self-discipline and the fastest rate of 4-phase spontaneous depolarization, and determines heart rate.