Describe the pathway of a nerve impulse in a reflex arc when a person accidentally touches a sharp object with their finger. Name each type of neurone involved and state the role of each one.

Eduqas A-Level Biology — 4.3 Variation and evolution · 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. A sharp object stimulates cutaneous receptors in the finger.
2. A sensory neuron carries the action potential to the spinal cord.
3. In the spinal cord a relay (interneuron) passes the impulse to a motor neuron.
4. The motor neuron carries the impulse to the effector muscle.
5. The response is involuntary; the impulse does not reach the brain, allowing a rapid withdrawal.

Examiner tips

  • Use the exact terms: sensory, relay (interneuron), motor; include the fact that the brain is bypassed for speed.
  • Number each step to show the sequence and match the marks for each point.

Common mistakes

  • Calling the relay neuron a sensory neuron; confusing the roles of interneuron and motor neuron.
  • Omitting that the brain is not involved in the reflex arc.

Mark scheme (5 marks)

  1. The sharp object acts as a stimulus that is detected by receptors in the finger / skin.
  2. The sensory neurone carries the impulse from the receptor to the spinal cord.
  3. The relay neurone (in the spinal cord / CNS) passes the impulse from the sensory neurone to the motor neurone.
  4. The motor neurone carries the impulse from the spinal cord / CNS to the effector (e.g. a muscle).
  5. The impulse does not travel to the brain (the response is involuntary / automatic), allowing a faster response to remove the finger from danger.

Key terms in this question

reflex arc

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