AS discussed in the first section, the action potential recorded from individual cardiac cells is caused by ionic currents generated by the movement of sodium and calcium across the cell membrane from the extracellular to the intracellular compartment, and the movement of potassium across the cell membrane from the intracellular to the extracellular compartment. These ionic currents are directly responsible for the upstroke ( Na and Ca), plateau ( Ca and K) and repolarization limb (K) of the cellular action potential, and indirectly responsible for the QRS complex, ST segment and T wave of the body surface electrocardiogram.