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 · View as Markdown

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

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.

Model answer (4 marks)

The myelin sheath is an electrical insulator, so ion flow across the membrane is blocked in the internodal regions.

Depolarisation can only occur at the nodes of Ranvier where the membrane is exposed.

The action potential therefore jumps from one node to the next – a process called saltatory conduction.

Because depolarisation is limited to the nodes, fewer ion‑channel openings are required, so the impulse travels along the axon more quickly overall.

Examiner tips

  • Use the term "saltatory conduction" – examiners look for this keyword. Show the causal link: insulation → limited depolarisation → fewer channel openings → faster conduction. Keep the answer concise – 4 points, one sentence each.
  • common_mistakes
  • :
  • Confusing myelin as a conductor instead of an insulator. Mentioning that the action potential travels faster because of a larger diameter rather than the insulating effect. Failing to state that depolarisation is restricted to the nodes of Ranvier.

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 in this question

myelinated · axon

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