When a nerve cell is stimulated, the cell membrane becomes MORE permeable to Na+.
Explanation:
When the action potential is fired, opening of Na+ ion channels in membrane makes it more permeable to the Na+ ions. The Na+ ions enter the axon and makes the inside of the membrane more positive. This is called as depolarization. Depolarization of axon is followed by depolarization during which K+ ion channels are opening to allow movement of K+ ion outside the axon and to make the inner side of axon membrane negative again.
