Myofibril Contraction: The Sliding Filament Model Revision Notes | A Level AQA Biology

1. The Molecular Machine Inside Every Muscle

Every time you blink, breathe, or lift a bag, millions of molecular events fire in sequence inside your muscle fibres. The question is: what actually pulls what, and how does a nerve signal become movement?

The answer lies in the sliding filament model — the mechanism by which two sets of protein filaments slide past each other to shorten a myofibril. No filament actually gets shorter; they overlap more. That distinction matters enormously in an exam.

To understand the model, you need to know the four key players: actin, myosin, calcium ions, and ATP. Each has a precise, non-interchangeable role, and examiners test all four.

2. The Structure You Need to Picture

Picture a myofibril as a series of repeating units called sarcomeres. Each sarcomere contains two types of filament arranged in parallel: thick filaments made of myosin, and thin filaments made of actin. The myosin filaments sit in the middle; the actin filaments extend inward from each end.

When a sarcomere contracts, the actin filaments are pulled inward by the myosin heads, so the two sets of filaments overlap more. The sarcomere shortens, the myofibril shortens, and the muscle fibre shortens — that is contraction.

Running alongside the actin filament is a protein called tropomyosin. At rest, tropomyosin sits directly over the myosin-binding sites on actin, physically blocking any cross-bridge from forming. This is the molecular lock that keeps your muscles relaxed until a signal arrives.

3. Calcium Ions and Tropomyosin: Unlocking the Filament

When a motor neurone fires, an action potential travels along the sarcolemma and into the T-tubules, triggering the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the sarcoplasm. This is the signal that starts everything.

Calcium ions bind to a protein complex associated with tropomyosin. This causes tropomyosin to shift its position along the actin filament, exposing the myosin-binding sites on actin. The molecular lock is now open.

The AQA specification states that you need to know the roles of calcium ions and tropomyosin in this process. The role of troponin — the protein that calcium actually binds to — is not required, so you do not need to name it. Focus on the outcome: calcium displaces tropomyosin, binding sites are exposed, cross-bridges can form.

Once the binding sites are exposed, myosin heads are free to attach. This is where ATP and the power stroke come in — the mechanism that actually generates force.

4. The Cross-Bridge Cycle: ATP and the Power Stroke

The cross-bridge cycle is a repeating sequence of events that drives filament sliding. Each cycle uses one molecule of ATP and moves the actin filament a small distance. Thousands of myosin heads cycling simultaneously produce the force needed to shorten a sarcomere.

Worked example: The Actinomyosin Cross-Bridge Cycle

Trace the sequence of molecular events from an exposed actin binding site through one complete cross-bridge cycle.

1. Exposed binding site on actin + myosin head → actinomyosin bridge formed — This is only possible because calcium ions have displaced tropomyosin. The myosin head at this point already has ADP and inorganic phosphate (Pi) attached from the previous cycle — it is in a high-energy, cocked position.
2. ADP + Pi released from myosin head → myosin head pivots, pulling actin filament toward centre of sarcomere — The release of ADP and Pi triggers a conformational change in the myosin head — it bends, pulling the actin filament inward. This is the power stroke. The filaments slide past each other; the sarcomere shortens.
3. ATP binds to myosin head → actinomyosin bridge breaks — ATP binding causes the myosin head to detach from actin. Without ATP, the myosin head cannot detach — this is why rigor mortis occurs after death when ATP is no longer produced.
4. ATP → ADP + Pi (catalysed by ATPase on myosin head) → myosin head returns to cocked, high-energy position — The energy from ATP hydrolysis re-cocks the myosin head, ready to bind again. The cycle repeats as long as calcium ions keep the binding sites exposed and ATP is available.

Answer: One complete cycle: binding site exposed → actinomyosin bridge forms → power stroke (ADP + Pi released) → ATP binds, bridge breaks → ATP hydrolysed, head re-cocked → cycle repeats.

Notice that ATP has two distinct roles here: it provides the energy for the power stroke (via hydrolysis), and it is required for the myosin head to detach from actin. Both roles are examinable and students frequently only state one.

5. Relaxation: Reversing the Process

Contraction continues only while calcium ions remain in the sarcoplasm. When nerve stimulation stops, calcium ions are actively pumped back into the sarcoplasmic reticulum — a process that itself requires ATP.

As calcium ion concentration in the sarcoplasm falls, tropomyosin moves back to cover the myosin-binding sites on actin. Cross-bridge formation stops, and the muscle relaxes. The actin filaments slide back to their original position, partly due to the elastic recoil of the muscle and the action of antagonistic muscles.

This means ATP is required at three points in the overall process: to re-cock the myosin head, to detach the myosin head from actin, and to pump calcium ions back into the sarcoplasmic reticulum during relaxation. Examiners frequently ask students to account for ATP use in muscle contraction — three distinct roles is the complete answer.

6. Putting It All Together: The Full Sequence

It helps to see the entire sequence as one connected chain of cause and effect. Each event creates the conditions for the next — disrupt any link and contraction fails.

• Action potential arrives at neuromuscular junction → acetylcholine released → action potential in sarcolemma
• Action potential travels into T-tubules → calcium ions released from sarcoplasmic reticulum
• Calcium ions bind (to associated protein) → tropomyosin shifts → myosin-binding sites on actin exposed
• Myosin heads (carrying ADP + Pi) bind to actin → actinomyosin bridges form
• Power stroke: ADP + Pi released → myosin head pivots → actin filament pulled toward centre of sarcomere
• ATP binds to myosin head → bridge breaks → myosin head detaches from actin
• ATP hydrolysed by ATPase → myosin head re-cocked → cycle repeats while calcium ions remain elevated
• Stimulation stops → calcium ions pumped back (ATP required) → tropomyosin covers binding sites → relaxation

7. Exam Technique and Common Mistakes

AQA questions on this topic most commonly use the command words describe and explain. A describe answer gives the sequence of events; an explain answer adds the mechanistic reason for each step. Practise distinguishing between the two.

For longer-mark questions, examiners award marks for specific molecular detail. Vague phrases such as 'energy is released' or 'the muscle contracts' will not score. Name the proteins, name the ions, and state the direction of movement.