# 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.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.3 Nervous system and reflex action · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The tap on the tendon acts as a stimulus, which is detected by a receptor (in the muscle/tendon).
2. A nerve impulse travels along a sensory neurone to the spinal cord (coordinator).
3. In the spinal cord, a relay neurone connects the sensory neurone to the motor neurone.
4. A nerve impulse travels along a motor neurone to the effector (muscle in the leg).
5. 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

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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