DEPOLARIZATION
If the stimulus is strong enough, the neuron fires an action potential. When the amount of stimulus causes the membrane to depolarize to threshold, large numbers of voltage-activated Na⁺ channels open and the rapid movement of Na⁺ into the cell causes a rapid depolarization. That is, as Na⁺ enter the cell, the interior becomes more positively charged. Depolarization abruptly stops when the Na⁺ channels inactivate as a result of the voltage changes.