A student is investigating how muscles contract. Explain how a muscle contracts when a nerve impulse arrives at the neuromuscular junction, including the role of ATP in this process.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.2 Muscle contraction · Explain · 5 marks · View as Markdown

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

Muscles are made up of muscle fibres that contain protein filaments. When stimulated, these filaments slide past each other to shorten the muscle. This process requires energy from respiration.

Model answer (5 marks)

A nerve impulse travelling along the motor neuron reaches the neuromuscular junction and causes the release of acetylcholine into the synaptic cleft. The acetylcholine binds to nicotinic receptors on the sarcolemma, depolarising the muscle fibre and triggering an action potential that propagates along the fibre and into the T‑tubules. This depolarisation releases Ca²⁺ from the sarcoplasmic reticulum, allowing the myosin heads to bind to actin and perform a power stroke. Each power stroke requires ATP to detach the myosin head and re‑catalyse it for the next cycle, so ATP is the energy source for filament sliding. ATP is produced by aerobic respiration in the mitochondria, where glucose is oxidised with oxygen to give ATP, CO₂ and H₂O.

Examiner tips

  • Use the sequence: nerve impulse → acetylcholine release → receptor binding → depolarisation → Ca²⁺ release → cross‑bridge cycle → ATP requirement → aerobic ATP production.
  • Mention the role of ATP in both power stroke and re‑catalysis of myosin heads. Include mitochondria and glucose/oxygen. Keep answer concise to fit 5 marks.

Common mistakes

  • Confusing acetylcholine with other neurotransmitters; not naming the receptor type; omitting the role of Ca²⁺; failing to state that ATP is produced aerobically in mitochondria.

Mark scheme (5 marks)

  1. A nerve impulse triggers the release of a neurotransmitter at the neuromuscular junction
  2. The neurotransmitter binds to receptors on the muscle fibre, causing the muscle fibre to be stimulated
  3. Protein filaments (actin and myosin) slide past each other, causing the muscle fibre to shorten / contract
  4. ATP provides the energy needed for the filaments to slide / for muscle contraction
  5. ATP is produced by aerobic respiration (in mitochondria) using glucose and oxygen

Key terms in this question

neuromuscular junction · ATP

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