22 - Chapter 22 SKELETAL MUSCLE STRUCTURE AND FUNCTION A...

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Chapter 22. SKELETAL MUSCLE STRUCTURE AND FUNCTION Muscle cells, myocytes or muscle fibers (clarified below), are specialized contractile cells capable of performing mechanical work in response to excitatory events at their surface membrane. The prefix sarco - is used to specify muscle, so the excitable membrane is termed the sarcolemma . All muscle types - skeletal, cardiac, and smooth - depend on excitatory events at the membrane to trigger the biochemical processes that bring about the mechanical response. The linkage between AP discharge at the sarcolemma and contraction of the cell is referred to as excitation-contraction coupling . The steps involved will be examined in some detail in this section. It is convenient to begin with skeletal muscle, the most abundant type in the body. Adult-type cells are unusual in being multinucleated , and are called fibers . This condition arises from the fusion of multiple individual cells during muscle growth, or the subsequent donation of nuclei from specialized satellite cells . These processes are discussed in Chapter 57. The mature fibers can be extremely large, both in length and diameter. These dimensions are of consequence for excitation-contraction coupling. It is generally true that skeletal muscles are seldom fully relaxed, and because muscles are often arranged in antagonistic pairs , a balance usually exists between low-level activity in each of the members of the pair. This low-grade contractile activity is sometimes called tone , or tonus . A typical muscle is shown dissected to progressively finer levels of detail in Figure 22-1. In skeletal muscle, the bulk of the fiber is occupied by highly organized myofibrils , or longitudinal assemblies of contractile filaments . At the more gross myofibrilar level, characteristic striations or banding are evident and are due to the arrangement of the filaments. The interdigitation of thick filaments, made up of myosin, with thin filaments, which are mainly actin , is strictly controlled, with the thin filaments being anchored at the Z-line . As shown in Figure 22-1, the sarcomere is the length of myofibril between two adjacent Z-lines. There is variation in the length of the sarcomere, depending on the state of relaxation (resting length) or contraction (shortened length). In certain situations, such as when the muscle is stretched, the sarcomere may be longer. However, there is both a minimum length, beyond which further shortening cannot occur, and a maximum length in stretched muscle, beyond which the sarcomere is torn or disrupted and the fiber is damaged. Contraction results from a fascinating series of interactions between the contractile filaments, the unraveling of which was a major biologic discovery that brought into play biochemistry, biophysics, electron microscopy, and crystallography.
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