Explain how voltage-gated sodium ion channels on the membranes of neurones generate an action potential.

At rest there is a difference in the concentration of sodium ions (Na+) across the membrane of the neurone.

Outside the neurone in the extracellular fluid (ECF) the concentration of Na+ is around 140 mEq/L whereas inside in the intracellular fluid (ICF) it is about 14 mEq/L.

This difference in concentration of Na+ creates a strong concentration gradient across the membrane.

When there is a change in voltage, the voltage-gated Na+ channels open.  This allows the Na+ to flow through the membrane from outside to inside, down the concentration gradient.

Normally the inside of the neurone has a negative charge but, when large amounts of Na+ flood into the neurone when the channels open this makes the inside of the neurone have a positive charge.

This change in charge from negative to positive is knows as depolarisation and this generates an action potential that travels down the neurone (axon).