The A-band stays the same length while the I-band and H-zone shrink: the thick filaments do not shorten, the thin filaments slide into the middle. The real arm view shows the muscles of a human arm: the biceps lights up and bulges and the elbow bends with the same contraction, and tapping the biceps zooms into one of its sarcomeres. The sarcomere model is simplified: lever arms and filament radii are exaggerated, and each power stroke moves about 10 nm in a real muscle.
- How does a muscle contract?
- Myosin heads on the thick filaments grab binding sites on the thin actin filaments and pull them toward the middle of the sarcomere, the M-line. Each pull uses energy from ATP. Millions of these pulls shorten every sarcomere, so the whole muscle gets shorter.
- What does calcium do in muscle contraction?
- Calcium switches contraction on. In a relaxed muscle, tropomyosin covers the binding sites on actin and the myosin heads cannot attach. A nerve signal releases calcium into the fibre, tropomyosin moves aside, and the heads can grab actin and pull.
- Why does rigor mortis happen?
- After death the body makes no new ATP. Myosin heads that are attached to actin cannot let go without fresh ATP, so the muscles stay locked and stiff. The stiffness passes after the proteins break down, usually within a day or two.
- Do the filaments get shorter when a muscle contracts?
- No. The thick and thin filaments keep their length. The sarcomere shortens because the thin filaments slide inward, so the I-band and H-zone get narrower while the A-band stays 1.6 µm. This is the sliding filament theory.
- How long is a sarcomere?
- About 2.5 µm when relaxed, where 1 µm is a thousandth of a millimetre. In a strong contraction it can shorten to about 1.8 µm, and it can be stretched to about 3 µm, beyond which it produces less force.