How muscles contract

The thin filament is two chains of F-actin (each a string of round G-actin units) twisted around each other. Running along them like a rope are two strands of another protein, tropomyosin, and at intervals there are small clusters of troponin. At rest, part of the troponin covers (masks) the spots on actin where myosin would bind.

The thick filament is a bundle of many meromyosin molecules. Each has a round head with a short arm (heavy meromyosin, HMM) and a tail (light meromyosin, LMM). The heads stick out from the filament as cross arms. Each head is an enzyme (an ATPase) with a site for binding ATP and a site for binding actin.

How does a muscle contract? By the sliding filament theory: the thin filaments slide over the thick ones.

• A signal from the brain comes along a motor neuron. At the neuromuscular junction, the neuron releases a chemical, acetylcholine.
• This starts an electrical signal (action potential) that spreads through the muscle fibre and makes the sarcoplasmic reticulum release calcium ions (Ca²⁺).
• Ca²⁺ binds to troponin, which shifts and uncovers the binding sites on actin.
• Using energy from ATP, the myosin heads attach to actin, forming cross bridges, and pull the thin filaments towards the centre of the A band. The Z lines are pulled inwards, so the sarcomere shortens. The I band gets shorter, but the A band keeps its length.
• A new ATP makes the myosin head let go, and the cycle of attach, pull, release repeats.
• When the Ca²⁺ is pumped back into the sarcoplasmic reticulum, the actin sites are covered again and the muscle relaxes.

If a muscle works hard again and again, it runs short of oxygen and breaks down glycogen without oxygen, building up lactic acid. This causes fatigue.

Muscle fibres come in two kinds. Red fibres have a lot of the red, oxygen-storing pigment myoglobin and many mitochondria: they work aerobically and tire slowly. White fibres have little myoglobin and fewer mitochondria but more sarcoplasmic reticulum: they work anaerobically, fast but tiring quickly.