A student accidentally steps on a sharp drawing pin. Before the student is consciously aware of any pain, their foot is automatically pulled away from the pin. Explain how the nervous system produces this automatic response, naming the neurones involved and describing how the nerve impulse travels between them.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.3 Nervous system and reflex action · Explain · 5 marks · View as Markdown

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

Reflex actions allow the body to respond rapidly to potentially harmful stimuli without conscious thought. These responses are coordinated by reflex arcs.

Model answer (5 marks)

1. Receptors in the skin of the foot detect the sharp stimulus (pain/pressure). 2. A sensory neuron carries the impulse along its axon to the dorsal horn of the spinal cord. 3. The impulse crosses a synapse in the dorsal horn, where the sensory neuron releases a neurotransmitter (e.g. glutamate) onto the relay neuron. 4. The relay neuron (interneuron) conducts the impulse across the spinal cord and synapses onto a motor neuron. 5. The motor neuron carries the impulse to the muscle in the leg/foot, causing it to contract and pull the foot away.

Examiner tips

  • Use the exact sequence of neurons: sensory → relay (interneuron) → motor. Mention neurotransmitter release at the synapse. Show the direction of impulse travel. Keep each point brief and to the point.

Common mistakes

  • Confusing the sensory neuron with the motor neuron. Forgetting to mention the neurotransmitter at the synapse. Describing the impulse as travelling through the effector rather than the motor neuron.

Mark scheme (5 marks)

  1. Receptors in the skin of the foot detect the sharp stimulus (pain/pressure)
  2. A nerve impulse travels along a sensory neurone to the spinal cord
  3. The impulse is passed across a synapse (between neurones) using a chemical / neurotransmitter
  4. The relay neurone (in the spinal cord) passes the impulse to the motor neurone
  5. The motor neurone carries the impulse to the effector (muscle in the leg/foot) causing it to contract, pulling the foot away

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