The cardiac action potential is the recording of the changes of electrical potential over time recorded from inside a cardiac myocyte (heart muscle cell). The action potential is due to the flow of electrolytes through the plasma membrane changing the voltage across the cell. The inside of the myocyte changes its electrical potential with the inflow and outflow of electrolytes through ion channels. These ion channels serve as gates that open and close, thereby controlling the ionic flow. The gates respond to changes in the membrane potential (differences in voltage).