So what will begin to happen is the sodium ions will begin to move down their electrochemical gradient. So because they contain a positive charge and the inside is negatively charged, they will move this way from the outside to the inside. And because our concentration is high on the outside and low on the inside, they will move into our cell. And this is known as the electrochemical gradient. So, since the concentration of sodium is higher on the outside than the inside, and because the inside the cell is negatively charged, our sodium ions will move down their electrochemical gradient into the cell. Now, why do we call this our depolarization period?