Human Biology (Year 12)
How is a nerve impulse propagated?
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
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
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
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
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