English: Key:

a) Sodium (Na+) ion b) Potassium (K+) ion c) Sodium channel d) Potassium channel e) Sodium-Potassium Pump

In the stages of an action potential, the permeability of the membrane of the neuron changes. At the resting state (1), sodium and potassium ions are unable to pass through the membrane, and the neuron has a negative charge inside (mainly due to the large proteins that are negatively charged, as well as the lower amount of positive K+ ions inside the neuron). Once the action potential is triggered, the depolarization (2) of the neuron activates the sodium channel, allowing sodium ions to pass through the membrane of the neuron and results in a positive charge in the neuron and a negative charge in the extracellular fluid. After the action potential is reached, the neuron begins repolarization (3), where the sodium channels close and the potassium channels open, allowing potassium ions to cross the membrane and flood into the extracellular fluid, resulting in a positive charge in the extracellular fluid and a negative charge that is below the resting potential of the neuron. Finally, to return the neuron to that resting potential after the potassium pump closes, a sodium-potassium pump works to exchange three sodium ions per two potassium ions across the plasma membrane during the refractory period (4). Once the Na+ and K+ are back where they started, the neuron is back to its resting state (1), ready to do it all over again for the next action potential.