A student is studying how the nervous system coordinates a response. They learn that when a person smells smoke, the nervous system detects the stimulus and sends electrical impulses to the muscles, which contract to move the person away from danger. Explain how the nervous system coordinates this response, from detecting the stimulus to bringing about the muscular response.

OCR A-Level Biology B — Advancing Biology (H422) — 5.7 Nervous communication · Explain · 5 marks · View as Markdown

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

When a person smells smoke, receptor cells in the nose detect the chemical stimulus. A series of electrical impulses then travel through the nervous system until muscles contract, moving the person away from danger.

Model answer (5 marks)

1. Receptor cells in the olfactory epithelium bind the smoke molecules and generate a receptor potential.
2. The receptor potential is converted into an action potential that travels along sensory neurons to the olfactory bulb in the CNS.
3. The olfactory bulb relays the signal to the brain, where the information is processed and a motor programme is generated.
4. Motor neurons carry the action potential from the brain to the appropriate skeletal muscles.
5. The muscle fibres receive the impulse, contract and produce the movement that removes the person from the danger.

Examiner tips

  • Use the exact sequence: receptor → sensory neuron → CNS → motor neuron → effector.
  • Show each step as a separate point to match the 5‑mark structure.
  • Mention the olfactory bulb and brain as the processing centres.
  • Include the word ‘contraction’ to show muscular response.

Common mistakes

  • Confusing sensory and motor neurons; students often say the same neuron type for both.
  • Omitting the olfactory bulb or brain as the processing site.
  • Failing to mention that the muscle is the effector and that contraction causes movement.

Mark scheme (5 marks)

  1. Receptor cells in the nose detect the stimulus (smell of smoke)
  2. Electrical impulses travel along sensory neurones to the central nervous system (CNS / brain or spinal cord)
  3. The CNS (brain) processes the information and coordinates the response
  4. Electrical impulses travel along motor neurones from the CNS to the effector (muscle)
  5. The muscle (effector) contracts, bringing about the response / moving the person away from danger

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