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). 

AH
Answered by Aaron H. Human Biology tutor

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