Community Q&A
A-Level Biology May/June 2025 Q10(c): Vertebrate animals generally have a myelin sheath around the axons of motor neurones. E…
A-Level Biology · Paper 9700/41 · May/June 2025 · Question 10(c) · [3 marks]
Vertebrate animals generally have a myelin sheath around the axons of motor neurones. Explain why the presence of a myelin sheath around a motor neurone axon is an advantage.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The myelin sheath significantly speeds up the transmission of the nerve impulse along the axon. The sheath acts as an electrical insulator, preventing ion exchange across the axon membrane except at the gaps in the sheath, called the nodes of Ranvier. Consequently, the action potential appears to 'jump' from node to node in a process known as saltatory conduction. This allows for a faster response, such as a more rapid muscle contraction, which is a key advantage for vertebrates in processes like escaping predators or catching prey.
How the marks are awarded
- M1 — Stating that the primary advantage is the increased speed of nerve impulse transmission, as mentioned in 'speeds up the transmission of the nerve impulse'.
- M1 — Explaining the mechanism by which speed is increased, either by naming 'saltatory conduction' or by describing the sheath as an 'electrical insulator'.
- M1 — Linking the increased speed to a physiological benefit for the organism, as stated in 'allows for a faster response, such as a more rapid muscle contraction'.
Common mistakes
- Stating that myelin 'protects' the axon. While true, this is not the advantage related to impulse transmission speed and would not score a mark here.
- Confusing the mechanism by suggesting the impulse travels through the myelin sheath, rather than jumping between the nodes of Ranvier.
- Describing the process (e.g., saltatory conduction) without linking it to the final advantage for the animal (e.g., faster reaction times or quicker muscle contraction).
- Simply stating 'it makes it faster' without any explanation of how (e.g., insulation or saltatory conduction).
Examiner tip: For questions explaining an 'advantage', always aim to link the structure to its specific function and then explain the resulting physiological benefit for the organism.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
Your answer
Sign in to answer this question.