Myofilaments

1.Myofilaments. In skeletal muscle fibers, these are of 2 major types.
a. Thin filaments. Thin (actin) filaments have several components.

(1) Filamentous actin (F-actin) is a polymeric chain of glohular actin (G-actin) monomers. Each thin filament contains 2 of F-actin strands wound in a double helix.

(2) Tropomyosin is a long, thin, double-helical polypeptide that wraps around the actin double helix, lies in the grooves on its surface, and spans 7 G-actin monomers.

(3) Troponin is a complex of 3 globular proteins. TnT (troponin T) attaches each complex to a specific site on each tropomyosin molecule, TnC binds calcium ions, and TnI inhibits the interaction between the thin and thick filaments.

b. Thick filaments. A myosin molecule is a long, golf-club-shaped polypeptide. A thick (myosin) filament is a bundle of myosin molecules with their shafts pointing toward and overlapping in the bundle's middle and their heads projecting from the bundle's ends. This arrangement leaves a headless region in the center of each filament corresponding to the H band. Treating myosin molecules with papain (a proteolytic enzyme) cleaves them, at a point near the head, into 2 pieces. The piece containing most of the thin shaft is termed light meromysin; the head and associated section of the shaft make up heavy meromyosin. The head portion of heavy meromysin has an ATP-binding site and an actin-binding site, both necessary for contraction.

c. Organization of the myofilaments. The banding pattern of skeletal muscle reflects the grouping of its myofilaments into parallel bundles of thick and thin filaments called myofibrils. Each muscle fiber map contains several myofibrils, the number depending on its size.

(1) Appearance of the myofibrils in cross section. EM images of myofibrils in cross section reveal patterns of large and small dots corresponding to the thick and thin filaments, respectively. Sections containing both filament types have 6 thin filaments in hexagonal array around each thick filament. Each thick filament shares 2 of its surrounding thin filaments with each adjacent thick filament to form a repeating crystalline pattern.

(2) Appearance of the myofibrils in longitudinal section. At both light and EM levels, each myofibril exhibits repeating, linearly arranged, functional subunits called sarcomeres, which have bands (striations) running perpendicular to the long axis of the myofibril. The sarcomeres of each myofibril lie in register with those in adjacent myofibrils so that their bands appear continuous. The sarcomere is separated from its neighbors at each end by a dense Z line, or Z disk. A major protein of the Z disk, a-actinin, anchors one end of the thin filaments and helps maintain spatial distribution. The thin filaments extend toward the middle of the sarcomere.

The center of each sarcomere is marked by the M line, which holds the thick filaments in place. Desmin-containing intermediate filaments are found in both M lines and Z disks. The thick filament bundles lie at the center of each sarcomere, are bisected by the M line, and overlap the free ends of the thin filaments. The pattern of overlapping between the thick and thin filaments is responsible for the banding pattern and differs depend ing on the state of contraction of the myofibrils. d. Bands. With the light microscope, skeletal muscle exhibits alternating light- and dark staining bands running perpendicular to the long axis of the muscle fibers.
(1) I bands. The light-staining bands contain only thin filaments. They are known as I bands (isotropic) because they do not rotate polarized light. Each I band is bisected by a Z line. Thus each sarcomere has 2 half I bands, one at each end
(2) A bands, One dark-staining band lies in the middle of each sarcomere and shows the position of the thick filament bundles. This is known as an A band (anisotropic) because it is birefringent (rotates polarized light). At the EM level, each A band has a lighter-staining central region termed the H band, which is bisected by an M line. The H band lies between the free ends of the thin filaments and contains only the shafts of myosin molecules. The darker peripheral portions of the A bands are regions of overlap between the thick and thin filaments and contain the heads of the myosin molecules. The interaction between the myosin heads of the thick filaments and the free ends of the thin filaments causes muscle contraction.