thin filament

In muscle physiology, thin filaments are primarily composed of the protein actin. Actin molecules assemble into long strands, forming the thin filaments that interdigitate with thicker filaments made of myosin. This interaction is crucial for muscle contraction.

The thin filaments also contain tropomyosin and troponin. Tropomyosin is a rod-shaped molecule that runs along the actin filaments, while troponin is a complex of three proteins. Together, tropomyosin and troponin regulate muscle contraction by controlling the interaction between actin and myosin. When calcium ions bind to troponin, it triggers a series of events that allow myosin to bind to actin, leading to muscle shortening and contraction.123