How is a nerve impulse propagated?

Depolarisation

• Once a nerve impulse is generated, the permeability of the cell membrane changes with voltage-gated sodium channels opening, allowing sodium ions to flow into the cell

• The flow of ions causes a reversal in charges, with the positive charge now on the interior of the cell and the negative charge now on the exterior of the cell

• When this occurs, the cell is said to be depolarised

Repolarisation

• The voltage-gated sodium channels then close, and voltage-gated potassium channels open, allowing for potassium to flow out of the cell and restore the cell's interior back to its negative charge

• It is now said to be repolarised

Hyperpolarisation

• The voltage-gated potassium channels then shut, but the membrane potential drops below -70mV, which causes hyperpolarisation to occur before returning to the resting state

• This change from being polarised to depolarised (called an action potential) occurs as a 'wave', the nerve impulse moving along the axon in a single direction

• The sodium-potassium pump then continues to pump sodium out of the cell and potassium into the cell to restore ion concentrations

Refractory Period

• The refractory period is the period during which the neuron cannot be stimulated again

• This period begins at the start of the action potential and a short time after, until the resting membrane potential of -70mV is restored