actin-myosin contraction
/ AK-tin MY-oh-sin kun-TRAK-shun /
Imagine two rows of people facing each other, each row holding a rope, and one row reaching out hand over hand to pull the other row closer. The two rows do not get shorter, but the gap between them shrinks. Muscle works on exactly this principle. Actin-myosin contraction is how cells generate pulling force, most famously in muscle but also when a dividing cell pinches itself in two.
The two players are the protein filaments actin and myosin. Myosin has tiny heads that reach out, grab a neighboring actin filament, and pull. Each pull is one power stroke, fueled by the energy from splitting ATP; the head then lets go, resets, grabs further along, and pulls again. Because many myosin heads do this in an organized array, sliding the actin filaments past the myosin, the overall structure shortens. This is called the sliding filament mechanism — the filaments themselves do not shrink; they slide past one another.
In skeletal muscle, actin and myosin are packed into highly ordered repeating units called sarcomeres, lined up so neatly that the muscle looks striped under a microscope. A nerve signal triggers a rise in calcium, which uncovers the actin so myosin can grab it, and the muscle contracts. The very same actin-and-myosin pulling — though in a looser arrangement — also drives the contractile ring that splits one cell into two at the end of cell division, showing that muscle is a refined version of a much older cellular trick.
Rigor mortis, the stiffening of muscles after death, happens because cells run out of ATP, so myosin heads grab actin and cannot release — locking the muscle until the proteins break down.
Releasing the grip needs ATP too — contraction is not the only step that costs energy.
The filaments do not shorten — they slide past each other. And myosin needs ATP both to pull and to let go, which is why a muscle with no ATP locks up rather than going limp.