A student is working in a laboratory and accidentally knocks a glass beaker. Without thinking, they quickly blink and flinch away before the beaker hits their face. Explain why the student responds so rapidly, describing the roles of the receptor, coordinator, and effector in this reflex action.
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 are coordinated through a reflex arc involving specific types of neurones.
Model answer (5 marks)
1. A receptor in the skin and eyes detects the sudden movement of the beaker.
2. A sensory neurone carries the impulse from the receptor to the spinal cord.
3. The spinal cord acts as the coordinator, relaying the impulse via a relay neurone to the motor neurone.
4. A motor neurone carries the impulse from the spinal cord to the facial and arm muscles.
5. The response is rapid because the impulse bypasses the brain, using a short spinal pathway that does not require conscious thought.
2. A sensory neurone carries the impulse from the receptor to the spinal cord.
3. The spinal cord acts as the coordinator, relaying the impulse via a relay neurone to the motor neurone.
4. A motor neurone carries the impulse from the spinal cord to the facial and arm muscles.
5. The response is rapid because the impulse bypasses the brain, using a short spinal pathway that does not require conscious thought.
Examiner tips
- Use the exact terms: receptor, sensory neurone, spinal cord (coordinator), relay neurone, motor neurone, effector. Show the pathway in order and explain why bypassing the brain gives speed. Keep each point short and to the point to hit the 5 marks.
Common mistakes
- Confusing the spinal cord with the brain as the coordinator. Omitting the role of the receptor or effector. Failing to mention that the pathway bypasses the brain for speed.
Mark scheme (5 marks)
- A receptor detects the stimulus (the approaching beaker / light stimulus / sudden movement)
- A sensory neurone carries the electrical impulse from the receptor to the spinal cord / coordinator
- The spinal cord acts as the coordinator / relay neurone in the spinal cord passes the impulse to the motor neurone
- A motor neurone carries the impulse from the spinal cord to the effector (muscle)
- The response is rapid because the impulse bypasses the brain / does not require conscious thought / shorter pathway means faster response
Key terms in this question
receptor · coordinator · effector
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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