Thick and thin filaments

Within each sarcomere the relative sliding of thick and thin filaments is brought about by "cross-bridges," parts of the myosin molecules that stick out from the myosin filaments and interact cyclically with the thin actin filaments, transporting them hy a kind of rowing action. During this process, the hydrolysis of ATP to ADP and phosphate couples the conformational... 

FIGURE 17.21 A drawing of the arrangement of the elastic protein titin in the skeletal mnscle sarcomere. Titin filaments originate at the periphery of the M band and extend along the myosin filaments to the Z lines. These titin filaments produce the passive tension existing in myofibrils that have been stretched so that the thick and thin filaments no longer overlap and cannot interact. (Adapted from Ohtsuki, ., Maruyama, K, and Ebashi,. S ., 1986. Advances ia Protein Chemisti y 38 1—67.)... 

FIGURE 17.29 A drawing of the thick and thin filaments of skeletal mnscle in cross-section showing the changes that are postulated to occur when Ca binds to troponin C. 

Figure 1. Muscle development. A skeletal muscle fiber is formed by the fusion of many single cells (myoblasts) into a multinucleated myotube. Myotubes then develop into the muscle fiber (see text). Sarcomeres form in longitudinal structures called myofibrils. The repeating structure of the sarcomere contains interdigitating thick and thin filaments.

Figure 3. Structure of a muscle sarcomere. In a polarizing microscope muscle appears to have dark (A) and light (I) bands. The l-band region only contains thin filaments. The A-band region contains both thick and thin filaments. One sarcomere is the distance between two Z-lines. In cross section, the hexagonal packing of the thick and thin filaments can be seen.

Both the thick and thin filaments contain other proteins. For example, the thick filament contains titin (molecular weight about 3,000,000) and the thin filament contains nebulin (although not in cardiac muscle), and the regulatory proteins troponin (molecular weight about 33,000) and tropomyosin (molecular weight about 70,000). Nebulin and titin are thought to be ruler proteins, that is, they determine the overall length of the thin and the thick filament, respectively. The... 

X-ray diffraction of live muscle (H.E. Huxley and Brown, 1967) showed the structure of the thick and thin filaments and how they changed when the muscle contracted, or was put into rigor (in rigor muscle, ATP is absent from the muscle. 

Release of Ca from terminal cisterns of sarcoplasmic reticulum and diffusion to thick and thin filaments... 

The myofibrils of skeletal muscle contain thick and thin filaments. The thick filaments contain myosin. The thin filaments contain actin, tropomyosin, and the troponin complex (troponins T, I, and C). 

Each fibre contains an array of parallel myofibrils each consisting of overlapping thick and thin filaments that form repeating units (sarcomeres) along the length of the fibre (Figure 13.5). The thick filaments are composed almost entirely of the protein myosin, whereas the thin filaments contain actin as well as troponin and tropomyosin. 

Sliding of the filaments is caused by cycles of attachment and detachment of the cross-bridges linking thick and thin filaments (see Figure 13.10)... 

When appropriately arranged in the context of the thick and thin filaments of the myofibril, repeated cycles of reaction steps 1-6 result in contraction. Other forms of myosin are responsible for contractile steps that are associated with the crawling of nonmuscle cells. See also Force Effects on Molecular Motors... 

Figure 5.14 Schematic diagram showing interaction of thick and thin filaments in skeletal muscle contraction. (Adapted from Stryer, 1975.)...

FIGURE 5-31 Structure of skeletal muscle, (a) Muscle fibers consist of single, elongated, multinucleated cells that arise from the fusion of many precursor cells. Within the fibers are many myofibrils (only six are shown here for simplicity) surrounded by the membranous sarcoplasmic reticulum. The organization of thick and thin filaments in the myofibril gives it a striated appearance. When muscle contracts, the I bands narrow and the Z disks come closer together, as seen in electron micrographs of (b) relaxed and (c) contracted muscle. 

FIGURE 5-32 Muscle contraction. Thick filaments are bipolar structures created by the association of many myosin molecules, (a) Muscle contraction occurs by the sliding of the thick and thin filaments... 

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