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When you first hear that a muscle contains only two main working proteins, actin and myosin, it is tempting to picture two simple sticks. In fact each is a carefully built-up structure, a bit like a rope woven from many smaller threads, and the way each is assembled is exactly what allows a muscle to be switched on and off. Getting to know the filament structure is really getting to know the machinery of movement.

The thin filament is made of actin and its helper proteins. Two strands of filamentous actin, called F-actin, are wound helically around each other. Each of these strands is itself a long chain, a polymer built from many identical small units called globular actin, or G-actin. Lying alongside the F-actin, running the whole length of the filament, are two filaments of another protein, tropomyosin, and at regular intervals along the tropomyosin sits a complex protein called troponin.

This extra scaffolding is not decoration — it is a control switch. In the resting muscle, a part of the troponin covers the active binding sites on actin, the very places where myosin would need to attach. As long as those sites stay masked, myosin cannot grip the thin filament and the muscle stays relaxed.

The thick filament is made of myosin, and it too is a polymer of many smaller units called meromyosins. Each meromyosin has two parts:

• A globular head with a short arm, together called the heavy meromyosin, which projects outward from the filament at regular intervals as the cross arm.
• A tail, called the light meromyosin, which lies along the body of the filament. …