Skeletal muscle consists of parallel bundles of musclefibers, each fiber a single, very large, multinucleatedcell, 20 to 100 µm in diameter, formed from many cellsfused together and often spanning the length of the muscle.Each fiber, in turn, contains about 1,000 myofibrils,2 µm in diameter, each consisting of a vast numberof regularly arrayed thick and thin filamentscomplexed to other proteins. A system offlat membranous vesicles called the sarcoplasmicreticulum surrounds each myofibril. Examined underthe electron microscope, muscle fibers reveal alternatingregions of high and low electron density, called theA bands and I bands. The A and I bandsarise from the arrangement of thick and thin filaments, which are aligned and partially overlapping. The I bandis the region of the bundle that in cross section wouldcontain only thin filaments. The darker A band stretchesthe length of the thick filament and includes the regionwhere parallel thick and thin filaments overlap. Bisectingthe I band is a thin structure called the Z disk, perpendicularto the thin filaments and serving as an anchorto which the thin filaments are attached. The Aband too is bisected by a thin line, the M line or M disk,a region of high electron density in the middle of thethick filaments. The entire contractile unit, consistingof bundles of thick filaments interleaved at either endwith bundles of thin filaments, is called the sarcomere.The arrangement of interleaved bundles allows the thickand thin filaments to slide past each other (by a mechanismdiscussed below), causing a progressive shorteningof each sarcomere.The thin actin filaments are attached at one end tothe Z disk in a regular pattern. The assembly includesthe minor muscle proteins α-actinin, desmin, and vimentin.Thin filaments also contain a large proteincalled nebulin(~7,000 amino acid residues), thought tobe structured as an αhelix that is long enough to spanthe length of the filament. The M line similarly organizesthe thick filaments. It contains the proteins paramyosin,C-protein, and M-protein. Another class of proteinscalled titins, the largest single polypeptide chains discoveredthus far (the titin of human cardiac muscle has26,926 amino acid residues), link the thick filaments tothe Z disk, providing additional organization to the overallstructure. Among their structural functions, the proteinsnebulin and titin are believed to act as “molecular rulers,” regulating the length of the thin and thick filaments,respectively. Titin extends from the Z disk to theM line, regulating the length of the sarcomere itself andpreventing overextension of the muscle. The characteristicsarcomere length varies from one muscle tissueto the next in a vertebrate organism, a finding attributedin large part to the different titin variants in thetissues.
The (blue-stained) endomysiumin Figure 1 below, which contains capillariesand nerve fibers, should be noted.
Composition of myofibrils
- Contractile proteins
- Regulatory proteins
- Structural proteins
Contractile proteins: myosin and actin.
• Thick filaments- Myosin (motor protein converts ATP into movement). 300 molecules of -myosin=1 thick filament.
• Thin filaments- are anchored to z-discs and are composed of F-actin and regulatory proteins (tropomyosin and troponin).
- Actin contains a myosin binding site.
– Tropomyosin and troponin
– Tropomyosin blocks myosin binding to actin by blocking the myosin site on actin in relaxed muscle.
– The tropomyosin is held in place by troponin.
– Titin-anchors thick filament to a z-disc and M line
– Other proteins contribute to alignment, stability, and elasticity.