Sliding filament theory: WACE Physical Education Studies Unit 3
“Explain the process of the sliding filament theory, including the role of calcium, actin, myosin and ATP in producing a muscle contraction”
The sliding filament theory says a muscle shortens when myosin cross bridges grab actin and pull the thin filaments toward the centre of the sarcomere. A nerve impulse releases calcium, which exposes the binding sites on actin, and ATP powers the repeated grab, pull and release of the cross bridges, so the filaments slide over each other without changing length.
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.
Students remember ATP powers the power stroke but forget it is also needed to break the cross bridge so the head can let go. This is why rigor sets in after death when ATP runs out: the heads cannot detach. In the exam, give ATP two jobs, splitting to re-cock the head for the pull, and binding fresh to release the head between pulls.
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.
The single most common error is writing that the actin or myosin filaments get shorter. They do not change length at all. The sarcomere shortens because the filaments slide over and past each other, increasing their overlap. State clearly that it is the degree of overlap, and therefore the length of the sarcomere, that changes, while the filaments themselves stay the same size.
Step 1: start with the trigger
Open with the nerve impulse arriving and spreading along the sarcolemma and down the T tubules, signalling the sarcoplasmic reticulum to release calcium. Examiners want the contraction to begin with a clear stimulus.
Step 2: explain how the block is removed
State that calcium binds to troponin, which moves tropomyosin off the actin binding sites. This is the step most students skip; naming troponin and tropomyosin separates a full-mark answer from a vague one.
Step 3: describe the cross bridge cycle
Myosin heads attach to the exposed sites to form cross bridges, then perform the power stroke that pulls the thin filaments toward the sarcomere centre. Make clear this sliding increases overlap and shortens the sarcomere.
Step 4: place ATP at the correct points
Give ATP two jobs: it is split to re-cock the head for the power stroke, and a fresh ATP binds to detach the head so the cycle repeats. Then finish with relaxation, where ATP pumps calcium back and the binding sites are re-covered.
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.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style7 marksDescribe the sliding filament theory of muscle contraction, in sequence, naming the roles of calcium, actin, myosin and ATP.Show worked answer →
A 7 mark describe answer needs the full sequence in order, with each named component doing its job.
- Impulse and calcium release
- A nerve impulse spreads along the sarcolemma and down the T tubules, triggering the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.
- Calcium exposes binding sites
- Calcium binds to troponin on the thin (actin) filament, causing tropomyosin to move aside and expose the myosin binding sites on actin.
- Cross bridge and power stroke
- Energised myosin heads attach to actin, forming cross bridges, then pivot in the power stroke, pulling the thin filaments toward the centre of the sarcomere. This sliding increases overlap.
- Role of ATP
- A fresh ATP binds to the myosin head to break the cross bridge so it detaches; ATP is then split to re-cock the head ready to attach again further along actin.
- Outcome
- The cross bridge cycle repeats, the sarcomere shortens and the muscle contracts. The filaments do not change length; they slide past one another.
Markers reward the correct order, calcium exposing the binding sites, myosin pulling actin in the power stroke, and ATP for both the power stroke and detachment.
WACE 2024-style3 marksExplain why ATP is required both to produce a muscle contraction and to allow the muscle to relax.Show worked answer →
A 3 mark explain answer needs ATP given more than one role.
- Contraction
- ATP is split by the myosin head to provide the energy that re-cocks the head for the power stroke that pulls actin inward.
- Detachment within the cycle
- Fresh ATP must also bind to the myosin head to break each cross bridge so the head can release and reattach, allowing the cycle to repeat.
- Relaxation
- ATP also powers the active pumping of calcium back into the sarcoplasmic reticulum; once calcium is removed, the binding sites are recovered and the muscle relaxes.
Markers reward ATP powering the power stroke, ATP breaking the cross bridge, and ATP-driven calcium re-uptake for relaxation.