Myofilaments

Myofilaments are the contractile filaments of muscle fibers and form the core of the contractile apparatus in muscle. They are categorized into thin filaments, mainly actin, and thick filaments, mainly myosin. In skeletal and cardiac muscle, these filaments are organized into sarcomeres, the repeating units within myofibrils that produce contraction.

Thin filaments consist of actin polymers with regulatory proteins. Actin monomers assemble into F-actin; along the filament run tropomyosin molecules that cover actin’s myosin-binding sites and the troponin complex (TnI, TnT, TnC) that responds to calcium to move tropomyosin away from the binding sites, enabling cross-bridge formation.

Thick filaments are primarily myosin II molecules with one head and one tail, arranged in a bipolar fashion. Each myosin head can bind to actin and possesses ATPase activity, allowing it to hydrolyze ATP and generate force during the power stroke. Cross-bridges formed between actin and myosin pull the filaments past one another.

Contraction follows the sliding filament theory. When intracellular calcium rises, calcium binds to troponin C, causing conformational changes that move tropomyosin aside. Myosin heads attach to actin, perform a power stroke, and then detach after ATP binds. Repeated cycles shorten the sarcomere and the muscle fiber. ATP availability and calcium regulation are essential for the cycle.

In smooth muscle, actin and myosin are present but regulation differs: there is no troponin complex, and contraction is controlled by phosphorylation of myosin light chains via myosin light chain kinase, driven by calcium-calmodulin signaling. Myofilament organization and dynamics underpin muscle strength, speed, and endurance, and their function is central to muscle physiology and related diseases.