Biology › Nervous coordination, receptors, muscles and behaviour › Sliding filaments: shortening a muscle without shortening anything

Sliding filaments: shortening a muscle without shortening anything

A muscle contracting to two thirds of its length contains no molecule that has changed length at all. Two sets of filaments slide past each other, dragged by thousands of protein heads rowing in shifts, and the banding pattern down a microscope is the evidence.

Before this Synaptic transmission and the neuromuscular junction · Aerobic and anaerobic respiration

What you should be able to do

• Describe the structure of skeletal muscle from whole muscle down to the sarcomere.
• Explain how an impulse at the neuromuscular junction leads to calcium release inside the fibre.
• Describe the sliding filament mechanism, including the roles of calcium, tropomyosin, ATP and the myosin head.
• State what happens to the A band, I band and H zone during contraction, and why.
• Compare slow and fast twitch fibres, and explain how ATP is supplied to a working muscle.

From muscle to sarcomere

Skeletal muscle is built in nested layers, and it is worth being able to name them in order because questions about structure usually start at one level and ask about the next.

A muscle is a bundle of muscle fibres. Each fibre is a single cell, but an unusual one: it is up to several centimetres long, it is multinucleate, its cell surface membrane is called the sarcolemma, its cytoplasm the sarcoplasm, and it is packed with mitochondria. Folds of the sarcolemma run deep into the fibre as transverse tubules, and a specialised endoplasmic reticulum, the sarcoplasmic reticulum, wraps around everything and stores calcium ions.

Inside each fibre lie hundreds of myofibrils, and each myofibril is a chain of sarcomeres joined end to end at Z lines. The sarcomere is the unit that contracts, and everything else in this lesson happens inside one.

Two kinds of filament fill it. Thick filaments are made of myosin, whose molecules have long tails bundled together and globular heads projecting outwards. Thin filaments are made of actin, wound with a fibrous protein called tropomyosin and studded with troponin. The thin filaments are anchored to the Z lines; the thick ones sit in the middle.

Sarcomere

The functional unit of a myofibril, running from one Z line to the next.

A band

The dark band, the full length of the thick filaments, including any overlap with thin filaments.

I band

The light band, containing thin filaments only, spanning a Z line.

H zone

The lighter region at the centre of the A band, containing thick filaments only.

Sarcoplasmic reticulum

The membrane system inside a muscle fibre that stores and releases calcium ions.

The bands, and the classic question

The banding pattern is the evidence for the whole mechanism, and it comes up in exams so reliably that it is worth being able to derive rather than recall.

The A band is the length of the thick filaments. During contraction the thick filaments neither shorten nor move relative to the centre of the sarcomere, so the A band does not change. That is the answer candidates most often get wrong, and it is the one that shows whether you have understood sliding.

The I band contains only thin filaments. As the thin filaments are pulled in towards the centre, more of their length lies inside the A band, so the region containing thin filaments alone gets shorter.

The H zone contains only thick filaments — the part of the A band that the thin filaments have not yet reached. As they slide further in, that gap closes, so the H zone gets shorter too, and in a strong contraction it can disappear entirely.

The sarcomere as a whole shortens by exactly the amount the I band and H zone lose, because the Z lines are dragged towards each other. A muscle shortens because millions of sarcomeres in series each shorten by a fraction of a micrometre.

How the sliding is driven

The mechanism is a cycle, and the order matters more than any single step. Start where the previous lesson finished, at a neuromuscular junction.

Acetylcholine released there depolarises the sarcolemma, and the depolarisation spreads along it and down the transverse tubules into the depths of the fibre. That triggers the sarcoplasmic reticulum to release calcium ions into the sarcoplasm.

Calcium binds to troponin, which changes shape and pulls the tropomyosin aside, exposing the myosin binding sites on the actin. Until this moment the binding sites were covered and no cross-bridge could form, which is why a resting muscle does not contract despite having everything else it needs.

Now the cycle proper. A myosin head, carrying ADP and phosphate from a previous round, attaches to the exposed binding site and forms a cross-bridge. The head then bends — the power stroke — releasing ADP and phosphate and pulling the thin filament about 10 nm towards the centre of the sarcomere. An ATP molecule then binds to the head, which causes it to detach from the actin. Hydrolysis of that ATP by the head's own ATPase activity returns it to its upright position, ready to attach further along the filament.

Thousands of heads on each thick filament do this out of step with one another, so at any instant some are attached while others are detached and the filament is never released. As long as calcium remains and ATP is available, the cycle repeats and the filaments keep sliding.

Relaxation is not passive either. When impulses stop, calcium ions are actively pumped back into the sarcoplasmic reticulum — which costs ATP — calcium leaves troponin, tropomyosin slides back over the binding sites, and cross-bridges can no longer form. The muscle is then lengthened by its antagonistic partner or by gravity, because a muscle can pull and cannot push.

Rigor mortis makes the role of ATP unusually clear. After death, respiration stops and ATP runs out. Without ATP the myosin heads cannot detach from actin, and without ATP calcium cannot be pumped back, so the cross-bridges stay formed and the muscles are locked rigid until the proteins themselves begin to break down.

Two kinds of fibre, and how they are paid for

Skeletal muscles contain a mixture of fibre types, and the proportions differ between muscles and between individuals. The comparison is examined as a table, so learn it as one, but notice that every row is a consequence of the first.

Slow twitch fibres are built around a continuous aerobic supply: many mitochondria to make ATP, dense capillaries to bring oxygen and glucose, and myoglobin to store oxygen inside the fibre. Fast twitch fibres are built for a short burst instead, relying on anaerobic glycolysis and a large glycogen store, and accumulating lactate as they go — which is why they tire quickly.

The ATP a muscle contains at rest lasts about two seconds of hard work, which is nowhere near enough for anything. Three supplies extend it. Phosphocreatine, stored in the sarcoplasm, transfers its phosphate group directly to ADP to regenerate ATP without any respiration at all; it is instant, it is anaerobic, and it is spent in about ten seconds. Anaerobic respiration — glycolysis with the pyruvate reduced to lactate — supplies ATP quickly but only two molecules per glucose, and the lactate accumulating limits it to a minute or two. Aerobic respiration is slower to get going and needs oxygen delivered, but yields around 30 ATP per glucose and can continue indefinitely.

In the exam

• The A band does not change. If you write nothing else about the bands, write that, and add that the I band and H zone both get shorter.
• Say that the filaments slide past one another. Any answer containing 'the filaments shorten' or 'the actin contracts' will be marked wrong however good the rest of it is.
• Calcium binds to troponin, not to tropomyosin. Troponin changes shape and moves tropomyosin off the binding site — two proteins, two different jobs.
• ATP is used three times over: for the myosin head to detach, for hydrolysis to re-cock it, and for pumping calcium back into the sarcoplasmic reticulum during relaxation. The third one is the mark most often missed.
• For fibre-type comparisons, give a structural difference and the functional consequence together — 'more mitochondria, so more aerobic ATP, so resistant to fatigue' — rather than a list of features.
• Phosphocreatine regenerates ATP from ADP directly and anaerobically. It is not a form of respiration and it does not produce lactate.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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