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The body is composed of four basic tissues: epithelium, connective tissue, muscle tissue, and nervous tissue. Of these four tissues, muscle tissue has a simple but vital role—to contract.
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Skeletal muscle powers behavior; it moves limbs, propels the breath, and is involved in producing speech.
Cardiac muscle pumps blood.
Smooth muscle controls blood pressure in vessels and helps to move food through the digestive tract.
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Contractions of these three muscle types differ in speed, force, frequency, and extent, but all are initiated by increases in Ca2+ concentration, which triggers the movements of the actomyosin filament systems and shortens the cells. The different modes of contraction by the three types of muscle result both from variations in the composition and array of their actomyosin systems and from differences in membrane organization and organelle distribution. The large-scale, regular array of the filaments in skeletal and cardiac muscle produces ordered structures, known as striations, which are visible in the light microscope. Actomyosin arrays in smooth muscle are less abundant, and no special structures are seen on standard sections.
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Although contraction is the obvious and main function of muscle, these cells play other roles in the complex economy of the body. The abundant protein in skeletal muscle can be catabolized as required to support maintenance of other tissues. Cardiac muscle cells produce hormones important for the regulation of fluid balance in the body. Smooth muscle in the large blood vessels produces elastic fibers important for their resilience. Myoepithelial cells are a fourth type of contractile cell (see Chapter 1).
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This chapter begins with a description of the anatomy of skeletal muscle, followed by a review of the molecular components of contraction common to all muscle and an examination of the specialized features found in skeletal muscle. The discussion then turns to cardiac and smooth muscle. Features of growth and repair are considered for each muscle type.
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This section includes an overview of the components of the molecular machinery that produce contraction in all muscle as well as a discussion of some of the specializations found in skeletal muscle. The anatomy of skeletal muscle will be described first to introduce some specific terminology.
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ORGANIZATION OF SKELETAL MUSCLE
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Muscles that move the skeleton are composed of long individual cells called myofibers. Skeletal muscles are built from myofibers and a set of three supportive connective tissue wrappings: endomysium, perimysium, and epimysium (Figure 5-1A).
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