Describe how the nervous system carries out a reflex arc when a person touches a sharp object with their finger. Include the names of the structures and neurones involved.

Eduqas A-Level Biology — 4.2 Inheritance · Describe · 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 mechanoreceptors in the skin of the finger.
1. A sensory neuron (afferent) carries the impulse to the spinal cord.
2. In the spinal cord a relay (interneuron) connects the sensory neuron to a motor neuron.
3. The motor neuron (efferent) carries the impulse to the effector – a muscle in the arm/hand.
4. The muscle contracts, withdrawing the finger. The response is involuntary and does not reach the brain first.

Examiner tips

  • Use the word ‘sensory’ for the afferent neuron and ‘motor’ for the efferent neuron. Mention the relay neuron in the spinal cord. State that the effector is a muscle and that the response is involuntary.
  • common_mistakes
  • :
  • Confusing sensory with motor neuron. Omitting the relay neuron. Forgetting to note that the response is involuntary.

Mark scheme (5 marks)

  1. The sharp object acts as a stimulus, detected by receptors in the finger.
  2. An impulse travels along a sensory neurone to the spinal cord.
  3. A relay neurone in the spinal cord passes the impulse to the motor neurone.
  4. The impulse travels along the motor neurone to an effector (e.g. a muscle in the arm/hand).
  5. The effector (muscle) contracts to withdraw the finger, and the response does not travel to the brain first / is involuntary.

Key terms in this question

reflex arc

Related

More Inheritance questions

▶ Try answering this question with AI marking (free) →