Explain how a simple reflex arc allows the body to respond rapidly to a harmful stimulus. Include the sequence of structures involved in the reflex arc in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A person accidentally touches a very hot surface and quickly pulls their hand away before they are consciously aware of the pain.
Model answer (5 marks)
1. A receptor in the skin detects the heat and generates a nerve impulse.
2. The impulse travels along a sensory neuron to the spinal cord.
3. In the spinal cord a relay neuron receives the impulse and immediately passes it to a motor neuron.
4. The impulse travels along the motor neuron to the muscle effector.
5. The muscle contracts rapidly, withdrawing the hand – the response is fast because the impulse is processed in the spinal cord and does not reach the brain, so no conscious thought is involved.
2. The impulse travels along a sensory neuron to the spinal cord.
3. In the spinal cord a relay neuron receives the impulse and immediately passes it to a motor neuron.
4. The impulse travels along the motor neuron to the muscle effector.
5. The muscle contracts rapidly, withdrawing the hand – the response is fast because the impulse is processed in the spinal cord and does not reach the brain, so no conscious thought is involved.
Examiner tips
- Use the exact sequence of structures: receptor → sensory neuron → spinal cord relay neuron → motor neuron → effector.
- Show that the reflex bypasses the brain to justify the rapid response.
- Keep each point concise and numbered to match the 5 marks.
Common mistakes
- Confusing the relay neuron with the motor neuron or omitting the relay step.
- Failing to mention that the impulse does not travel to the brain, thus no conscious awareness.
Mark scheme (5 marks)
- A receptor detects the stimulus (heat/pain) and generates a nerve impulse
- The impulse travels along a sensory neurone to the spinal cord / relay neurone
- The relay neurone (in the spinal cord) passes the impulse to the motor neurone
- The impulse travels along a motor neurone to the effector (muscle)
- The response is rapid / does not involve conscious thought because the impulse does not travel to the brain (bypasses the brain / is processed in the spinal cord)
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →