How a muscle shortens when none of its filaments does

How to use this simulation

1. Leave the sarcomere at its resting length and read the three bands in the chart at the bottom: the A band is 1.60 µm, the I band is 0.90 µm and the H zone is 0.50 µm. Show labels is on, so every part of the diagram is named.
2. Press Contract. Calcium switches on, the purple strand beside every thin filament slides aside, the myosin heads in the overlap zone start to cycle and the two Z lines glide toward each other.
3. Keep your eye on the bars. The sarcomere bar, the I band bar and the H zone bar shrink, but the A band bar never moves. That single observation is the sliding-filament theory.
4. Switch Calcium ions present off in the middle of a contraction. The strand slips back over the binding sites, the heads let go and the muscle stops pulling. The step reading says Rest.
5. Switch ATP present off while calcium is on. The heads latch onto actin and stay there: this is rigor, the same state as rigor mortis. Switch ATP back on and the cycle resumes.
6. Drag the sarcomere length slider to the far right. As the thin filaments pull out of the thick ones, the relative force reading falls toward zero. Drag it left past 2.0 µm and the force falls again as the filaments crowd.

The bag of rice problem

You lift a bag of rice from the floor to the kitchen shelf. Your biceps bulges, your elbow bends, the bag rises. The muscle got shorter and fatter, and it did it in about a second. The question that puzzled biologists for a long time was how.

The obvious guess is that the muscle is made of long molecular ropes that scrunch up like a spring when you tell them to. That guess is wrong, and the experiment that overturned it is lovely. Under a microscope the bands of a contracting muscle do not all get shorter. One dark band stays exactly the same width. The light bands on either side of it narrow, and the pale stripe in the middle of the dark one closes up.

In 1954 two teams, Hugh Huxley with Jean Hanson, and Andrew Huxley with Rolf Niedergerke, explained this at once. The muscle is built of two interleaved sets of filaments, and contraction is those sets sliding past each other like two combs being pushed together. Nothing in either set shrinks.

This page lets you run that machine. You will squeeze a sarcomere, watch the bands change, break the system by taking away calcium or ATP, and calculate how many tiny pulls a single myosin head needs to shorten a muscle.

From muscle to sarcomere: the zoom ladder

A skeletal muscle looks like one piece of meat, but it is a bundle of bundles. Zoom in step by step and the structure becomes clear.

• A whole muscle (the biceps) is wrapped in connective tissue and made of bundles called fascicles.
• Each fascicle holds many muscle fibres, which are single giant cells with many nuclei, up to several centimetres long.
• Each fibre is packed with rod-shaped myofibrils running along its length, a few micrometres thick.
• Each myofibril is a chain of identical units called sarcomeres, joined end to end. This is the unit that does the shortening.
• Each sarcomere holds two kinds of protein filament: thin filaments of actin, about 1.0 µm long, and thick filaments of myosin, 1.6 µm long.

Words you need first

Most of the marks in this chapter are definitions and labelled diagrams. Learn each term with the one next to it, because examiners like to ask for pairs.

Reading the bands of a sarcomere

Under a microscope a sarcomere shows alternating dark and light bands. Each band is simply a region where the filaments overlap or do not. This is the part of the chapter that decides whether the sliding-filament theory makes sense to you, so go slowly.

The A band (dark) is as long as the thick filaments, 1.6 µm. It contains the thick filaments and any thin filaments that have slid into them. The I band (light) holds only thin filaments, so it is the part of the sarcomere that is not the A band. The H zone is the pale patch in the middle of the A band where there are thick filaments only, and the M line runs through it.

The sliding-filament model in plain words

Picture two combs facing each other with their teeth interleaved. Push the combs together and the whole thing gets shorter, but no tooth got shorter. The combs are the Z lines with their thin filaments, and the teeth between them are the thick filaments.

In a sarcomere the thin filaments hang from the two Z lines and the thick filaments float in the middle, held by the M line. Myosin heads stick out of each thick filament toward the thin ones. When the heads grab actin and bend, they drag the thin filaments toward the M line. The Z lines come closer, so the sarcomere shortens from 2.5 µm toward 2.0 µm.

Notice what shrinks. The I band shrinks because the thin filaments slide into the A band. The H zone shrinks because the tips of the thin filaments move toward each other. Neither the thick nor the thin filaments change length at all.

Now multiply. A myofibril has thousands of sarcomeres in a row, and the fibre has hundreds of myofibrils side by side. When every sarcomere shortens by a little, the whole muscle shortens by a lot.

The cross-bridge cycle, step by step

One myosin head moves the thin filament only a few nanometres each time it works. Muscle gets its power from thousands of heads doing this together, each one repeating the same four-step cycle. The simulation shows the cycle on its heads in the overlap zone; watch the Current step reading as the heads move.

