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? Well, we call it the depolarization period because the inside of the cell basically becomes positive. Why? Well, as our sodium ions flow into the cell, our sodium ions each carry a positive charge.