Explain how the proteins troponin, tropomyosin, actin and myosin regulate muscle contraction.

When the muscle is relaxed, tropomyosin fibres wrap around the actin filament, covering up the sites at which myosin can bind with actin.

During muscle contraction, Ca2+ ions bind to troponin complexes on the tropomyosin, causing the tropomysosin fibres to change shape. This exposes the myosin binding sites on the actin filament.

The heads on the myosin filament bind to the actin filament, forming cross bridges. Inside each myosin head is a molecule of ADP and Pi (organic phosphate). The ADP is released, causing the myosin head to change shape and pull the actin filament along the myosin filament. This is called the "power stroke".

A new molecule of ATP binds to the myosin head, causing it to detach from the binding site on the actin filament. The ATP molecule is then hydrolysed to form ADP and Pi, which changes the shape of the myosin head so that it is ready to bind to another binding site futher along the actin filament.

This happens multiple times, shortening the muscle filaments and thereby contracting the muscle.