Muscle tissue muscle tissues are highly specialized

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Muscle Tissue Muscle tissues are highly specialized to contract, or shorten, to produce movement. Types of Muscle Tissue The three types of muscle tissue are illustrated in Figure 3.20. Notice their similarities and differ- ences as you read through the descriptions that follow. Skeletal Muscle Skeletal muscle tissue is packaged by connective tissue sheets into organs called skeletal muscles, which are attached to the skeleton. These muscles, which can be controlled voluntarily (or con- sciously), form the flesh of the body, the so-called muscular system (see Chapter 6). When the skele- tal muscles contract, they pull on bones or skin. The result of their action is gross body movements or changes in our facial expressions. The cells of skeletal muscle are long, cylindrical, multinucleate, and they have obvious striations (stripes). Because skeletal muscle cells are elongated to provide a long axis for contraction, they are often called muscle fibers . Cardiac Muscle Cardiac muscle, covered in more detail in Chap- ter 11, is found only in the heart. As it contracts, the heart acts as a pump and propels blood through the blood vessels. Like skeletal muscle, cardiac muscle has striations, but cardiac cells are uninucleate, relatively short, branching cells that fit tightly together (like clasped fingers) at junctions called intercalated disks. These intercalated disks contain gap junctions that allow ions to pass freely from cell to cell, resulting in rapid conduction of the exciting electrical impulse across the heart. Cardiac muscle is under involuntary control, which means that we cannot consciously control Chapter 3: Cells and Tissues 95
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FIGURE 3.20 Types of muscle tissue and their common locations in the body. Cell division typically yields two daughter cells, each with one nucleus. How is the multinuclear condition of skeletal muscle explained? A Skeletal muscle cells repeatedly undergo mitosis unaccompanied by cytokinesis. Q Photomicrograph: Skeletal muscle (approx. 300x). Photomicrograph: Cardiac muscle (800x). Photomicrograph: Sheet of smooth muscle (approx. 600x). Nuclei Intercalated discs Nucleus Smooth muscle cell Nuclei Part of muscle fiber (a) Diagram: Skeletal muscle (b) Diagram: Cardiac muscle (c) Diagram: Smooth muscle
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Chapter 3: Cells and Tissues 97 the activity of the heart. (There are, however, rare individuals who claim they have such an ability.) Smooth Muscle Smooth, or visceral, muscle is so called because no striations are visible. The individual cells have a single nucleus and are spindle-shaped (pointed at each end). Smooth muscle is found in the walls of hollow organs such as the stomach, bladder, uterus, and blood vessels. As smooth muscle contracts, the cavity of an organ alternately becomes smaller (constricts on smooth muscle contraction) or en- larges (dilates on smooth muscle relaxation) so that substances are propelled through the organ along a specific pathway. Smooth muscle contracts much more slowly than the other two muscle types.
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