Describe how an action potential is generated?

An action potential is an all or none response which has a threshold voltage which needs to be reached by the stimulus. A stimulus will call depolarisation of a nerve axon. This is caused on the entry of sodium into the cell through voltage gated sodium channels which open when there is a stimulus. If there is a strong enough signal the voltage inside the cell will increase from the resting membrane potential -80mv to +40mv (action potential peak). After this point there the sodium channels will begin to close and potassium channels will open resulting in an efflux of potassium from the cell thus causing repolarisation. This will return the voltage inside the cell back to resting potential. There is an overshoot of potassium efflux which can cause hyperpolarisatiom and the potassium channels will then close and the sodium will begin to open returning the voltage to resting membrane potential.