A cyclist is riding along a road when they see a red traffic light ahead. They decide to apply the brakes. Explain how the nervous system coordinates this response, from detecting the red light to the muscles in the hand acting on the brake lever.

Eduqas A-Level Biology — 3.8 The nervous system · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The eye acts as the receptor, detecting the red light as a stimulus.
A sensory neuron carries a nerve impulse to the brain (CNS).
The brain processes the information and coordinates a response.
A motor neuron sends a nerve impulse from the brain to the effector.
The hand muscles contract to act on the brake lever.

Examiner tips

  • Use the sequence: receptor → sensory neuron → CNS → motor neuron → effector.
  • Mention the specific role of each part (eye, sensory neuron, brain, motor neuron, muscles).

Common mistakes

  • Omitting the eye as the receptor; writing only ‘light detected’. Confusing sensory and motor neurons. Failing to state that the hand muscles are the effector.

Mark scheme (5 marks)

  1. The eye acts as the receptor, detecting the red light (stimulus)
  2. A nerve impulse is sent along a sensory neurone to the brain / central nervous system
  3. The brain (CNS) processes the information and coordinates a response
  4. A nerve impulse is sent along a motor neurone from the brain to the effector
  5. The muscles in the hand act as the effector and contract to apply the brakes

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