The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system. Describe the roles of sensory neurones, relay neurones, and motor neurones in coordinating a response to a stimulus.

WJEC A-Level Biology (Wales) — 3.8 The nervous system · Describe · 5 marks · View as Markdown

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

A student hears a loud bang and immediately turns their head towards the source of the sound.

Model answer (5 marks)

Sensory neurones carry impulses from the receptors that detect the loud bang to the CNS. In the CNS, relay neurones receive these impulses and transmit them to the appropriate motor neurones. Motor neurones then carry the impulses from the CNS to the effectors – in this case the neck muscles – which contract and turn the head towards the sound. The sequence of the pathway is therefore receptor → sensory neurone → relay neurone → motor neurone → effector.

Examiner tips

  • Use the exact terms: sensory, relay, motor neurones, effectors
  • Show the correct sequence of the pathway
  • Mention that the effectors are muscles in this example
  • Keep the answer concise – 5 marks only

Common mistakes

  • Confusing sensory and motor neurones, or omitting relay neurones
  • Not specifying that effectors are muscles
  • Writing a long narrative instead of a concise description

Mark scheme (5 marks)

  1. Sensory neurones carry impulses from receptors to the CNS
  2. Relay neurones are located in the CNS and pass impulses between sensory and motor neurones
  3. Motor neurones carry impulses from the CNS to effectors
  4. Effectors (muscles or glands) produce the response
  5. The pathway follows the correct sequence: receptor → sensory neurone → relay neurone → motor neurone → effector

Key terms in this question

sensory neurone · relay neurone · motor neurone · CNS

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