# Explain the role of calcium ions in initiating skeletal muscle contraction following the arrival of an action potential at the neuromuscular junction.

> IB DP Biology Higher Level (2023 syllabus) — B3.3 Muscle and motility (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Action potential travels along the T-tubules (transverse tubules) into the interior of the muscle fibre.
2. The action potential in the T-tubules triggers the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
3. Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
4. Exposure of actin binding sites allows myosin heads to attach, forming cross-bridges, enabling the power stroke and muscle contraction.

## Key terms

- [calcium ions](https://www.gradenine.co.uk/glossary/calcium-ions)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-role-of-calcium-ions-c77af3d2) · Published by Druglandscape Ltd.