The most important function of heart cells is to contract rhythmically and systematically. The contraction of the heart as a whole is as a direct result of the contraction of all of the tiny cells of the heart muscle. These contractions are triggered by an electrical impulse known as the action potential. Before discussing the cardiac action potential, we must first know what an action potential is:
Each cell in our body is surrounded by a thin membrane. Different ions (small charged molecules can move across the cell membrane through the special ion channels. The channels can freely let one type of ions go through the membrane and block passage of other types of ions. Due to such a specific permeability a concentration gradient is established across the cellular membrane. Because ions are charged molecules, an electrical gradient is also established across the cell membrane, transforming each cell into a tiny battery. This imbalance in electrical charge across the cell membrane is known as the membrane potential. Every cell in our body is slightly more negative inside then outside - to be more exact it has a resting membrane potential of approximately (- 0.1 V or -100mV).
Some of the cells (called excitable cells) are capable to rapidly reverse their resting membrane potential from negative resting values to slightly positive values. This rapid change in membrane potential is called an action potential. The action potential is brought on by a rapid change in membrane permeability to certain ions. Excitable cells include neurons (nerve cells) and muscle cells.
Let's consider an action potential generated by a nerve cell, a considerably simpler event than a cardiac action potential. Inside all cells there is a high concentration of potassium ions and low concentration of sodium ions. The extracellular space contains the opposite: a high concentration of sodium ions and a low concentration of potassium ions. The imbalance between ions inside and outside the cells creates a resting membrane potential which is about -60 mV. Upon activation of certain ion channels potassium ions diffuse out of the cell and sodium ions move into the cell, moving from high concentrations of ions to low concentrations. As more positively charged sodium ions get into the cell, the membrane potential becomes less negative and reaches a threshold at -45 mV. Once this occurs, many more sodium channels are open and the membrane potential begins rising more rapidly (rising phase) until it reaches the peak of the action potential. By that time the sodium channels are closed and cell begins to repolarize, returning the membrane potential to its original resting state. A brief period of overcompensation, called hyperpolarization, occurs when the membrane potential becomes more negative then its original resting state. Thus, the action potential of the cell is the change in voltage of the membrane potential that causes it to go from its negative resting state to a positive value for a very brief time.
In order to understand how this relates to the heart, go to The Cardiac Action Potential.