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Skeletal muscle consists of long cylindrical cells that, under a microscope, appear striated with bands perpendicular to the length of the cell. The many nuclei in each cell (multinucleated cells) are located near the outside along the plasma membrane, which is called the sarcolemma. Skeletal muscle is attached to bones and causes movements of the body. Because it is under conscious control, it is also called voluntary muscle.
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Cardiac muscle, like skeletal muscle, is striated. However, cardiac muscle cells have a single, centrally located nucleus, and the muscle fibers branch often. Where two cardiac muscle cells meet, they form an intercalated disc containing gap junctions, which bridge the two cells. Cardiac cells are the only cells that pulsate in rhythm.
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Smooth muscle consists of cells with a single, centrally located nucleus. The cells are elongated with tapered ends and do not appear striated. Smooth muscle lines the walls of blood vessels and certain organs such as the digestive and urogenital tracts, where it serves to advance the movement of substances. Smooth muscle is called involuntary muscle because it is not under direct conscious control.
Figure 1. Three kinds of muscle tissue exist: skeletal muscle, cardiac muscle, and smooth muscle.