Striated muscle structure

The cells of the latter three types contain only a single nucleus and are called myocytes. The cells of skeletal muscle are long and multinucleate and are referred to as muscle fibers. At the microscopic level, skeletal muscle and cardiac muscle display alternating light and dark bands, and for this reason are often referred to as striated muscles. The different types of muscle cells vary widely in structure, size, and function. In addition, the times required for contractions and relaxations by various muscle types vary considerably. The fastest responses (on the order of milliseconds) are observed for fast-twitch skeletal... 

In addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

In striated muscles, SR is well developed to surround the myofibrils and is divided into two parts, the terminal cisternae (TC) and longitudinal tubules (LT). TC forms triad (skeletal muscle) or dyad (heart) structure with transverse tubules. The ryanodine receptor is located only in the TC, whereas the Ca2+ pump/SERCA is densely packed in both TC and LT. 

Nonmuscle/smooth muscle myosins-Il are structurally similar to striated muscle myosin-II, but they have slower rates of ATP hydrolysis than do their striated muscle counterparts. Nonmuscle/smooth muscle myosin-II is also regulated differently than striated muscle myosin-II. Nonmuscle myosin-II is divided into the invertebrate and vertebrate branches (Cheney et al., 1993). This group is ubiquitous because it is present in most lower organisms, such as slime molds, amoeba, sea urchins, etc., and in virtually all mammalian nonmuscle cells. Smooth muscle myosin-II is also somewhat heterogeneous in that at least three separate forms of smooth muscle heavy chains, with molecular weights of 196,000, 200,000, and 204,000 have been identified (Kawamoto and Adelstein, 1987). The physiological properties of these separate myosin heavy chains are not yet known. 

Smooth muscles, as the name implies, do not contain sarcomeres. In fact, it was initially difficult to demonstrate the presence of thick filaments in smooth muscle, although their presence is now well-established. On the other hand, it is very difficult to demonstrate thick filaments in highly motile cells, such as macrophages and neutrophils, and this may reflect the necessity to rapidly form and redistribute cytoskeletal elements during migration. Thick filaments in smooth muscles appear to be considerably longer than those in striated muscles. They run diagonally in smooth muscle cells and attach to the membrane at structures known as dense bodies. Thus, there is a cork-screw effect when smooth muscles contract (Warshaw etal., 1987). 

The superstructure of smooth muscle actin filaments is differentiated from those of striated muscle by the absence of the troponins and the lateral organization by association of the filaments with dense bodies instead of with the Z-line. How these differences are encoded is again not at all clear. However, the myofibrillar structure and the alignment of the alternating actin and myosin filaments is apparently due primarily to dense bodies and the actin-actinin macrostructures. As the bent dumbbell shaped actins assemble into filaments they are all oriented in the same direction. The S-1 fragments of myosin will bind to actin filaments in vitro and in... 

Huxley, H.E. (1963). Electron microscope studies on the structure of natural and synthetic protein filaments from striated muscle. J. Mol. Biol. 7, 281-308. 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

Internally, muscle fibers are highly organized. Each fiber contains numerous myofibrils — cylindrical structures that also lie parallel to the long axis of the muscle. The myofibrils are composed of thick filaments and thin filaments. It is the arrangement of these filaments that creates alternating light and dark bands observed microscopically along the muscle fiber. Thus, skeletal muscle is also referred to as striated muscle. 

Analysis of muscle structure began about the middle of the last century. Microscopists reported a transverse striated appearance when muscle fibers were examined by ordinary light microscopy. With the... 

Until the past decade, the cytoplasm was widely considered to be structurally unorganized with the main division of labor at the organellar level. Certainly, relatively little was known about the nature of the cyto-skeleton (with the notable exception of the mitotic apparatus and striated muscle), and the dynamics of cytoplasmic behavior were conceptualized vaguely in terms of sol-gel transitions without a sound molecular foundation. Substantial improvements in electron, light, and fluorescence microscopy, as well as the isolation of discrete protein components of the cytoskeleton, have led the way to a much better appreciation of the structural organization of the cytoplasm. Indeed, the lacelike network of thin filaments, intermediate filaments, and microtubules in nonmuscle cells is as familiar today as the organelles identified... 

