Describe the roles of the three types of neurone found in the nervous system, and explain one structural adaptation of a neurone that allows nerve impulses to travel quickly.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Sensory neurone carries impulses from a receptor to the spinal cord or brain (CNS). Relay neurone carries impulses between different parts of the CNS. Motor neurone carries impulses from the CNS to an effector such as a muscle or gland.
A key structural adaptation is the myelin sheath, produced by Schwann cells, which surrounds the axon. The myelin sheath provides electrical insulation, allowing the action potential to jump between the nodes of Ranvier (saltatory conduction) and therefore travel much faster than it would along an unmyelinated axon.
A key structural adaptation is the myelin sheath, produced by Schwann cells, which surrounds the axon. The myelin sheath provides electrical insulation, allowing the action potential to jump between the nodes of Ranvier (saltatory conduction) and therefore travel much faster than it would along an unmyelinated axon.
Examiner tips
- Use the exact terms ‘sensory’, ‘relay’ and ‘motor’ neurone and state their pathways clearly. Include the word ‘myelin sheath’ and explain insulation and saltatory conduction. Keep the answer concise to fit the 5 marks.
Common mistakes
- Confusing sensory with motor neurone functions, or omitting the CNS pathway. Not naming the myelin sheath or misidentifying Schwann cells as oligodendrocytes. Failing to explain how insulation speeds conduction.
Mark scheme (5 marks)
- Sensory neurone carries impulses from a receptor to the spinal cord / brain (CNS)
- Relay neurone carries impulses between different parts of the central nervous system
- Motor neurone carries impulses from the CNS to an effector (muscle or gland)
- Named structural adaptation: myelin sheath (made of Schwann cells) surrounds the axon
- Explanation of how the named adaptation speeds up the impulse: myelin sheath provides electrical insulation which speeds up the nerve impulse / long axon reduces the number of synapses so fewer slow-downs occur / dendrites pick up impulses from multiple neurones
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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