Describe how a reflex arc works when a person accidentally touches a sharp object with their finger. Include the names of the neurones involved and explain why the response does not require a decision from the brain.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A sharp object is the stimulus, detected by receptors in the finger.
1. A sensory neurone carries the impulse to the spinal cord.
2. In the spinal cord a relay neurone passes the impulse to a motor neurone.
3. The motor neurone carries the impulse to an effector (muscle) that withdraws the hand.
4. The entire pathway is in the spinal cord, so the brain is not involved.
5. Because the brain is bypassed, the response is almost instantaneous and automatic, protecting the organism.
1. A sensory neurone carries the impulse to the spinal cord.
2. In the spinal cord a relay neurone passes the impulse to a motor neurone.
3. The motor neurone carries the impulse to an effector (muscle) that withdraws the hand.
4. The entire pathway is in the spinal cord, so the brain is not involved.
5. Because the brain is bypassed, the response is almost instantaneous and automatic, protecting the organism.
Examiner tips
- Use the exact terms: sensory, relay, motor neurone, effector; mention spinal cord; explain bypass of brain for speed.
- Show the sequence clearly, numbering each step to match the marks.
- Include the reason for speed – no brain decision needed.
Common mistakes
- Confusing motor and sensory neurone; forgetting the relay neurone; claiming the brain is involved; omitting the effector muscle.
Mark scheme (5 marks)
- The sharp object acts as a stimulus, which is detected by receptors in the finger.
- A nerve impulse travels along a sensory neurone to the spinal cord.
- A relay neurone in the spinal cord passes the impulse to a motor neurone.
- The impulse travels along the motor neurone to an effector (e.g. a muscle), which produces the response / causes the hand to withdraw.
- The impulse is processed in the spinal cord rather than the brain, so the response is faster / the brain is not needed for the decision, allowing an almost instantaneous / automatic response to protect the organism.
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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