Striated skeletal muscle

What we commonly call movement depends on mnscle cells. There are several types of muscle in the hnman body heart mnscle striated skeletal muscle, which includes the large muscles of the body and smooth mnscle of the stomach, intestines, arteries and veins, and nterns. There is a fnndamental difference between striated and smooth muscle. You can control the former bnt not the latter. Yon can pretty mnch will what yon do with yonr arms and legs, for example. The activities of your stomach, veins and arteries, and intestines are pretty mnch beyond yonr ability to control by force of will. Heart is an exception heart mnscle is basically a striated muscle but we are at a loss to willfully control the rate or force of its contractions. 

C.L. Davey, Significance of carnosine and anserine in striated skeletal muscle.. Arch. Biochem. Biophys., 89 (1960) 303-308. 

Striated skeletal muscle fibers are bound together by collagenous connective tissue to form individual muscles. The connective tissue covering a muscle is known as the epimysium. This forms a resilient elastic sheath covering that separates the muscle from surrounding structures such as tendons and bone. 

Active movement is one of the signs of life. The universal actuator in the animal kingdom is the muscle. The striated skeletal muscle - which is the focus of the following discussion is the evolutionary highlight of biological actuators. Early stages functioning as contractile elements can even be seen in protozoa such as amoebas. 

The structure of heart muscle is similar to striated skeletal muscle but has significantly more mitochondria and sarcoplasm. 

Muscle is an apparatus for the rapid conversion of chemical energy into heat and mechanical work. As regards rate and continuity, this proceeds almost uniformly in cardiac and un-striated muscle, but striated skeletal muscle is under voluntary control and subject to abrupt and occasional demands from the organism. Skeletal muscle, in consequence, requires a store of energy that can be released rapidly under anaerobic conditions, and replenished under aerobic conditions in the resting tissue. 

Muscle tissue is unique in its ability to shorten or contract. The human body has three basic types of muscle tissue histologically classified into smooth, striated, and cardiac muscle tissues. Only the striated muscle tissue is found in all skeletal muscles. The type of cells which compose the muscle tissue are known as contractile cells. They originate from mesenchymal cells which differentiate into myoblasts. Myoblasts are embryonic cells which later differentiate into contractile fiber cells. 

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... 

FIGURE 17.10 The four classes of muscle cells iu mammals. Skeletal muscle and cardiac muscle are striated. Cardiac muscle, smooth muscle, and myoepithelial cells are mononucleate, whereas skeletal muscle is multinucleate. 

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. 

The general picture of muscle contraction in the heart resembles that of skeletal muscle. Cardiac muscle, like skeletal muscle, is striated and uses the actin-myosin-tropomyosin-troponin system described above. Unlike skeletal muscle, cardiac muscle exhibits intrinsic rhyth-micity, and individual myocytes communicate with each other because of its syncytial nature. The T tubular system is more developed in cardiac muscle, whereas the sarcoplasmic reticulum is less extensive and consequently the intracellular supply of Ca for contraction is less. Cardiac muscle thus relies on extracellular Ca for contraction if isolated cardiac muscle is deprived of Ca, it ceases to beat within approximately 1 minute, whereas skeletal muscle can continue to contract without an extraceUular source of Ca +. Cyclic AMP plays a more prominent role in cardiac than in skeletal muscle. It modulates intracellular levels of Ca through the activation of protein kinases these enzymes phosphorylate various transport proteins in the sarcolemma and sarcoplasmic reticulum and also in the troponin-tropomyosin regulatory complex, affecting intracellular levels of Ca or responses to it. There is a rough correlation between the phosphorylation of Tpl and the increased contraction of cardiac muscle induced by catecholamines. This may account for the inotropic effects (increased contractility) of P-adrenergic compounds on the heart. Some differences among skeletal, cardiac, and smooth muscle are summarized in... 

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. ... 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

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. 

