The formation of the thin filament involves the aggregation of actin molecules, known as globular actin or G-actin, into a helical filament called filamentous actin or F-actin. Nebulin, an elongated cytoskeletal protein, extends along the length of the thin filament and may play a role in regulating its length. Tropomyosin, which consists of protein dimers, covers the myosin binding sites on the actin molecules. Each tropomyosin dimer spans seven actin molecules and is arranged in a head-to-tail configuration. The troponin complex, consisting of troponin T, troponin I, and troponin C, is present on each tropomyosin dimer and influences the position of tropomyosin on the actin filament. This, in turn, affects the ability of tropomyosin to inhibit the binding of myosin to the actin filament at low cytosolic calcium concentrations. The binding of cytosolic calcium to troponin C promotes the movement of tropomyosin on the actin filament, exposing myosin binding sites and facilitating actin-myosin interaction and muscle contraction. Other proteins associated with the thin filament include tropomodulin, α-actinin, and CapZ protein. Tropomodulin is located at the end of the thin filament and may play a role in determining its length. The force of muscle contraction is transmitted longitudinally to the tendon at the myotendinous junction and laterally to adjacent extracellular connective tissue at costameres. The force is transmitted from the muscle fiber to the tendon through connections with collagen fibers, while folds in the sarcolemma at the end of the muscle fiber result in an interdigitation of the muscle fiber with the tendon, forming the myotendinous junction. Costameres, which are located on the sides of the muscle fibers, act as bridges between the Z lines in the subsarcolemmal myofibrils and the extracellular connective tissue. They facilitate the lateral transmission of force during muscle contraction, which helps stabilize the sarcolemma. The dystrophin-associated glycoprotein complex (DGC) is also involved in this process. CapZ protein and α-actinin serve to anchor the thin filaments to the Z line, ensuring the proper alignment of sarcomeres across muscles and the efficient transmission of force.
The thin filament is formed by aggregation of actin molecules (termed globular actin or G-actin) into a two-stranded helical filament called filamentous actin or F-actin. The elongated cytoskeletal protein nebulin extends along the length of the thin filament and may participate in regulation of the length of the thin filament. Dimers of the protein tropomyosin extend over the entire actin filament and cover myosin binding sites on the actin molecules. Each tropomyosin dimer extends across seven actin molecules, with sequential tropomyosin dimers arranged in a head-to-tail configuration. A troponin complex consisting of three subunits (troponin T, troponin I, and troponin
C) is present on each tropomyosin dimer and influences the position of the tropomyosin molecule on the actin filament and hence the ability of tropomyosin to inhibit binding of myosin to the actin filament at low cytosolic Ca concentrations (see the section “Actin-Myosin Interaction:
Cross-Bridge Formation”).
Binding of cytosolic Ca++ to troponin C promotes the movement of tropomyosin on the actin filament that exposes myosin binding sites on actin, thereby facilitating actin-myosin interaction, and hence contraction (see the section “Actin-Myosin Interaction: ”). Additional proteins associated with the thin filament include tropomodulin, α-actinin, and CapZ protein. Tropomodulin is located at the end of the thin filament, toward the center of the sarcomere, and may participate in setting the length of the thin filament.
• Fig. 12.4. The force of contraction of the muscle fiber is transmitted both longitudinally to the tendon (at the myotendinous junction) and laterally to adjacent extracellular connective tissue (at costameres). The force of contraction is transmitted from the end of the muscle fiber (M) to the tendon by connections with numerous collagen fibers (A, tip of arrow). Folds in the sarcolemma at the end of the muscle fiber (B) result in an interdigitation of the muscle fiber with the tendon, and represents the myotendinous junction. Costameres are located on the sides of the muscle fibers, and represent the bridges between the Z lines in the subsarcolemmal myofibrils and the extracellular connective tissue (C). Costameres facilitate the lateral transmission of force of contraction, which helps stabilize the sarcolemma. DGC, dystrophinassociated glycoprotein complex. (A and B, From Tidball JG. Myotendinous junction: morphological changes and mechanical failure associated with muscle cell atrophy. Exp Molec Pathol. 1984;40:1-12. C, From Hughes D, Wallace M, Baar K. Effects of aging, exercise, and disease on force transfer in skeletal muscle. Am J Physiol Endocrin Metab. 2015;309:E1-E10.)
CapZ protein and α-actinin serve to anchor the thin fila-of sarcomeres across muscles and the lateral transmission of ment to the Z line. force (described later in this section).