What happens during an action potential?

An action potential is a brief electrical event that occurs when a cell’s membrane potential rises and falls rapidly. 

These are the main events that occur during an action potential:

1. Stimulus: A stimulus or triggering event causes the sodium voltage-gated channels to open.
2. Depolarization: Positively charged Sodium ions (NA+), which are present in abundance outside the cell, rush into the neuron, which has a negative charge relative to the outside. This causes the depolarization of the neuron, in which the membrane potential reaches zero and then swings past equilibrium and acquires a positive charge as the action potential passes through.  
3. Repolarization: When the potential difference reaches +40 mV, the sodium channels shut down, halting the inward rush of positive sodium ions. Simultaneously, the potassium channels open, enabling a large efflux of potassium ions (K+) from the inside to the outside of the cell. This leads to repolarization, in which the cell loses positively charged ions, the membrane potential is reduced, and the cell returns to its resting cell. 
4. Hyperpolarization: The large efflux of positive potassium (K+) ions causes an overshoot of the potential difference where the cell is more negative than its typical resting membrane potential. Potassium channels stay open a little bit longer as the action potential passes through, continuing to allow positive ions to leave the neuron, which causes the cell to temporarily become even more negative than its resting state. This phenomenon is called hyperpolarization. As the potassium channels start to shut down, the sodium-potassium pump works to reestablish the resting state.
5. Resting state: In the last stage, the neuron returns to its resting membrane potential where no action potential is generated.