hi! am a student learning this currently and also a bit confused but this is what i understand... action potential is the impulse generated after the resting membrane potential (~ -70mV) of the neuron has reached threshold voltage level. this may be caused by external stimulus. upon reaching threshold level (~ -50-55mV), voltage-gated sodium channels are opened, allowing for the influx of sodium ions within axon.... this process is known as depolarisation. after reaching a peak voltage of approx +30-40mV, these voltage-gated sodium channels are closed and voltage-gated potassium channels are opened to allow for resting membrane potential to be achieved again caused by efflux of potassium ions....this process is known as repolarisation. hyperpolarisation occurs due to excess loss of potassium ions but membrane potential quickly is returned to normal due to workings of sodium-potassium pumps...