How muscles contract: myofibrils and the sliding filament model
A skeletal muscle shortens because millions of tiny protein filaments slide past each other. Myosin heads grab actin, pull, let go and grab again, powered by ATP and switched on by calcium. The filaments themselves never get shorter; they overlap more.
The short version
- A muscle fiber (cell) is packed with myofibrils, each a chain of repeating units called sarcomeres, about 2 µm long, running from one Z-line (Z-disc) to the next.
- Thin filaments (mostly actin) are anchored at the Z-lines; thick filaments (myosin) sit in the middle.
- In contraction the Z-lines move closer; the I band and H zone shrink, the A band stays the same.
- Calcium binds troponin, which moves tropomyosin off actin's binding sites so myosin can attach.
- ATP is needed to release myosin from actin and to re-cock the head, and to pump calcium back.
Inside a sarcomere
Under a microscope, skeletal muscle looks striped (striated) because the sarcomeres in neighboring myofibrils line up. Each stripe has a name:
| Region | What it contains | During contraction |
|---|---|---|
| Z-line (Z-disc) | The boundary of the sarcomere; thin filaments attach here | Neighboring Z-lines move closer together |
| I band (light) | Thin filaments only, on both sides of a Z-line | Gets shorter |
| A band (dark) | The full length of the thick filaments, including where they overlap thin filaments | Stays the same length |
| H zone | The middle of the A band: thick filaments only | Gets shorter, can disappear |
| M-line | The center of the sarcomere, where thick filaments are held together | Stays in the center |
The fact that the A band keeps its length is the key evidence for the sliding filament model, proposed in 1954 by two research teams (Andrew Huxley and Rolf Niedergerke; Hugh Huxley and Jean Hanson). If the filaments themselves shortened, the A band would shrink too. Instead, the thin filaments slide toward the M-line, increasing overlap.
The thin filament is not just actin. Wrapped along it is tropomyosin, a long strand that covers the spots where myosin would bind, and attached to tropomyosin at intervals is troponin, a small complex that can bind calcium. Together they act as the on/off switch.
From nerve signal to calcium
The chain of events that links an electrical signal to the mechanical pull is called excitation–contraction coupling:
- A motor neuron releases acetylcholine at the neuromuscular junction.
- Acetylcholine binds receptors on the muscle membrane (sarcolemma), which depolarizes and fires an action potential.
- The action potential spreads along the membrane and down the T-tubules, inward folds that carry it deep into the fiber.
- Voltage sensors in the T-tubule membrane are linked to calcium channels in the neighboring sarcoplasmic reticulum (SR), a calcium store. The channels open and Ca²⁺ floods into the cytoplasm.
- Ca²⁺ binds troponin; troponin shifts tropomyosin; actin's binding sites are exposed. Cross-bridge cycling starts.
- When nerve signals stop, acetylcholinesterase breaks down acetylcholine, and ATP-powered pumps move Ca²⁺ back into the SR. Tropomyosin covers the sites again and the muscle relaxes.
The cross-bridge cycle
Each myosin head works like a tiny rowing oar. One round goes like this:
- Attach. A "cocked" myosin head, holding ADP and phosphate, binds to an exposed site on actin, forming a cross-bridge.
- Power stroke. Phosphate is released and the head pivots toward the M-line, pulling the thin filament about 10 nm. ADP is released at the end of the stroke.
- Detach. A fresh ATP binds the head, and this makes myosin let go of actin.
- Re-cock. The head splits (hydrolyzes) ATP into ADP and phosphate, and the energy swings it back to the high-energy, cocked position, ready to bind again further along.
As long as calcium is present and ATP is available, the cycle repeats. Different heads are at different stages at any moment, so the filament is pulled smoothly rather than in jerks, a bit like a team pulling a rope hand over hand.
Common mistake. Many students think ATP is needed to make myosin grab actin. In fact, binding ATP makes myosin let go; splitting ATP re-cocks the head. The power stroke itself happens as phosphate leaves.
Rigor mortis
After death, cells stop making ATP. Calcium leaks out of the sarcoplasmic reticulum and cannot be pumped back, so binding sites stay exposed and myosin attaches to actin. With no ATP, the heads cannot detach (step 3), so the muscles lock stiff. The stiffness fades over the following days as the muscle proteins themselves break down.
Memory aid
Remember the bands by their vowels: in the light I band the thin filaments are Isolated; the dark A band is the length of the thick filament, and it is Always the same. The H zone is the Hole in the middle where thin filaments have not yet reached; it closes up as they slide in.
For the cycle, think "Bind, Pull, Release, Reset", with the rule "ATP in = let go".
A verse to remember it
Check yourself
Which sarcomere band stays the same length during contraction, and why?
The A band, because it is the length of the thick (myosin) filaments, and those do not shorten. Only the overlap between thick and thin filaments changes.
What exactly does calcium do?
It binds troponin, which pulls tropomyosin away from the myosin-binding sites on actin, so cross-bridges can form.
Name two jobs of ATP in contraction and relaxation.
ATP binding detaches myosin from actin, and ATP hydrolysis re-cocks the myosin head. ATP also powers the pumps that return calcium to the sarcoplasmic reticulum so the muscle can relax.
Why does rigor mortis happen?
Without ATP, myosin heads that are bound to actin cannot detach, so the cross-bridges stay locked.