Somlyo But Ogawa recently published a paper about skeletal muscle, showing that there is some thing similar happening there SOCs mediate Ca2+ entry (Kurebayashi Ogawa 2001). InsP3 does not release Ca from the SR of striated muscle. 

Skeletal muscle, also known as striated muscle because of the microscopic appearance, is responsible for locomotion and those fine, voluntary movements of the body which are under conscious control. Smooth muscle exerts automatic, involuntary... 

Skeletal muscle is under conscious control. Each fibre is an enormous, multi-nucleate cell, formed by fusing hundreds of myoblasts end-to-end. They show a striated pattern, reflecting the regular arrangement of sarcomeres within each cell. 

Cardiac muscle is similar to skeletal muscle, but is not under conscious control. These mono-nucleate cells are much smaller, but still show a striated pattern. The cells are in electrical contact through communicating gap junctions. These are important for the orderly spread of excitation through the heart. Spontaneous electrical depolarization of the specialized pacemaker cells together with conducting fibres activate the bulk of the ventricular muscle in the chamber walls, in each case through direct electrical contacts. 

Fig. 2. Macroscopic and microscopic structure of muscle (a) Entire muscle and its cross-section with fatty septa, (b) Fascicle with several muscle fibres (cells). A layer of fat along the fascicle is indicated, (c) Striated myofibre corresponding with one single muscle cell containing several nuclei. The lengths of a myofibre can be several tens of centimetres, (d) Myofibril inside a myocyte. It is one contractile element and contains actin and myosin and further proteins important for the muscular function, (e) Electron myograph of human skeletal muscle showing the band structure caused by the contractile myofilaments in the sarcomeres. One nucleus (Nu) and small glycogen granules (arrow, size <0.1 pm) are indicated.

Cardiac muscle is present only in the heart. Its fibres are striated but, unlike skeletal muscle, they are short and branched. The force it produces is less than that of skeletal muscle but it is the endurance muscle par... 

Figure 13.8 Cardiac, skeletal and smooth muscle fibres. Cardiac and skeletal muscle fibres appear striated because each fibre is packed with longitudinally arranged myofibrils which lie side by side in almost perfect register (Figure 13.5). In contrast, smooth muscle fibres contain fewer myofibrils that are uneven in diameter and length and are not in register.

Smooth muscle differs from skeletal muscle in various ways. Smooth muscles—which are found, for example, in blood vessel walls and in the walls of the intestines—do not contain any muscle fibers. In smooth-muscle cells, which are usually spindle-shaped, the contractile proteins are arranged in a less regular pattern than in striated muscle. Contraction in this type of muscle is usually not stimulated by nerve impulses, but occurs in a largely spontaneous way. Ca (in the form of Ca -calmodulin see p.386) also activates contraction in smooth muscle in this case, however, it does not affect troponin, but activates a protein kinase that phosphorylates the light chains in myosin and thereby increases myosin s ATPase activity. Hormones such as epinephrine and angiotensin II (see p. 330) are able to influence vascular tonicity in this way, for example. 

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. 

A third tissue is muscle, which is classified into three types striated (voluntary skeletal muscle), cardiac (involuntary striated muscle), and smooth (involuntary) muscle. There are two major groups of cells in nervous tissue, the fourth tissue type. Neurons are the actual conducting cells whose cell membranes carry nerve impulses. Several kinds of glial cells lie between and around the neurons. 

There is probably no biological phenomenon that has excited more interest among biochemists than the movement caused by the contractile fibers of muscles. Unlike the motion of bacterial flagella, the movement of muscle is directly dependent on the hydrolysis of ATP as its source of energy. Several types of muscle exist within our bodies. Striated (striped) skeletal muscles act under voluntary control. Closely related are the involuntary striated heart muscles, while smooth involuntary muscles constitute a third type. Further distinctions are made between fast-twitch and slow-twitch fibers. Fast-twitch fibers have short isometric contraction times, high maximal velocities for shortening, and high rates of ATP hydrolysis. 

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