Describe how a nerve impulse travels from a receptor to an effector in the nervous system.

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).

Model answer (5 marks)

1. The receptor detects a stimulus and generates an action potential.
2. The action potential travels along a sensory neuron to the central nervous system (brain or spinal cord).
3. In the CNS the impulse is transmitted to a relay neuron.
4. The relay neuron sends the impulse along a motor neuron back to the effector.
5. The effector (muscle or gland) responds to the impulse, producing a contraction or secretion.

Examiner tips

  • Use the exact sequence of cells (receptor → sensory neuron → relay neuron → motor neuron → effector).
  • Show the direction of the impulse (sensory to CNS, then motor to effector).
  • Include the type of effector (muscle or gland).

Common mistakes

  • Mixing up the order of sensory and motor neurons.
  • Omitting the relay neuron in the CNS.
  • Not specifying the effector type.

Mark scheme (5 marks)

  1. Receptor detects a stimulus
  2. Nerve impulse travels along a sensory neurone to the CNS (brain or spinal cord)
  3. Impulse passes through a relay neurone in the CNS
  4. Impulse travels along a motor neurone from the CNS to the effector
  5. Effector (muscle or gland) responds to the impulse

Key terms in this question

receptor · effector · nerve impulse

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