Depolarizing the membrane during an action potential first involves the flow of sodium (Na) ions into the membrane.
An action potential is a rise and subsequent fall in voltage or membrane potential across a cellular membrane. At the resting potential (high concentration of Na+ outside the cell and low of the K+, different inside the cell), K channels are closed. When Na channels open (by stimulus), the membrane depolarizes. When depolarization reaches the threshold potential, it triggers an action potential. Generation of the action potential brings the closing of Na channels. When Na channels close and K channels open, the membrane repolarizes.