Explain how a nerve impulse travels from a receptor to an effector during a reflex arc when a person touches a sharp object.

WJEC A-Level Biology (Wales) — 4.2 Inheritance · 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 sharp object is the stimulus, detected by sensory receptors.
The sensory neurone carries the impulse to the spinal cord.
Within the spinal cord a relay (interneuron) transmits the impulse to a motor neurone.
The motor neurone carries the impulse to the effector muscle.
Because the impulse never reaches the brain, the response is rapid and involuntary.

Examiner tips

  • Use the word ‘stimulus’ for the sharp object. Show the pathway: receptor → sensory neuron → spinal cord → relay neuron → motor neuron → effector. Mention the absence of brain involvement to justify speed and involuntariness.
  • common_mistakes
  • :
  • Confusing the sensory neuron with the motor neuron. Forgetting to state that the impulse does not reach the brain. Using vague terms like ‘nerve’ instead of specifying sensory, relay and motor neurons.

Mark scheme (5 marks)

  1. The sharp object acts as a stimulus which is detected by receptors
  2. An impulse travels along a sensory neurone to the spinal cord / CNS
  3. The relay neurone carries the impulse within the spinal cord / CNS to the motor neurone
  4. The impulse travels along the motor neurone to the effector (e.g. a muscle)
  5. The impulse does not travel to the brain, so the response is faster / involuntary

Key terms in this question

receptor · effector · reflex arc

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