Explain how the ultrastructure of a myelinating Schwann cell is adapted for its function in the peripheral nervous system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Schwann cells wrap multiple layers of their plasma membrane around the axon, forming a thick myelin sheath. During this wrapping the cytoplasm is largely displaced or compressed, leaving tightly packed lipid bilayers that act as electrical insulation. Gaps between adjacent Schwann cells create nodes of Ranvier, which allow saltatory conduction and greatly increase the speed of nerve impulse transmission. Schwann cells are rich in mitochondria and have high metabolic activity to support the energy demands of membrane synthesis and maintenance of the sheath, and to provide active support to the axon.
Examiner tips
- Use the word ‘wrap’ to show the process of myelination. Mention the displacement of cytoplasm to explain insulation. Explain nodes of Ranvier and saltatory conduction. Show that mitochondria supply energy for membrane maintenance.
Common mistakes
- Confusing Schwann cells with oligodendrocytes. Forgetting to mention the nodes of Ranvier. Using vague terms like ‘insulation’ without linking to lipid bilayers.
Mark scheme (4 marks)
- Schwann cells wrap multiple layers of their plasma membrane around the axon, forming the myelin sheath
- The cytoplasm is largely displaced / compressed out during wrapping, leaving tightly packed lipid bilayers that act as electrical insulation
- Gaps between adjacent Schwann cells form nodes of Ranvier, allowing saltatory conduction which greatly increases the speed of nerve impulse transmission
- Schwann cells are rich in mitochondria / have metabolic activity to support the energy demands of membrane synthesis and maintenance of the sheath / active support of axon
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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