Our sodium voltage gated channels on the membrane are very quick to open and basically they allow for the rapid movement of the sodium ions inside the cell and that causes the inside of the cell to become positive. And that leads to our depolarization period. Now, when we reach about positive 30 millivolts, the sodium voltage gated channels close, while the potassium voltage gated channels begin to open. But they open very slowly at first. At the same time, we also open the calcium voltage gated channels and calcium voltage gated channels are very slow to actually close. So basically what happens is because we have this additional voltage gated channel, and because the calcium ions go into the cell while the potassium goes out of the cell, and because the potassium has a larger charge, we see that the inner portion of the membrane stays positive for a longer period.