# Explain how a skeletal muscle fibre relaxes after stimulation ceases.

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

## Mark scheme (4 marks)

1. Action potentials at the neuromuscular junction cease, so no further acetylcholine is released (or acetylcholine is broken down by acetylcholinesterase), stopping depolarisation of the sarcolemma / T-tubules.
2. The sarcoplasmic reticulum actively pumps calcium ions (Ca²⁺) back into its lumen using ATP / Ca²⁺-ATPase pumps, lowering cytoplasmic Ca²⁺ concentration.
3. With low cytoplasmic Ca²⁺, troponin releases calcium ions and reverts to its original conformation, causing tropomyosin to move back and block the myosin-binding sites on actin.
4. With binding sites blocked, myosin heads cannot attach to actin and cross-bridge cycling stops; the thin filaments slide back (or elastic recoil of titin / connective tissue) returns the sarcomere to its resting length.

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