Describe the pathway of a reflex arc when a person steps on a sharp object. Include the role of each type of neurone involved.

Eduqas A-Level Biology — 4.4 Application of reproduction and genetics · Describe · 5 marks · View as Markdown

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

When a person accidentally steps on a sharp object, they quickly pull their foot away before they are fully aware of the pain.

Model answer (5 marks)

The sharp object is the stimulus, detected by receptors in the skin of the foot.
1. A sensory (afferent) neuron carries the impulse to the spinal cord.
2. In the spinal cord a relay (interneuron) passes the impulse to a motor (efferent) neuron.
3. The motor neuron carries the impulse to the leg/foot muscle, the effector.
4. The response is involuntary; the impulse does not reach the brain, so the foot is pulled away before conscious awareness.

Examiner tips

Use the word "sensory" for the afferent neuron, "relay" for the interneuron, and "motor" for the efferent neuron.
Show the sequence: stimulus → receptor → sensory neuron → spinal cord → relay neuron → motor neuron → effector.
Mention that the response is involuntary and does not involve the brain.

Common mistakes

Confusing sensory and motor neurons; students often label both as sensory.
Omitting the relay neuron or the fact that the impulse does not reach the brain.
Using vague terms like "nerve cell" instead of the specific types.

Mark scheme (5 marks)

The sharp object acts as a stimulus which is detected by receptors in the skin / foot
A nerve impulse travels along the sensory neurone to the spinal cord (CNS)
The relay neurone in the spinal cord / CNS passes the impulse to the motor neurone
The motor neurone carries the impulse from the spinal cord / CNS to an effector (muscle in the leg / foot)
The impulse does not travel to the brain / the response is involuntary, so the foot is pulled away before conscious awareness

Key terms in this question

reflex arc