Difference in electrical charge between two differently charged bodies. Normally, the inside of heart cells is negatively charged (—90 millivolts) in comparison to the outside. When nerve impulses pass through the heart, the cells become more positive inside due to an influx of sodium and calcium ions, which are positively charged (called depolarization). This change in electrical potential leads to contraction of the cardiac muscle cells. Following contraction, the intracellular chemical milieu returns to baseline and the electrical potential returns to —90 millivolts.