Action potentials are the rapid changes in charge across the membrane that occur when a neuron is firing

• During an action potential, the inside of the neuron becomes temporarily more positive relative to the outside (approximately +30 mV)

The change in membrane potential is triggered by the opening of sodium and potassium channels along the axon

• The positively charged ions will move along the concentration gradient that was established by the resting potential, creating an electrical change

• The sodium ions first move into the neuron to create a positive membrane potential (depolarisation), before the potassium ions flow out to re-establish a negative membrane potential (repolarisation)

Action Potential Stages

Depolarisation
Sodium Channel

Repolarisation
Potassium Channel

The ion channels are voltage-gated, meaning a change in voltage at one point of an axon will trigger the opening of ion channels in the next segment of the axon

• This causes the action potential to be propagated along the length of the axon in a unidirectional wave
  

Once an action potential has occurred, the resting potential must be restored by the sodium-potassium pump before another action potential can happen

• This brief period of time between nerve impulses is called the refractory period
  

Propagation Along an Axon