Explain how the structure of a myelinated motor neurone allows for rapid transmission of nerve impulses compared with an unmyelinated neurone.

IB DP Biology Higher Level (2023 syllabus) — C2.2 Neural signalling · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Myelin sheaths, produced by Schwann cells, wrap tightly around the axon and act as an electrical insulator. Because the membrane is covered, depolarisation can only occur at the exposed gaps – the nodes of Ranvier. The action potential therefore jumps from one node to the next (saltatory conduction), covering a larger distance per depolarisation event. This greatly increases the conduction velocity compared with an unmyelinated axon, where depolarisation must propagate continuously along the entire membrane.

Examiner tips

  • Mention Schwann cells and insulation first; then nodes of Ranvier; describe saltatory conduction; finish with speed comparison. Use the exact terms ‘myelin sheath’, ‘nodes of Ranvier’, ‘saltatory conduction’, ‘conduction velocity’.
  • Keep the answer concise – 4 points can be covered in 3–4 sentences. Avoid unnecessary detail such as ion channel types. Use UK spelling (insulator, velocity).

Common mistakes

  • Confusing Schwann cells with oligodendrocytes; or saying the whole axon is insulated. Mixing up ‘myelinated’ with ‘unmyelinated’ in the wrong context. Writing a long paragraph that repeats the same idea instead of distinct points.

Mark scheme (4 marks)

  1. Myelin sheath is formed by Schwann cells wrapping around the axon, acting as an electrical insulator
  2. Depolarisation / ion exchange can only occur at nodes of Ranvier (gaps between myelin sheaths) where the axon membrane is exposed
  3. The impulse effectively 'jumps' from node to node (saltatory conduction), covering larger distances per depolarisation event
  4. This significantly increases conduction velocity compared with an unmyelinated axon, in which depolarisation must occur continuously along the entire length of the membrane

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