Explain how myelination of a neuron affects the speed of action potential propagation along its axon.

IB DP Biology Standard 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 is an insulating sheath produced by Schwann cells that surrounds the axon. Because the myelin blocks ion flow, depolarisation can only occur at the gaps between the sheaths – the nodes of Ranvier. The action potential therefore "jumps" from one node to the next (saltatory conduction), bypassing the myelinated sections of the axon. This means that fewer membrane sites must be depolarised and the signal travels much faster than in an unmyelinated axon.

Examiner tips

  • Use the term "myelin sheath" and "nodes of Ranvier". Explain that ion exchange is restricted to the nodes. Describe the jump of the action potential (saltatory conduction). Show that this increases speed by reducing membrane depolarisation.
  • common_mistakes
  • :
  • Confusing Schwann cells with oligodendrocytes (the latter are for CNS). Saying that myelin increases ion exchange rather than blocking it. Forgetting to mention that the action potential jumps between nodes.

Mark scheme (4 marks)

  1. Myelin sheath is an insulating layer formed by Schwann cells wrapped around the axon
  2. Ion exchange / depolarisation can only occur at nodes of Ranvier (gaps between myelin sheaths)
  3. The action potential 'jumps' between nodes of Ranvier (saltatory conduction), bypassing the myelinated internodes
  4. Saltatory conduction increases the speed of propagation compared with unmyelinated axons, because less membrane needs to be depolarised

Key terms in this question

action potential propagation

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