Describe the pathway of a nerve impulse through a reflex arc when a person's hand touches a sharp object.

Cambridge International IGCSE Biology (0610) — 14.1 Nervous control in humans · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The sharp object is the stimulus detected by receptors in the skin.
The sensory neurone carries the impulse to the spinal cord.
In the spinal cord a relay neurone passes the impulse to a motor neurone.
The motor neurone carries the impulse to the muscle, which contracts to withdraw the hand.

Examiner tips

Use the word "receptor" for the skin, "sensory neurone" for the afferent, "relay neurone" for the interneuron, and "motor neurone" for the efferent.
Show the sequence clearly – stimulus → receptor → sensory neurone → spinal cord → relay neurone → motor neurone → effector.
Use the term "effector" (muscle) and describe the contraction.
Keep the answer concise – 4 marks, 1 point each.

Common mistakes

Mixing up sensory and motor neurones, or omitting the relay neurone.
Using the wrong terminology (e.g., "nerve" instead of "neurone").
Leaving out the effector or not stating the contraction response.

Mark scheme (4 marks)

The sharp object acts as a stimulus that is detected by receptors in the skin.
A nerve impulse travels along the sensory neurone to the spinal cord (CNS).
The relay neurone in the spinal cord passes the impulse to the motor neurone.
The impulse travels along the motor neurone to an effector (muscle), which responds by contracting to withdraw the hand.

Key terms in this question

reflex arc · nerve impulse