Skeletal Muscle Fiber

 Figure 1 shows the organization of skeletal muscle, demonstrating that all skeletal muscles are composed of numerous fibers ranging from 10 to 80 micrometers in diameter. Each of these fibers is made up of successively smaller subunits, also shown in Figure 1 and described in subsequent paragraphs.

Fig1. Organization of skeletal muscle, from the gross to the molecular level. F, G, H, and I are cross sections at the levels indicated. 

In most skeletal muscles, each fiber extends the entire length of the muscle. Except for about 2 percent of the fibers, each fiber is usually innervated by only one nerve ending, located near the middle of the fiber.

The Sarcolemma Is a Thin Membrane Enclosing a Skeletal Muscle Fiber. The sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide mate rial that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber. The tendon fibers in turn collect into bundles to form the muscle tendons that then connect the muscles to the bones.

Myofibrils Are Composed of Actin and Myosin Filaments. Each muscle fiber contains several hundred to several thousand myofibrils, which are illustrated in the cross-sectional view of Figure 1C. Each myofibril (Figure 1D and E) is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that are responsible for the actual muscle contraction. These filaments can be seen in longitudinal view in the electron micrograph of Figure 2 and are represented diagrammatically in Figure 1, parts E through L. The thick filaments in the diagrams are myosin, and the thin filaments are actin.

Fig2. An electron micrograph of muscle myofibrils showing  the detailed organization of actin and myosin filaments. Note the  mitochondria lying between the myofibrils. (From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

Note in Figure 6-1E that the myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark bands, as illustrated in Figure 2. The light bands contain only actin filaments and are called I bands because they are isotropic to polarized light. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called A bands because they are anisotropic to polarized light. Note also the small projections from the sides of the myosin filaments in Figure 1E and L. These projections are cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction.

Figure 1E also shows that the ends of the actin filaments are attached to a Z disk. From this disc, these filaments extend in both directions to interdigitate with the myosin filaments. The Z disk, which is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber. Therefore, the entire muscle fiber has light and dark bands, as do the individual myofibrils. These bands give skeletal and cardiac muscle their striated appearance.

The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z disks is called a sarcomere. When the muscle fiber is contracted, as shown at the bottom of Figure 3, the length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. As discussed later, at this length the muscle is capable of generating its greatest force of contraction.

Fig3. Relaxed and contracted states of a myofibril showing  (top) sliding of the actin filaments (pink) into the spaces between the  myosin filaments (red) and (bottom) pulling of the Z membranes  toward each other. 

Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place. The side-by-side relationship between the myosin and actin filaments is maintained by a large number of filamentous molecules of a protein called titin (Figure 4). Each titin molecule has a molecular weight of about 3 million, which makes it one of the largest protein molecules in the body. Also, because it is filamentous, it is very springy. These springy titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work. One end of the titin molecule is elastic and is attached to the Z disk, acting as a spring and changing length as the sarcomere contracts and relaxes. The other part of the titin molecule tethers it to the myosin thick filament. The titin molecule also appears to act as a template for initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments.

Fig4. Organization of proteins in a sarcomere. Each titin molecule extends from the Z disk to the M line. Part of the titin molecule  is closely associated with the myosin thick filament, whereas the rest  of the molecule is springy and changes length as the sarcomere  contracts and relaxes.

Sarcoplasm Is the Intracellular Fluid between Myofibrils. The many myofibrils of each muscle fiber are sus pended side by side in the muscle fiber. The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mitochondria that lie parallel to the myofibrils. These mitochondria supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria.

Sarcoplasmic Reticulum Is a Specialized Endoplasmic Reticulum of Skeletal Muscle. Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive reticulum (Figure 5), called the sarcoplasmic reticulum. This reticulum has a special organization that is extremely important in regulating calcium storage, release, and reuptake and therefore muscle contraction, as discussed in Chapter 7. The rapidly contracting types of muscle fibers have especially extensive sarcoplasmic reticula. 

Fig5. Sarcoplasmic reticulum in the extracellular spaces  between the myofibrils, showing a longitudinal system paralleling the  myofibrils. Also shown in cross section are T tubules (arrows) that  lead to the exterior of the fiber membrane and are important for  conducting the electrical signal into the center of the muscle fiber.  (From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)

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