Calcium and ATP: the two switches

A resting muscle is not slack by accident. Two things must be present before the cycle can run, and the simulation lets you remove either one.

First, calcium. At rest the strand of tropomyosin lies across the binding sites on actin, so the heads have nothing to grab. A nerve impulse reaches the muscle fibre through the neuromuscular junction, the fibre’s membrane fires, and the sarcoplasmic reticulum releases calcium ions into the cell. Calcium binds to troponin, troponin shifts tropomyosin aside, and the sites are open. Pump the calcium back and the sites are covered again.

Second, ATP. ATP does two jobs: its splitting cocks the head, and its fresh binding releases the head from actin. Without ATP the heads attach and cannot detach, so the muscle goes stiff. That is rigor, and it is why a dead body stiffens: ATP production stops, the pumps stop, calcium leaks out of the reticulum, and the heads freeze onto actin until the proteins start to break down.

• Nerve impulse arrives at the neuromuscular junction (acetylcholine released).
• The impulse spreads along the fibre membrane and into the cell.
• Calcium is released from the sarcoplasmic reticulum.
• Calcium binds troponin, tropomyosin moves aside, binding sites open.
• Cross-bridge cycle runs for as long as calcium and ATP are present.
• Calcium is pumped back, sites close, the muscle relaxes.

How long is the sarcomere, and how hard can it pull?

Force depends on how many heads can reach actin, and that depends on the overlap between the filaments. If the sarcomere is stretched until thin and thick filaments barely touch, there are almost no cross-bridges and almost no force. At about 2.0 to 2.2 µm the overlap is full and the force is at its peak. If the sarcomere is squeezed much shorter, the thin filaments crowd past each other in the middle and the thick filaments hit the Z lines, so the force falls again.

In the simulation the relative force at the resting length of 2.5 µm is 79 % of the maximum. You can check this with the relative-force reading: at 2.1 µm it reads 100 %, at 3.0 µm it reads 45 %, and by 3.2 µm only 31 % is left. The curve is idealised, but the shape is the real one.

Try it in the simulation

Spend ten minutes on these. Each one is a question an examiner could ask, answered by what you see on the diagram.

• Press Contract on the defaults. Which two bands shrink and which one does not move?
• Pause during the contraction and name the step of the cross-bridge cycle shown in the Current step reading. Resume and watch it change.
• While contracting, switch Calcium ions present off. What do the heads do and what happens to the sarcomere?
• Switch ATP present off with calcium on. What does the step reading say, and why does the muscle not relax?
• Drag the sarcomere length to 2.0 µm. What is the H zone now?
• Drag it to 3.4 µm. How does the relative force compare with the value at 2.1 µm?
• Hide the labels and name the Z line, M line, A band, I band and H zone from memory, then show them to check.

Solved problems

Each solution states the rule first, then substitutes the numbers, the order you should write it in an exam. Every figure is computed by the same code that drives the simulation.

Common mistakes

These are the slips that cost marks every year.

• Saying the filaments shorten. They do not; they slide. The A band is the proof.
• Saying the A band shortens. The I band and H zone shorten, the A band does not.
• Writing that ATP is needed only to make the muscle contract. ATP is also needed for the head to let go and for the calcium pump.
• Forgetting that calcium acts on troponin, and troponin moves tropomyosin. Calcium does not bind actin directly.
• Confusing the Z line with the M line. Z lines are the sarcomere’s ends; the M line is in the middle.
• Drawing the thin filaments attached to the M line. They hang from the Z lines.

Where you see it in real life

Rigor mortis begins a few hours after death and fades after a day or two. The reason is exactly what you simulated: no ATP, so the heads cannot detach. When the muscle proteins later break down, the body relaxes again. Forensic scientists use the stiffening to estimate the time of death.

A cramp is a sustained involuntary contraction. Often it comes from tired, ion-imbalanced muscle that keeps firing, so the heads keep cycling when you want them to stop. Stretching helps because it pulls the thin filaments back out and breaks the overlap.

Heart muscle uses the same sliding filaments. Cardiac fibres are striated too, and they also depend on a rapid pulse of calcium for each beat, which is why calcium-channel blockers are a common heart medicine.

Exam corner

Typical questions, and what a full-mark answer contains.

Revision summary

A sarcomere runs from Z line to Z line and shortens from about 2.5 µm during contraction. Thin actin filaments hang from the Z lines; thick myosin filaments (1.6 µm) sit in the middle. Calcium, released from the sarcoplasmic reticulum, binds troponin and moves tropomyosin so that myosin heads can attach. Each head attaches, makes a power stroke of about 10 nm, detaches when new ATP binds, and re-cocks by splitting it. The A band stays constant; the I band and H zone shorten. With no calcium the muscle relaxes, with no ATP it locks in rigor, and the force is greatest around 2.0 to 2.2 µm.

Frequently asked questions