A doctor tests a patient's knee-jerk reflex by tapping the tendon just below the kneecap. The lower leg kicks forward automatically, without the patient deciding to move it. Explain why the knee-jerk response occurs without conscious thought, and describe the roles of the neurones involved in producing this response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The knee-jerk reflex is a spinal reflex — the nerve impulse travels through a reflex arc that does not require the brain to initiate the response.
Model answer (5 marks)
1. The tap on the tendon is the stimulus, detected by a muscle spindle receptor.
2. A sensory neurone carries the impulse to the spinal cord.
3. In the spinal cord a relay (inter) neurone connects the sensory neurone to a motor neurone.
4. The motor neurone sends the impulse to the quadriceps muscle, causing it to contract.
5. The response occurs without conscious thought because the reflex arc bypasses the brain – the impulse is processed entirely in the spinal cord before the muscle contracts.
2. A sensory neurone carries the impulse to the spinal cord.
3. In the spinal cord a relay (inter) neurone connects the sensory neurone to a motor neurone.
4. The motor neurone sends the impulse to the quadriceps muscle, causing it to contract.
5. The response occurs without conscious thought because the reflex arc bypasses the brain – the impulse is processed entirely in the spinal cord before the muscle contracts.
Examiner tips
- Use the exact sequence of neurones: sensory → relay → motor. Mention the muscle spindle as the receptor. Explain that the brain is bypassed. Keep each point brief and to the point.
Common mistakes
- Confusing the receptor with the effector. Forgetting to state that the brain is bypassed. Using vague terms like "nerve cell" instead of specifying sensory, relay, and motor neurones.
Mark scheme (5 marks)
- The tap on the tendon acts as a stimulus, which is detected by a receptor (in the muscle/tendon).
- A nerve impulse travels along a sensory neurone to the spinal cord (coordinator).
- In the spinal cord, a relay neurone connects the sensory neurone to the motor neurone.
- A nerve impulse travels along a motor neurone to the effector (muscle in the leg).
- The response occurs without conscious thought because the reflex arc bypasses the brain / the impulse does not need to travel to the brain before a response is produced.
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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