# A student accidentally steps on a sharp drawing pin. Before the student is consciously aware of any pain, their foot is automatically pulled away from the pin. Explain how the nervous system produces this automatic response, naming the neurones involved and describing how the nerve impulse travels between them.

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

> Reflex actions allow the body to respond rapidly to potentially harmful stimuli without conscious thought. These responses are coordinated by reflex arcs.

## Mark scheme (5 marks)

1. Receptors in the skin of the foot detect the sharp stimulus (pain/pressure)
2. A nerve impulse travels along a sensory neurone to the spinal cord
3. The impulse is passed across a synapse (between neurones) using a chemical / neurotransmitter
4. The relay neurone (in the spinal cord) passes the impulse to the motor neurone
5. The motor neurone carries the impulse to the effector (muscle in the leg/foot) causing it to contract, pulling the foot away

## 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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