Contraction of Skeletal Muscle

About 40 per cent of the body is skeletal muscle, and perhaps another 10 per cent is smooth and cardiac muscle. Some of the same basic principles of contraction apply to all these different types of muscle. In this chapter, function of skeletal muscle is considered mainly; the specialized functions of smooth muscle are discussed in Chapter 8, and cardiac muscle is discussed in Chapter 9.

Physiologic Anatomy of Skeletal Muscle

Skeletal Muscle Fiber

Figure 6-1 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 6-1 and described in subsequent paragraphs.

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

Sarcolemma. The sarcolemma is the cell membrane of the muscle fiber. The sar-colemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons that then insert into the bones.

Myofibrils; Actin and Myosin Filaments. Each muscle fiber contains several hundred to several thousand myofibrils, which are demonstrated by the many small open dots in the cross-sectional view of Figure 6-1C. Each myofibril (Figure 6-1D 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 can be seen in longitudinal view in the electron micrograph of Figure 6-2 and are represented diagrammatically in Figure 6-1, parts E through L. The thick filaments in the diagrams are myosin, and the thin filaments are actin.

Note in Figure 6-1E that the myosin and actin filaments partially interdigi-tate and thus cause the myofibrils to have alternate light and dark bands, as illustrated in Figure 6-2. 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 6-1E and L. These are cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction.

Figure 6-1E also shows that the ends of the actin filaments are attached to a so-called Z disc. From this disc, these filaments extend in both directions to

Figure 6-1

Organization of skeletal muscle, from the gross to the molecular level. F, G, H, and I are cross sections at the levels indicated. (Drawing by Sylvia Colard Keene. Modified from Fawcett DW: Bloom and Fawcett: A Textbook of Histology. Philadelphia: WB Saunders, 1986.)

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