A doctor is testing a patient's nervous system. The doctor taps the patient's knee with a rubber hammer, and the patient's leg kicks forward automatically. This is called a reflex action. Explain how a nerve impulse travels from the receptor in the knee to the effector muscle, causing the leg to kick forward.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Reflex actions are rapid, automatic responses to stimuli that do not require conscious thought. They involve a specific pathway of neurones.
Model answer (5 marks)
1. A receptor in the knee senses the tap and generates a nerve impulse.
2. The impulse travels along a sensory neurone to the spinal cord.
3. It passes across a synapse onto a relay neurone in the spinal cord.
4. A motor neurone carries the impulse from the spinal cord to the quadriceps muscle.
5. The muscle contracts, causing the leg to kick forward.
2. The impulse travels along a sensory neurone to the spinal cord.
3. It passes across a synapse onto a relay neurone in the spinal cord.
4. A motor neurone carries the impulse from the spinal cord to the quadriceps muscle.
5. The muscle contracts, causing the leg to kick forward.
Examiner tips
- Use the exact sequence of steps; each step earns one mark.
- Show the pathway: receptor → sensory neurone → synapse → relay neurone → motor neurone → effector.
- Use correct terminology: receptor, sensory neurone, synapse, relay neurone, motor neurone, effector muscle.
- Keep the answer concise – one sentence per point.
Common mistakes
- Mixing up sensory and motor neurones; forgetting the relay neurone.
- Using vague terms like ‘nerve’ instead of ‘sensory/motor neurone’.
- Omitting the synapse step or the effector muscle contraction.
Mark scheme (5 marks)
- A receptor in the knee detects the stimulus (the tap)
- A nerve impulse travels along a sensory neurone to the spinal cord (relay neurone)
- The impulse passes across a synapse (between neurones)
- A motor neurone carries the impulse from the spinal cord to the effector (muscle)
- The effector muscle contracts, causing the leg to kick forward
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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