Electrocardiogram (ECG) illustration comparing action potentials of ventricular muscle and the sinoatrial node. heart tissue, the membrane potential of the 4-phase action potential of some cells is unstable and can spontaneously depolarize to produce an impulse. This is called automaticity and is found in cells such as the pacemaker cells of the sinus node. The 4-phase resting potential of some cells is stable. They cannot spontaneously produce impulse, and therefore have no automaticity.