Smooth muscle is composed of myocytes with a fusiform shape, about 30 to 200 ^m long, 5 to 10 ^m wide at the middle, and tapering to a point at each end. There is only one nucleus, located near the middle of the cell. Although thick and thin filaments are both present, they are not aligned with each other and produce no visible striations or sarcomeres; this is the reason for the name smooth muscle. Z discs are absent; instead, the thin filaments are attached by way of the cytoskeleton to dense bodies, little masses of protein scattered throughout the sarcoplasm and on the inner face of the sarcolemma.
The sarcoplasmic reticulum is scanty, and there are no T tubules. The calcium needed to activate smooth muscle contraction comes mainly from the extracellular fluid (ECF) by way of calcium channels in the sarcolemma. During relaxation, calcium is pumped back out of the cell. Some smooth muscle has no nerve supply, but when nerve auto = self fibers are present, they are autonomic (like those of cardiac muscle) and not somatic motor fibers.
Unlike skeletal and cardiac muscle, smooth muscle is capable of mitosis and hyperplasia. Thus, an organ such as the pregnant uterus can grow by adding more myocytes, and injured smooth muscle regenerates well.
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