Unlike the action potential in skeletal muscle cells, the cardiac action potential is not initiated by nervous activity. Instead, it arises from a group of specialized cells known as pacemaker cells, that have automatic action potential generation capability. In healthy hearts, these cells form the cardiac pacemaker and are found in the sinoatrial node in the right atrium. They produce roughly 60–100 action potentials every minute. The action potential passes along the cell membrane causing the cell to contract, therefore the activity of the sinoatrial node results in a resting heart rate of roughly 60–100 beats per minute. All cardiac muscle cells are electrically linked to one another, by intercalated discs which allow the action potential to pass from one cell to the next. This means that all atrial cells can contract together, and then all ventricular cells. SA node is the main pacemaker of the heart having maximum P cells.

Rate dependence of the action potential is a fundamental property of cardiac cells and alterations can lead to severe cardiac diseases including cardiac arrhythmia and sometimes sudden death. Action potential activity within the heart can be recorded to produce an electrocardiogram (ECG). This is a series of upward and downward spikes (labelled P, Q, R, S and T) that represent the depolarization (voltage becoming more positive) and repolarization (voltage becoming more negative) of the action potential in the atria and ventricles.

Similar to skeletal muscle, the resting membrane potential (voltage when the cell is not electrically excited) of ventricular cells is around −90 millivolts (mV; 1 mV = 0.001 V), i.e. the inside of the membrane is more negative than the outside. The main ions found outside the cell at rest are sodium (Na⁺), and chloride (Cl⁻), whereas inside the cell it is mainly potassium (K⁺).

The action potential begins with the voltage becoming more positive; this is known as depolarization and is mainly due to the opening of sodium channels that allow Na⁺ to flow into the cell. After a delay (known as the absolute refractory period), the action potential terminates as potassium channels open, allowing K⁺ to leave the cell and causing the membrane potential to return to negative, this is known as repolarization. Another important ion is calcium (Ca²⁺), which can be found inside the cell in the sarcoplasmic reticulum (SR) where calcium is stored, and is also found outside of the cell. Release of Ca²⁺ from the SR, via a process called calcium-induced calcium release, is vital for the plateau phase of the action potential (see phase 2, below) and is a fundamental step in cardiac excitation-contraction coupling.

There are important physiological differences between the pacemaker cells of the sinoatrial node, that spontaneously generate the cardiac action potential and those non-pacemaker cells that simply conduct it, such as ventricular myocytes). The specific differences in the types of ion channels expressed and mechanisms by which they are activated results in differences in the configuration of the action potential waveform, as shown in figure 2.

Cardiac automaticity also known as autorhythmicity, is the property of the specialized conductive muscle cells of the heart to generate spontaneous cardiac action potentials. Automaticity can be normal or abnormal, caused by temporary ion channel characteristic changes such as certain medication usage, or in the case of abnormal automaticity the changes are in electrotonic environment, caused, for example, by myocardial infarction.

The standard model used to understand the cardiac action potential is that of the ventricular myocyte. Outlined below are the five phases of the ventricular myocyte action potential, with reference also to the SAN action potential.

In the ventricular myocyte, phase 4 occurs when the cell is at rest, in a period known as diastole. In the standard non-pacemaker cell the voltage during this phase is more or less constant, at roughly -90 mV. The resting membrane potential results from the flux of ions having flowed into the cell (e.g. sodium and calcium), the flux of ions having flowed out of the cell (e.g. potassium, chloride and bicarbonate), as well as the flux of ions generated by the different membrane pumps, being perfectly balanced.^{[citation needed]}