Explain how a reflex arc protects the body from harm, referring to the role of neurones involved and the speed of the response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student accidentally touches a hot surface and quickly pulls their hand away before they are consciously aware of the pain.
Model answer (4 marks)
Sensory neurones carry the impulse from the pain receptor to the spinal cord. The impulse is transmitted across a synapse to motor neurones, which then carry it from the spinal cord to the muscle, causing the hand to withdraw. Because the reflex arc uses only the spinal cord and does not involve the brain, the signal is processed very quickly. The rapid response means the hand is pulled away before significant tissue damage occurs, protecting the body from harm.
Examiner tips
- Use the exact terms ‘sensory neuron’, ‘motor neuron’, ‘spinal cord’, ‘effector’ and ‘reflex arc’.
- Explain that the brain is bypassed, which gives the fastest response.
- Show the sequence: receptor → sensory neuron → spinal cord → motor neuron → effector.
- Mention the protective outcome – withdrawal before damage.
Common mistakes
- Confusing sensory with motor neurons or vice versa.
- Forgetting to state that the brain is bypassed or that the response is rapid.
- Using vague terms like ‘quickly’ without linking to the reflex arc mechanism.
Mark scheme (4 marks)
- Sensory neurones carry the nerve impulse from the receptor to the central nervous system (spinal cord)
- Motor neurones carry the nerve impulse from the CNS to the effector (muscle), causing a response
- The reflex arc bypasses the brain / does not involve conscious processing, so the response is faster
- The faster response means the hand is withdrawn before significant tissue damage occurs, protecting the body from harm
Key terms in this question
Related
- Revision notes: The nervous system →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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