The process of membrane depolarization / repolarization is controlled by the movement of three cations, Na+, Ca+2, and K+, in and out of the cell. At the resting stage, the concentration of Na+ is high on the outside. On membrane depolarization sodium fluxes-in leading to an immediate elevation of the action potential. Elevated intracellular Na+ triggers the influx of free of Ca++ that occurs more slowly. The higher intracellular [Ca++] results in the efflux of K+. The reestablishment of the action potential occurs later by the reverse of the Na+-K+ exchange.