Voltage-gated ion channels open or close in response to changes in the potential difference across a neuron's plasma membrane. By allowing specific ions to diffuse down their electrochemical gradients, sodium and potassium channels generate the rapid voltage changes of an action potential.
The mechanism
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At the resting potential, voltage-gated sodium and potassium channels are closed. Ion concentration gradients are maintained by the sodiumpotassium pump and membrane permeability to ions.
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A stimulus causes some sodium ions to enter, producing depolarization. If the membrane reaches the threshold potential, voltage-gated sodium channels open rapidly.
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Sodium ions diffuse into the axon down their electrochemical gradient. The inside becomes less negative and then positive, causing further sodium channels to open. This positive feedback produces the rising phase of the action potential.
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Voltage-gated sodium channels then become inactivated, while voltage-gated potassium channels open more slowly. Potassium ions diffuse out of the axon, causing repolarization.
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Potassium channels close slowly, so excess potassium leaves and briefly causes hyperpolarization. The channels then close, and the resting conditions are re-established.
| Channel | Response to threshold | Effect on membrane potential |
|---|---|---|
| Voltage-gated sodium channel | Opens rapidly, then becomes inactivated | Sodium enters; rapid depolarization occurs |
| Voltage-gated potassium channel | Opens more slowly and closes slowly | Potassium leaves; repolarization and brief hyperpolarization occur |
Depolarization in one region creates local currents that bring the adjacent membrane to threshold, propagating the action potential along the axon. The refractory period caused partly by sodium-channel inactivation ensures one-way propagation.
A common misconception is that the sodiumpotassium pump directly produces each rapid action potential. It maintains the ion gradients over time, but diffusion through voltage-gated channels causes the immediate voltage changes.
Exam technique
For an IB AHL explain question, link each channel state to ion movement and its effect: threshold → sodium channels open → sodium influx → depolarization; sodium channels inactivate and potassium channels open → potassium efflux → repolarization. Always state that ions move down their electrochemical gradients.