# A student investigating action potentials places two recording electrodes on the outer surface of a myelinated neurone. The first electrode is placed at node A and the second at node B, further along the axon. When a single action potential is initiated, electrode A records a brief negative deflection before electrode B records the same deflection. Explain why the action potential travels faster along a myelinated axon than along an unmyelinated axon of the same diameter.

> OCR A-Level Biology A (H420) — 5.3 Neuronal communication · Explain · 4 marks

> Myelinated neurones are surrounded by a myelin sheath formed from Schwann cells wrapped repeatedly around the axon, leaving small exposed gaps called nodes of Ranvier spaced at intervals along the axon's length.

## Mark scheme (4 marks)

1. The myelin sheath acts as an electrical insulator, preventing ion movement across the membrane in the internodal regions
2. Depolarisation / action potentials can only occur at the nodes of Ranvier where the membrane is exposed
3. The impulse effectively jumps from one node to the next — saltatory conduction
4. Fewer sites of depolarisation means less time is spent on sequential ion channel opening and closing, so the impulse travels along the axon more quickly overall

## Key terms

- [myelinated](https://www.gradenine.co.uk/glossary/myelinated)
- [axon](https://www.gradenine.co.uk/glossary/axon)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-investigating-action-potentials-places-ed91e93c) · Published by Druglandscape Ltd.