A cyclist is riding along a path when a dog suddenly runs in front of them. The cyclist sees the dog and quickly squeezes the brakes. Explain how the nervous system coordinates this response, from the detection of the stimulus to the effector producing a response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The nervous system is made up of receptors, sensory neurones, relay neurones, motor neurones, and effectors. Electrical impulses travel along neurones and pass across synapses between neurones.
Model answer (5 marks)
Receptors in the cyclist’s eyes detect the dog and the light it reflects.
A sensory neurone carries the electrical impulse to the central nervous system.
The impulse crosses a synapse in the CNS, where neurotransmitters are released.
A motor neurone then carries the impulse from the CNS to the hand/arm muscles.
The effector muscles contract, allowing the cyclist to squeeze the brakes.
A sensory neurone carries the electrical impulse to the central nervous system.
The impulse crosses a synapse in the CNS, where neurotransmitters are released.
A motor neurone then carries the impulse from the CNS to the hand/arm muscles.
The effector muscles contract, allowing the cyclist to squeeze the brakes.
Examiner tips
- Use the exact sequence: receptor → sensory neurone → CNS → synapse → motor neurone → effector
- Mention neurotransmitters at the synapse
- Keep answer concise, 5 points total
Common mistakes
- Omitting the synapse or neurotransmitter step
- Using vague terms like ‘brain’ instead of CNS
- Not specifying the effector as hand/arm muscles
Mark scheme (5 marks)
- Receptors in the eyes detect the stimulus (the dog / light from the dog)
- A nerve impulse travels along a sensory neurone to the central nervous system (CNS) / brain
- The impulse passes across a synapse (between neurones) using chemicals / neurotransmitters
- A nerve impulse travels along a motor neurone from the CNS to the effector (muscles in the hand/arm)
- The effector (muscle) contracts / responds, causing the cyclist to squeeze the brakes
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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