The resting potential of a cardiac muscle cell is about –90 mV: the inside of the cell is negative with respect to the outside (phase 4). The threshold for the generation of an action potential is –60 mV. Once the threshold is reached, the following events occur:

• Rapid depolarization occurs as fast sodium channels open and Na⁺ enters the cell (phase 0). The membrane potential thus becomes +30 mV and the voltage-dependent sodium channels close and are inactivated.
• Partial repolarization occurs as potassium channels open and transiently let K⁺ exit the cell (phase 1).
• Slow, voltage-dependent calcium channels open and Ca²⁺ enters the cell, causing the membrane potential to fall more slowly (phase 2). This phase is known as the plateau.
• When the membrane potential further decreases, the calcium channels close and the slow potassium channels open. The outflow of K⁺ causes a rapid repolarisation (phase 3).