Action potentials are electric potentials that is usually activated by nerve impulses. The correct sequence is 2-4-1-3. The first phase is depolarization (2) which involves activation of sodium channels which let the sodium ions inside the cell, making the cell membrane more positive in the interior (4). After this happens, the sodium channels will now close and the potassium channels will now open (1) (stimulated by the high intracellular sodium), and potassium ions will leak out the cell, making the cell membrane more negative in the interior (3).
