The sliding filament theory of muscle contraction

The sliding filament theory explains how a muscle contracts. Thin actin filaments slide past thick myosin filaments, pulled by the heads of the myosin molecules, so each sarcomere gets shorter while the filaments themselves stay the same length. Calcium ions switch the process on and ATP powers it.

What is the sliding filament theory?

Skeletal muscle is made of long cells called muscle fibres, and each fibre is packed with myofibrils. A myofibril is a chain of repeating units called sarcomeres, the units that actually contract. Inside each sarcomere are two kinds of protein filament:

• Thin filaments, made of actin, with the proteins tropomyosin and troponin wound along them. They are anchored to the Z lines at each end of the sarcomere.
• Thick filaments, made of myosin. Each myosin molecule has a head that sticks out and can bind to actin.

The theory says contraction happens because the myosin heads grab the actin and pull it towards the middle of the sarcomere, over and over. The filaments overlap more, the Z lines are drawn closer together, and the whole muscle shortens because thousands of sarcomeres in a row all shorten at once.

It was put forward in two papers published side by side in Nature on 22 May 1954: one by Andrew Huxley and Rolf Niedergerke, the other by Hugh Huxley and Jean Hanson. Their key observation was that the dark A band of a sarcomere keeps the same width when a muscle contracts or is stretched, which only makes sense if filaments of fixed length slide past each other.

The bands of a sarcomere

Under an electron microscope, skeletal muscle looks striped. Each stripe is named, and the sliding filament theory predicts exactly which ones change.

A quick way to remember it: the bands with an H or an I in them (H zone, I band) shrink, because they are the parts where the filaments don't yet overlap. The A band matches one myosin filament from end to end, and myosin doesn't change length.

Worked example: band lengths

Suppose a relaxed sarcomere is 2.6 µm long, each thick filament is 1.6 µm and each thin filament is 1.0 µm. The muscle contracts until the sarcomere is 2.2 µm. Find the A band, I band and H zone before and after.

• A band = length of a thick filament = 1.6 µm, both before and after.
• I band = sarcomere − A band. Before: 2.6 − 1.6 = 1.0 µm (0.5 µm at each end). After: 2.2 − 1.6 = 0.6 µm.
• H zone = sarcomere − two thin filaments. Before: 2.6 − 2 × 1.0 = 0.6 µm. After: 2.2 − 2.0 = 0.2 µm.

The sarcomere shortened by 0.4 µm, and the I band and H zone each lost exactly that 0.4 µm, while the A band didn't change. That pattern is the evidence for sliding.

Muscle contraction step by step

Here is the sequence from a nerve impulse to a contracting myofibril, as A-level specifications describe it.

1. An impulse arrives. An action potential reaches the neuromuscular junction, where the motor neurone releases acetylcholine. This depolarises the membrane of the muscle fibre (the sarcolemma).
2. Calcium ions are released. The depolarisation spreads deep into the fibre along T-tubules, and the sarcoplasmic reticulum, the fibre's store of calcium, releases Ca²⁺ ions into the cytoplasm.
3. The binding sites are uncovered. At rest, tropomyosin lies across the myosin binding sites on actin. Calcium ions bind to troponin, which pulls the tropomyosin out of the way.
4. Cross-bridges form. Myosin heads, already carrying ADP and phosphate, bind to the exposed sites on actin, forming actinomyosin cross-bridges.
5. The power stroke. Each myosin head tilts, pulling the actin filament towards the M line. ADP and phosphate are released.
6. The heads let go. A new ATP molecule binds to each myosin head, and the head detaches from the actin.
7. The heads reset. The myosin head hydrolyses the ATP to ADP and phosphate, and the energy released swings the head back to its starting angle, ready to bind further along the actin.

Steps 4 to 7 repeat many times a second for as long as calcium ions and ATP are present. Each cycle moves the actin a tiny distance, and millions of heads working out of step give a smooth pull.

Relaxation

When the nerve impulses stop, calcium ions are actively transported back into the sarcoplasmic reticulum, which uses ATP. Tropomyosin covers the binding sites again, no more cross-bridges form, and the muscle relaxes. It is pulled back to its full length by its antagonistic partner, because muscles can only pull, never push.

The role of ATP in muscle contraction

ATP is needed at three points, and it's a favourite exam question:

• Binding of ATP detaches the myosin head from actin.
• Hydrolysis of ATP resets the myosin head, ready for the next power stroke.
• ATP powers the active transport of calcium ions back into the sarcoplasmic reticulum during relaxation.

Muscles hold only a small amount of ATP, so they keep a reserve called phosphocreatine (creatine phosphate). It gives its phosphate to ADP to make ATP again very quickly, without needing oxygen. It runs out after roughly 15 seconds of hard work, after which respiration has to supply the ATP.

ATP's job in detaching the heads explains rigor mortis. After death, ATP runs out, the myosin heads stay locked onto actin, and the muscles become stiff.

Slow and fast twitch muscle fibres

Skeletal muscles contain a mix of fibre types, suited to different jobs.

Common mistakes

• Saying the filaments shorten. Actin and myosin stay the same length. The sarcomere shortens because they overlap more.
• Saying the A band shortens. It doesn't. The I band and H zone shorten; the A band spans a whole thick filament, which never changes length.
• Saying ATP is needed to form cross-bridges. ATP is needed to break them and to reset the head. That is why muscles lock in rigor mortis.
• Saying calcium binds to tropomyosin or actin. Calcium binds to troponin, which moves the tropomyosin.
• Saying myosin pulls towards the Z line. The heads pull actin towards the M line, the middle of the sarcomere, so the Z lines come closer together.

To practise muscle contraction questions alongside the rest of the second-year content, try today's A-level Biology: Year 2 short timed paper. Each paper mixes topics, so you won't see this one every day.

Read next

Biology · Nerves and homeostasisMuscle fibres: structure, types and how they get energyWhat a skeletal muscle fibre is, its parts and their jobs, slow and fast twitch fibres compared, and how fibres make ATP, with A-level worked examples.