Explain the process of the sliding filament theory, including the role of calcium, actin, myosin and ATP in producing a muscle contraction

What this dot point is asking

This is the mechanism behind every contraction in the course. WACE expects you to describe the sequence in order and to name the role of calcium, actin, myosin and ATP. The filaments themselves do not shorten; they slide past one another, which is the idea the theory is named for.

Setting the scene

Inside each sarcomere the thick myosin filaments and thin actin filaments overlap. At rest, the regulatory protein tropomyosin sits over the binding sites on actin so myosin cannot attach. A contraction is simply the controlled removal of that block, followed by repeated pulling.

The sequence of events

First, a nerve impulse arrives at the muscle and spreads along the sarcolemma and down the T tubules. This signals the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm.

Second, calcium binds to troponin on the thin filament. This causes tropomyosin to move aside and expose the myosin binding sites on actin.

Third, the energised myosin heads attach to actin, forming cross bridges. Each head then pivots in the power stroke, pulling the thin filament toward the centre of the sarcomere. This is the actual sliding.

Fourth, a new molecule of ATP binds to the myosin head, breaking the cross bridge so the head detaches. ATP is then split into ADP and a phosphate, which re-cocks the head ready to attach again further along the actin.

This grab, pull, release and re-cock is the cross bridge cycle, and it repeats many times per second. Because thin filaments are pulled in from both ends, the sarcomere shortens and the whole muscle contracts.

How the contraction ends

When the impulse stops, calcium is actively pumped back into the sarcoplasmic reticulum, again using ATP. With calcium gone, troponin and tropomyosin recover the binding sites, the cross bridges can no longer form, and the filaments slide back to their resting position as the muscle relaxes.

How this maps to the exam

This content is often worth several marks for a step by step explanation. Write it as a numbered sequence: impulse, calcium release, calcium binds troponin, tropomyosin moves, cross bridge forms, power stroke, ATP detaches the head, repeat. Naming calcium, actin, myosin and ATP at the correct step is where the marks sit.