A person is cooking and accidentally places their hand on a hot hob. Explain how the nervous system coordinates a rapid withdrawal response, naming the neurones involved and describing how the nerve impulse passes between them.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The withdrawal of the hand occurs before the person is consciously aware of the heat.
Model answer (5 marks)
A receptor in the skin detects the heat stimulus
A sensory neurone carries the impulse to the spinal cord
The impulse crosses a synapse in the spinal cord using the neurotransmitter acetylcholine
A relay (interneuron) in the spinal cord connects the sensory neurone to the motor neurone
The motor neurone carries the impulse to the muscle, causing contraction and withdrawal of the hand
A sensory neurone carries the impulse to the spinal cord
The impulse crosses a synapse in the spinal cord using the neurotransmitter acetylcholine
A relay (interneuron) in the spinal cord connects the sensory neurone to the motor neurone
The motor neurone carries the impulse to the muscle, causing contraction and withdrawal of the hand
Examiner tips
- Use the exact terms: receptor, sensory neurone, relay neurone, motor neurone, effector, neurotransmitter
- Show the pathway in order: receptor → sensory neurone → synapse → relay neurone → motor neurone → effector
- Mention acetylcholine as the common synaptic transmitter in the spinal cord
- Keep the answer concise – one sentence per point
Common mistakes
- Confusing the relay neurone with the motor neurone
- Omitting the neurotransmitter or using the wrong one
- Failing to mention the spinal cord as the site of the synapse
Mark scheme (5 marks)
- A receptor in the skin detects the heat stimulus
- A nerve impulse is sent along a sensory neurone to the spinal cord / relay neurone
- The impulse passes across a synapse (between neurones) using a chemical / neurotransmitter
- A relay neurone in the spinal cord connects the sensory neurone to the motor neurone
- The motor neurone carries the impulse to the effector (muscle), causing the hand to be withdrawn / muscle to contract
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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