Action potentials — CCEA A2 Biology Practice Questions

Question 1

Describe how an action potential is generated in an axon membrane.

Accepted Answer

At rest the membrane is at about −70 mV, maintained by the sodium–potassium pump moving 3 Na⁺ out for every 2 K⁺ in. A stimulus opens some voltage-gated sodium channels and sodium ions diffuse into the axon. If depolarisation reaches the threshold of about −55 mV, many more sodium channels open in a positive feedback cycle and the membrane potential rises to about +40 mV. The sodium channels then close and voltage-gated potassium channels open, so potassium ions diffuse out and the membrane repolarises. Potassium channels close slowly, causing a brief hyperpolarisation, and the sodium–potassium pump restores the resting potential. During the refractory period no new action potential can be generated, so the impulse travels in one direction only.

Question 2

Explain why an action potential travels faster along a myelinated axon than along an unmyelinated axon of the same diameter.

Accepted Answer

The myelin sheath acts as an electrical insulator, so sodium and potassium ions cannot cross the axon membrane where myelin is present. Depolarisation can therefore occur only at the nodes of Ranvier, and the local currents cause the impulse to jump from one node to the next in saltatory conduction. Because far less of the membrane has to be depolarised, the action potential travels much faster than in an unmyelinated axon of the same diameter.

Action potentials: CCEA Biology practice questions

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Describe how an action potential is generated in an axon membrane.

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