myofilament

Myofilament refers to the protein filaments that form the contractile machinery of muscle cells. The two principal types are thin (actin) filaments and thick (myosin) filaments. Thin filaments are primarily polymerized actin molecules and are associated with the regulatory proteins tropomyosin and troponin on their surface. Thick filaments are bundles of myosin molecules with protruding heads that can form cross-bridges with actin filaments during contraction.

In striated muscle, myofilaments are arranged in units called sarcomeres, the basic contractile units. Thin filaments extend from the Z-discs toward the center of the sarcomere, while thick filaments lie in the middle. The sarcomeric organization, along with elastic proteins such as titin, contributes to the characteristic striations and to passive stiffness.

Contraction occurs by sliding filaments past one another, driven by ATP-dependent cross-bridge cycling of myosin heads binding to actin. At rest, troponin-tropomyosin blocks myosin-binding sites on actin. When Ca2+ ions rise in the cytoplasm, they bind troponin, causing a shift in tropomyosin and exposure of binding sites. Myosin heads bind, swivel, and release after ATP binding, producing filament sliding and sarcomere shortening.

In smooth muscle, actin and myosin filaments are present but are organized differently and lack a regular sarcomeric arrangement. Nevertheless, the contractile process relies on these core filaments and their interactions.

Mutations or alterations in myofilament proteins can impair contraction and contribute to muscular diseases such as cardiomyopathies and various myopathies.