The conduction of nerve impulses relies upon the movement of electrically charge ions across the nerve cell membrane. When a nerve is resting, or polorized, there are more potassium ions than sodium ions inside the cell, with an opposite ratio outside. Sodium ions are actively kept out of the cell by an energy consuming pump mechanism. This maintains a negative charge on the inside of the cell and a positive charge on the outside. When an impulse travels along the nerve, sodium ions flood into the cell and make the inside of the cell positive with respect to the outside. This produces a rise in the electrical potential across the cell membrane. After the impulse has passed, potassium ions leave the cell, restoring the negative charge within the cell and the positive charge outside it. While this resting situation is being restored another impulse cannot be generated.