Many cells in the body have the ability to undergo a transient depolarization and repolarization that is either triggered by external mechanisms (e.g., motor nerve stimulation of skeletal muscle or cell-to-cell depolarization in the heart) or by intracellular, spontaneous mechanisms (e.g., cardiac pacemaker cells).
There are two general types of cardiac action potentials. Non-pacemaker action potentials, also called "fast response" action potentials because of their rapid depolarization, are found throughout the heart except for the pacemaker cells. The pacemaker cells generate spontaneous action potentials that are also termed "slow response" action potentials because of their slower rate of depolarization. These are normally found in the sinoatrial and atrioventricular nodes of the heart.
Both types of action potentials in the heart differ considerably from action potentials found in neural and skeletal muscle cells. One major difference is in the duration of the action potentials. In a typical nerve, the action potential duration is about 1 ms. In skeletal muscle cells, the action potential duration is approximately 2-5 ms. In contrast, the duration of cardiac action potentials range from 200 to 400 ms. Another difference between cardiac and nerve and muscle action potentials is the role of calcium ions in depolarization. In nerve and muscle cells, the depolarization phase of the action potential is caused by an opening of sodium channels. This also occurs in non-pacemaker cardiac cells; however, in cardiac pacemaker cells, calcium ions are involved in the initial depolarization phase of the action potential. In non-pacemaker cells, calcium influx prolongs the duration of the action potential and produces a characteristic plateau phase.