# A doctor tests a patient's knee-jerk reflex by tapping the tendon just below the kneecap. The lower leg kicks forward rapidly without the patient deciding to move it. Explain how the nervous system produces this rapid, automatic response, naming the components of the reflex arc involved.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.3 Nervous system and reflex action · Explain · 5 marks

> The knee-jerk reflex is a spinal reflex. When the tendon is tapped, the leg muscles are briefly stretched, triggering a response that does not require the brain to initiate the movement.

## Mark scheme (5 marks)

1. A receptor detects the stimulus (stretch of the muscle/tendon)
2. A sensory neurone carries the electrical impulse to the spinal cord (coordinator)
3. The spinal cord acts as the coordinator and passes the impulse to a motor neurone (via a relay/connector neurone)
4. A motor neurone carries the impulse from the spinal cord to the effector (muscle)
5. The effector (muscle) contracts, producing the rapid, automatic response without conscious control

## Key terms

- [reflex arc](https://www.gradenine.co.uk/glossary/reflex-arc)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-doctor-tests-a-patient-s-knee-jerk-3f0c2725) · Published by Druglandscape Ltd.