Muscle tissue, the fourth basic tissue type with epithelia, connective tissues, and nervous tissue, is composed of cells that optimize the universal cell property of contractility. As in all cells, actin microfilaments and associated proteins generate the forces necessary for the muscle contraction, which drives movement within organ systems, of blood, and of the body as a whole. Essentially all muscle cells are of mesodermal origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin.
Three types of muscle tissue can be distinguished on the basis of morphologic and functional characteristics (Figure 10–1), with the structure of each adapted to its physiologic role.
Light micrographs of each type, accompanied by labeled drawings. (a) Skeletal muscle is composed of large, elongated, multinucleated fibers that show strong, quick, voluntary contractions. (b) Cardiac muscle is composed of irregular branched cells bound together longitudinally by intercalated discs and shows strong, involuntary contractions. (c) Smooth muscle is composed of grouped, fusiform cells with weak, involuntary contractions. The density of intercellular packing seen reflects the small amount of extracellular connective tissue present. ([a, b]: X200; [c]: X300; All H&E)
Skeletal muscle contains bundles of very long, multinucleated cells with cross-striations. Their contraction is quick, forceful, and usually under voluntary control.
Cardiac muscle also has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs that are unique to cardiac muscle. Contraction is involuntary, vigorous, and rhythmic.
Smooth muscle consists of collections of fusiform cells that lack striations and have slow, involuntary contractions.
In all types of muscle, contraction is caused by the sliding interaction of thick myosin filaments along thin actin filaments. The forces necessary for sliding are generated by other proteins affecting the weak interactions in the bridges between actin and myosin.
As with neurons, muscle specialists refer to certain muscle cell organelles with special names. The cytoplasm of muscle cells is often called sarcoplasm (Gr. sarkos, flesh + plasma, thing formed), the smooth ER is the sarcoplasmic reticulum, and the muscle cell membrane and its external lamina are the sarcolemma (sarkos + Gr. lemma, husk).
The variation in diameter of muscle fibers depends on factors such as the specific muscle, age, gender, nutritional status, and physical training of the individual. Exercise enlarges the skeletal musculature by stimulating formation of new myofibrils and growth in the diameter of individual muscle fibers. This process, characterized by increased cell volume, is called hypertrophy (Gr. hyper, above + trophe, nourishment). Tissue growth by an increase in the number of cells is termed hyperplasia (hyper + Gr. plasis, molding), which takes place very readily in ...