A young child quickly blinks when a fly suddenly moves close to their eye. This blinking response is a reflex action. Explain why reflex actions are important for survival, and describe the roles of the sensory neurone and relay neurone in producing this response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When an object moves rapidly towards the eye, the eyelid closes automatically before a person consciously decides to blink.
Model answer (5 marks)
Reflex actions are faster than voluntary responses, so they protect the body from damage or harm.
They do not require conscious thought and therefore do not involve the conscious brain.
The sensory neurone carries the electrical impulse from the receptor (in the eye) to the relay neurone in the spinal cord.
The relay neurone passes the impulse through the CNS, connecting the sensory neurone to the motor neurone.
The impulse crosses the synapse between neurones by means of chemical transmitters (neurotransmitters) diffusing across the synaptic gap.
They do not require conscious thought and therefore do not involve the conscious brain.
The sensory neurone carries the electrical impulse from the receptor (in the eye) to the relay neurone in the spinal cord.
The relay neurone passes the impulse through the CNS, connecting the sensory neurone to the motor neurone.
The impulse crosses the synapse between neurones by means of chemical transmitters (neurotransmitters) diffusing across the synaptic gap.
Examiner tips
- Use the word "protect" or "prevent" when linking speed to survival. Mention that the reflex arc bypasses the cortex. Show the pathway: receptor → sensory neurone → relay neurone → motor neurone → effector.
Common mistakes
- Confusing the relay neurone with the motor neurone. Forgetting that the reflex arc does not involve the conscious brain. Using vague terms like "fast" without linking to survival.
Mark scheme (5 marks)
- Reflex actions are faster than voluntary responses (so protect the body from damage / harm)
- Reflex actions do not require conscious thought / do not involve the conscious brain
- The sensory neurone carries the electrical impulse from the receptor (in the eye / skin) to the relay neurone (in the spinal cord)
- The relay neurone passes the impulse through the spinal cord (CNS), connecting the sensory neurone to the motor neurone
- Impulses pass across synapses between neurones by means of chemical transmitters (neurotransmitters diffusing across the gap)
Key terms in this question
reflex action · sensory neurone · relay neurone
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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