Final answer:
Depolarization occurs when a stimulus makes the membrane permeable to Na+ ions, causing a reversal of charge across the membrane. Repolarization follows as the membrane becomes impermeable to Na+ and K+ ions move from high to low concentration.
Step-by-step explanation:
An action potential is an all-or-nothing event where the membrane potential of a neuron rapidly rises and falls. During depolarization, a stimulus makes the membrane permeable to Na+ ions, causing sodium ions to rush into the cell and reversing the charge across the membrane. Repolarization follows as the membrane becomes impermeable to Na+ and potassium ions (K+) move from high to low concentration.
In summary, depolarization and repolarization are the two main phases of an action potential, resulting from the movement of ions across the cell membrane.