The smallest structural unit of skeletal musculature is the striated muscle fiber. 

Einally, external urethral sphincter (voluntary sphincter, or rhabdosphincter) is a striated circumscribing structure emanating from the bladder neck and bladder base detrusor through the mid-urethra in the female and intermediate prostatic urethra in the male. While also surrounding Cowper s glands in the male, these rhabdosphincter subunits contract, most likely, only with ejaculation (Hutch, 1972 Elbadawi, 1980), along with simultaneous anal rhabdosphincter, bulbo-cavernosus muscle, and cremaster muscle contractions. 

Tire Structural Organization of Striated Muscle Stress-Prone Pigs... 

Electron micrograph of a striated muscle sarcomere showing the appearance of filamentous structures when cross-sectioned at the locations illustrated below. (Electron micrograph courtesy of Dr. Hugh Huxley, Brandeis University.)... 

Figure 16.2 (a) An electron micrograph of a longitudinal section of a striated muscle cell and (b) a schematic drawing of the structures observed within the cell. From Biochemistry, 4th Edition, by Stryer, pp 392 and 393. 1995, 1988, 1981, and 1975, by W. H. Freeman and Company. 

At the ultrastructural level, flatworm muscle resembles smooth muscle with individual, non-striated myofibrils being delimited by the sarcolemma and interconnected by gap junctions. Also, flatworm muscles lack a T-tubule system that is characteristic of striated muscle in other animal groups. The contractile portion of flatworm myofibrils contains thick myosin and thin actin filaments that connect with the sarcolemma via attachment plaques or desmo-somes. Actomyosin cross-bridges have been reported and where overlap has been observed, ratios that vary from 9 1 to 12 1 have been observed. Although flatworm muscle is mostly non-striated, pseudo-striated (e.g. in the tail of schistosome cercariae Dorsey et al., 2002 Mair et al., 2003) and obliquely striated (e.g. tentacular bulb of the trypanorhynch, Crillotia eri-naceus Ward et al., 1986) muscles have been reported. It is presumed that the role played by these structures has demanded the development... 

Li, Y., Mui, S., Brown, J. H., Strand, J., Reshetnikova, L., Tobacman, L. S., and Cohen, C. (2002). The crystal structure of the C-terminal fragment of striated-muscle a-tropomyosin reveals a key troponin T recognition site. Proc. Natl. Acad. Sci. 99, 7378-7383. 

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural... 

The cortical region of many cells is enriched in actin and associated actin-binding proteins, which function in motility, cell shape maintenance, and membrane protein distribution in polarized cells. In some cases, discrete structures anchor actin to the membrane, as is the case for intercellular adherens junctions and cell-substrate focal contacts. In certain special cell types, the fundamental blueprint for an adherens junction is taken to a new structural level, serving as scaffolding for cell-type specific complexes, such as the dystrophin-associated protein complex (DPC) in striated muscle. Although for years morphological studies have described a close association with IF with the actin-rich cortex, recent advances in methods to study protein-protein interactions have provided new insight into the intimate structural and functional relationship between IFs and these membrane domains. 

Fig. 4. Structure of striated muscle costameres and the DPC. A single membrane-associated costamere from a portion of a striated muscle fiber is magnified above to show the components of the dystrophin-associated protein complex that are involved in linking desmin intermediate filaments (IFs) to the muscle cell membrane. Additional actin-associated proteins present at these sites (including vinculin, talin, spectrin, and ankyrin) are not shown here. In addition to components of the DPC, plectin has also been localized to costameres, and likely contributes to linking desmin IFs to actin-associated structures.

Anyone eating a steak or a slice carved from roast beef knows that meat is fibrous in texture. These fibers, 20 to 100 fj.m in diameter and very long, are the multinucleate muscle cells of which skeletal muscles are composed (Fig. 1 A, B). Such fibers in the light microscope appear cross-striated and the muscles from which they are derived are known as striated muscles. The term striated also covers the muscles in animal hearts (Fig. 1C), but here the cells (the myocytes) are much shorter, they contain a single nucleus, and they are linked end to end by special structures known as... 

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