As the membrane potential is increased, sodium ion channels open, allowing the entry of sodium ions into the cell. This is followed by the opening of potassium ion channels that permit the exit of potassium ions from the cell.> Inward flow of sodium ions increases the concentration of positively-charged cations in the cell and causes depolarization, where the potential of the cell is higher than the cell's resting potential.> Sodium channels close at the peak of the action potential, while potassium continues to leave the cell.> Efflux of potassium ions decreases the membrane potential or hyperpolarizes the cell.> For small voltage increases from rest, the potassium current exceeds the sodium current and the voltage returns to its normal resting value, typically -70 mV.> However, if the voltage increases past a critical threshold, typically 15 mV higher than the resting value, the sodium current dominates.> This results in a runaway condition whereby the positive feedback from the sodium current activates even more sodium channels. Thus, the cell "fires," producing an action potential">