Final answer:
An action potential is an all-or-nothing event that requires the threshold of excitation to be reached. Depolarization occurs when sodium ions rush into the cell, and repolarization happens when potassium ions leave the cell. This process allows the membrane potential to return to the resting state.
Step-by-step explanation:
An action potential is an all-or-nothing event; it either happens or it does not. The threshold of excitation must be reached for the neuron to "fire" an action potential. As sodium ions rush into the cell, depolarization actually reverses the charge across the membrane form -70mv to +30mV. This change in the membrane potential causes voltage-gated K+ channels to open, and K+ begins to leave the cell, repolarizing it. At the same time, Na* channels inactivate so no more Na+ enters the cell. K+ ions continue to leave the cell and the membrane potential returns to the resting potential. At the resting potential, the K+ channels close and Na+ channels reset. The depolarization of the membrane proceeds in a wave down the length of the axon. It travels in only one direction because the sodium channels have been inactivated and unavailable until the membrane potential is near the resting potential again; at this point they are reset to closed and can be opened again.