Explain how the nervous system and the musculoskeletal system are integrated to allow a skeletal muscle to contract in response to a voluntary stimulus.

IB DP Biology Standard Level (2023 syllabus) — C3.1 Integration of body systems · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A motor neurone carries an action potential from the CNS to the muscle.
At the neuromuscular junction the action potential triggers release of acetylcholine into the synaptic cleft.
Acetylcholine binds to receptors on the motor end‑plate, generating a new action potential in the muscle fibre.
The muscle‑fibre action potential activates the sliding‑filament mechanism, where actin and myosin cross‑bridge, causing the muscle to shorten and move the skeleton.

Examiner tips

  • Use the exact sequence: motor neurone → ACh release → receptor binding → muscle action potential → sliding filament contraction.
  • Mention the neuromuscular junction and motor end‑plate explicitly.
  • Show the link between action potential and actin‑myosin interaction.

Mark scheme (4 marks)

  1. A motor neurone carries an action potential / nerve impulse from the central nervous system (brain or spinal cord) to the skeletal muscle.
  2. At the neuromuscular junction, the action potential triggers release of acetylcholine (a neurotransmitter) into the synaptic cleft.
  3. Acetylcholine binds to receptors on the muscle cell membrane (motor end plate / sarcolemma), generating a new action potential in the muscle fibre.
  4. The action potential in the muscle fibre leads to muscle contraction, involving the interaction of actin and myosin filaments (sliding filament mechanism) so that the muscle shortens and moves the skeleton.

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