Muscle tissue, the fourth basic tissue type with epithelia, connective tissues, and nervous tissue, consists of cells that optimize the universal cell property of contractility. As in all cells, actin microfilaments and associated proteins generate the forces necessary for muscle contraction, which drives movement within organ systems, of blood, and of the body as a whole. Essentially all muscle cells originate from embryonic mesoderm and differentiate by elongation and abundant synthesis of the myofibrillar proteins actin and myosin.
Three generally different types of muscle tissue develop based on their morphologic and functional characteristics (Figure 10–1), with the structure of each well adapted to its physiologic role.
Skeletal muscle contains bundles of very long, multinucleated cells with cross-striations and produces quick, forceful contractions, usually under voluntary control.
Cardiac muscle also has cross-striations but consists of less elongated, often branched cells bound to one another at structures called intercalated discs, unique to cardiac muscle. Contraction is involuntary, vigorous, and rhythmic.
Smooth muscle comprises collections of fusiform cells, which lack striations and generate slow, involuntary contractions.
Three types of muscle.
Key features shown in light micrographs and diagrams.
(a) Skeletal muscle contains large, elongated, multinucleated fibers with strong, quick, voluntary contractions.
(b) Cardiac muscle consists of irregular branched cells bound together longitudinally by intercalated discs and shows strong, involuntary contractions.
(c) Smooth muscle comprises groups of smaller, fusiform cells with weak, involuntary contractions. (a, b: ×200; c: ×300; All H&E)
In all types of muscle contraction involves the sliding interaction of thick myosin filaments along thin actin filaments. The forces necessary for this 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 components with special names. The cytoplasm of muscle cells often becomes sarcoplasm (Gr. sarkos, flesh + plasma, thing formed), smooth ER is the sarcoplasmic reticulum, and the muscle cell membrane is called 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 smooth muscle, where cells have not lost the capacity to divide by mitosis.
Skeletal (or striated) muscle consists of muscle fibers, which are ...