How Does an Action Potential Work?
Learn how a neurone maintains its resting potential and how sodium ion movement causes depolarisation and an action potential in AQA A-level Biology.
An action potential is a brief reversal of the electrical potential difference across a neurone’s membrane. At rest, the inside is negative relative to the outside. When a stimulus reaches the threshold, voltage-gated sodium ion channels open and sodium ions enter, causing depolarisation.
Resting potential
The resting potential is the potential difference across the membrane when the neurone is not transmitting an impulse. The sodium–potassium pump uses ATP to move three Na+\mathrm{Na}^{+}Na+ ions out for every two K+\mathrm{K}^{+}K+ ions moved in. The membrane also allows some K+\mathrm{K}^{+}K+ ions to diffuse out through leak channels, while relatively few Na+\mathrm{Na}^{+}Na+ ions enter. Together, these processes maintain an inside that is negative relative to the outside.
Depolarisation
A stimulus causes a small change in membrane potential. If this reaches the threshold—the level needed to trigger an action potential—voltage-gated Na+\mathrm{Na}^{+}Na+ channels open. Na+\mathrm{Na}^{+}Na+ ions diffuse into the neurone down their electrochemical gradient. This makes the inside less negative and then positive. More Na+\mathrm{Na}^{+}Na+ channels open as the membrane depolarises, producing the rapid rise of the action potential.
Afterwards, the Na+\mathrm{Na}^{+}Na+ channels close and voltage-gated K+\mathrm{K}^{+}K+ channels open. K+\mathrm{K}^{+}K+ ions leave, making the inside negative again; this is repolarisation. The membrane may briefly become more negative than its resting level before the resting potential is restored.
How the impulse travels
Depolarisation at one point creates local currents that bring the next section of membrane to threshold. Its voltage-gated sodium ion channels open, so an action potential is generated there. The recently activated section enters a refractory period: its sodium ion channels cannot immediately reopen. This helps keep the impulse moving forwards and limits how frequently action potentials occur. A stronger stimulus does not make each action potential larger; once threshold is reached, the response is all-or-nothing.
Exam mistake to avoid: the sodium–potassium pump helps maintain the ion gradients needed for an action potential, but the rapid depolarisation is caused by Na+\mathrm{Na}^{+}Na+ entering through voltage-gated channels—not by the pump.
