Bundles of myofibrils

Figure 8.44. Drawing of a muscle fiber containing six myofibrils with each surrounded by the sarcoplasmic reticulum that releases calcium ion to trigger contraction and that pumps calcium ion back out to allow for the relaxation in preparation for the next contraction/relaxation cycle. Just inside the sar-colemma of the muscle fiber that surrounds the bundles of myofibrils are the mitochondria that supply the ATP required to convert the fiber from a contracted state to a relaxed but energized state in wait for the next release of calcium ion for trigger-...

Figure 8.43. Musculature of a man that highlights the biceps muscle, perhaps best recognized for performing the mechanical work of lifting a weight, with a cutaway to an individual muscle fiber, presented as a bundle of myofibrils that contains the fundamental contractile element. (From Fundamentals of Bio-...

FIGURE 6.2 Structural organization of skeletal muscle from macro- to microlevel. Whole muscle (o), bundles of myofibrils (b), single myofibril (c), sarcomere (d)> and thick (myosin) filament and thin (actin) filament (e). All symbols are defined in the text. [Modified from Enoka (1994).]... 

Connective tissue Sarcoiemma Sarcoplasma Myofibrii Bundle of myofibrils-l Mitochondria Nudeus... 

Striated muscle is composed of multinucleated muscle fiber cells surrounded by an electrically excitable plasma membrane, the sarcolemma. An individual muscle fiber cell, which may extend the entire length of the muscle, contains a bundle of many myofibrils arranged in parallel, embedded in intracellular fluid termed sarcoplasm. Within this fluid is contained glycogen, the high-energy compounds ATP and phosphocreatine, and the enzymes of glycolysis. 

The anatomical unit of muscle is an elongated cell called a fibre. Each individual fibre cell consists of myofibrils which are bundles of contractile protein filaments composed of actin and myosin (Figure 7.1). Differences in structure indicate that muscles have evolved to perform particular functions. Although the structure of fibres, myofibrils and filaments of actin and myosin, is similar in all muscle types, their arrangement, action and control allow identification of three tissue types ... 

The muscle is composed of subunits called fascicles. The entire muscle and subunits with several fascicles are often surrounded by layers of connective tissue or fatty tissue. Fascicles are bundles of individual muscle fibres. Each fibre is one elongated cell that may extend for the length of the muscle. Each muscle fibre cell is segmented into distinct sectional bands. In contrast to most other tissues, cells of skeletal musculature have several nuclei. Within each muscle cell are numerous myofibrils, which also extend for the length of the muscle cell. Sarcomeres are the basic contractile subunit of myofibrils. 

Striated muscle consists of parallel bundles of muscle fibers. Each fiber is a single large mul-tinucleate cell. The cytoplasm in these cells contains myofibrils 2-3 pm thick that can extend over the full length of the muscle fiber. 

Myosin is quantitatively the most important protein in the myofibrils, representing 65% of the total. It is shaped like a golf club (bottom right). The molecule is a hexamer consisting of two identical heavy chains (2 X 223 kDa) and four light chains (each about 20 kDa). Each of the two heavy chains has a globular head at its amino end, which extends into a tail about 150 nm long in which the two chains are intertwined to form a superhelix. The small subunits are attached in the head area. Myosin is present as a bundle of several hundred stacked molecules in the form of a thick myosin filament. The head portion of the molecule acts as an ATPase, the activity of which is modulated by the small subunits. 

Skeletal muscles consist of bundles of long muscle fibers, which are single cells of diameter 10-100 pm formed by the fusion of many embryonic cells. The lengths are typically 2-3 cm in mammals but may sometimes be as great as 50 cm. Each fiber contains up to 100-200 nuclei. Typical cell organelles are present but are often given special names. Thus, the plasma membrane (plasmalemma) of muscle fibers is called the sarcolemma. The cytoplasm is sarcoplasm, and mitochondria may be called sarcosomes. The major characteristic of muscle is the presence of the contractile myofibrils, organized bundles of proteins 1-2 pm in diameter and not separated by membranes from the cytoplasm. Since they occupy most of the cytoplasm, a substantial number of myofibrils are present in each muscle fiber. 

Anatomical structure. The striated muscles of vertebrates consist of bundles of muscle fiber cells covered with connective tissue. A muscle fiber is composed of bundles of striated myofibrils surrounded by sarcoplasmic reticulum, mitochondria and other organelles. The unique structure of the striated myofibrils, composed of thin(actin)-filaments and thick(myosin)-filaments, is common to all striated muscles of vertebrates including fish (19,20). 

FIGURE 2.2 Schematic structure of skeletal muscle tendon (1), epimysium (2), perimysium (3), endomysium (4), sarcolemma (5), myofibril (6), muscle fiber (7), and bundle of muscle fibers (8). 

The fibers of both white and dark muscles consist of bundles of striated myofibrils each containing thin and thick filaments and various subcellular structures such as the sarcoplasmic reticulum and other organelles (9,10,11). The size and shape of fish muscle fibers and myofibrils differ somewhat from the corresponding components of mammalian muscles (9,11). 

The structure of muscle can be viewed at the electron micrograph level shown in Figure 8.9a, The muscle tissue is composed of bundles of muscle cells called muscle fibers. Within a muscle fiber are myofibrils, which are also arranged in bundles. Individual myofibrils contain the structurally distinct regions described below. Myofibrils have thin filaments composed of actin and thick filaments composed of myosin. Arrangement of the thick and thin filaments in a myofibril produces the distinctive pattern in Figure 8.9a and Figure 8.15. 

A striated muscle, such as the biceps, is comprised of bundles of muscle fibers. The fundamental unit of a muscle fiber is the myofibril composed of a series of repeating units called sarcomeres defined by the periodicity of Z lines (disks) at repeat distances of just over 2 pm (e.g., 2.3 pm). The structural relationships proceeding from the anatomical level of the biceps to the microscopic level of the sarcomere are shown in Figures 8.43, 8.44, and 8.45. ... 

Muscle cells contain contractile elements, myofibrils, which are essentially bundles of proteins found in the sarcoplasm (Figure 2.23). Muscle myofibrils contain isotropic sections (I-bands about 0.8 xm long) and anisotropic sections (A-bands about 1.5 xm long). The I-band is interrupted by a Z-line about 80 nm wide. The low-density central part of the A-band of is the H-zone. The M-line is situated in the centre of the H-zone. The entire structural unit of myofibrils, which is the span between two Z-lines, is called the sarcomere. In a relaxed state (relaxed muscle), it is about 2.5 [am long. During muscle contraction (shortening) it is shorter, 1.7-1.8 p.m. 

The myofibril also contains thin filaments of F-actin with tropomyosin dimers bound in the grooves and about one troponin complex per tropomyosin dimer. The thin filaments are also arranged in a bipolar fashion. All the pointed ends point away from each side of the Z-disk which contains a number of proteins including n-actinin. o-Actinin serves to bundle the actin filaments very close to their barbed ends and anchor them very firmly in the disk. 

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