What Happens in a Contracted Sarcomere

An important refinement of the slippery thread theory concerned the particular way in which myosin is able to pull on actin to shorten the sarcomere. Scientists have shown that the spherical end of each myosin protein closest to actin, called the S1 region, has several articulated segments that can bend and facilitate contraction (Hynes et al. 1987; Spudich, 2001). Bending the myosin S1 region helps explain how myosin moves or “walks” along the actin. The thinner and generally longer “tail” region of myosin (S2) also exhibits flexibility and rotates with S1 contraction (Figure 3A). MiRNA-1 and miRNA-133 have been shown to act as specific activators or suppressors of sarcoma formation and muscle gene expression. Deletion of miRNA-1-2 and miRNA-1-1 in mice (miRNA-1 zero) leads to sarcomeric disorders in cardiomyocytes and impaired heart function. All zero miRNA-1 mice died before weaning age (Heidersbach et al., 2013; Wei et al., 2014). miRNA-1 acts to negatively regulate myocardium, the main regulator of smooth muscle gene expression, and telokine, the specific smooth muscle inhibitor of phosphorylation MLC-2 (Heidersbach et al., 2013; Wystub et al., 2013). The upward regulation of myocardium and telokine in zero miRNA-1 cores may contribute in part to the defect of the sarcoma organization. In addition, studies by Wei et al.

have shown that miRNA-1 directly suppresses the estrogen-bound nuclear receptor β (Errβ). The high level of Errβ in the miRNA-1 zero heart activates the expression of genes associated with the fetal sarcoma (Wei et al., 2014). The sarcomere is the basic structural and functional unit of the fibril. It is bounded at each end by a Z-band with adjacent I-bands, and there is a central M-line with adjacent H-bands and partially overlapping A-bands. The Z band (or Z disk) is a dense fibrous structure of actin, α-actinin and other proteins. Thin filaments (or actin filaments) are anchored at one end in the Z strip. Titin is anchored in both the Z band and the M line. Thick filaments are anchored to the M line in the middle of the sarcomere. Band I is the area on both sides of a Z disc that contains only thin filaments and titine. This partial overlap of the filaments makes the A-band darker at its ends, leaving a bright area in the middle (H-band) where there is no overlap with the light bands.

An important indication of the contraction mechanism was the discovery that the H and I bands shorten during contraction, while the A bands do not. Figure 6.7. When (a) a sarcomere (b) contracts, the Z lines move closer together and the I band becomes smaller. The A-band remains the same width and at full contraction the thin filaments overlap. It starts with a signal from the nervous system. So it starts with a signal from your brain. The signal passes through your nervous system to your muscle. Your muscles contract and your bones move. And all of this happens incredibly fast.

Muscle contraction occurs when muscle fibers become shorter. In the true sense of the word, muscle fibers become smaller. To understand how this happens, you need to know more about the structure of muscle fibers. A sarcoma is the functional unit (contractile unit) of a muscle fiber. As shown in Figure 2-5, each sarcoma contains two types of myofilaments: thick filaments, which consist mainly of the contractile protein myosin, and thin filaments, which consist mainly of the contractile actin protein. Thin filaments also contain the regulatory proteins troponin and tropomyosin. When myofilaments are seen under an electron microscope, their arrangement gives the appearance of alternating bands of light and dark bands. The light strips are called I bands and contain only thin filaments. Dark bands are called A-bands and contain thick, thin filaments, with thick filaments encompassing the entire length of the A-band. Thus, the length of the thick filament determines the length of the A-band.

Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril consists of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z-lines (Figure 13.13.2). The region between two Z lines is called sarcomeres. In a sarcoma, the myosin filaments overlap the actin filaments. Myosin filaments have tiny structures called transverse bridges that can attach to actin filaments. .

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