MUSCLE TISSUE

Three types of muscle tissues exist: skeletal, cardiac, and smooth. Skeletal and cardiac muscles are striated muscles. The actin and myosin filaments are arranged in large par­allel arrays in bundles, giving the muscle fibers a striped or striated appearance when they are viewed through a microscope.

Skeletal muscle is the most abundant tissue in the body, accounting for 40% to 45% of the total body weight. Most skeletal muscles are attached to bones, and their con­tractions are responsible for movements of the skeleton. Skeletal muscle differs from cardiac and smooth muscle in that it is enervated by the somatic rather than the auto-nomic nervous system. Cardiac muscle, comprising the myocardium, is designed to pump blood continuously. It has inherent properties of automaticity, rhythmicity, and conductivity. The pumping action of the heart is con­trolled by impulses originating in the cardiac conduction system and is modified by blood-borne neural mediators and impulses from the autonomic nervous system. Smooth muscle is found in the iris of the eye, the walls of blood vessels, hollow organs such as the stomach and urinary bladder, and hollow tubes, such as the ureters, that con­nect internal organs.

Neither skeletal nor cardiac muscle can undergo the mitotic activity needed to replace injured cells. Smooth muscle, however, may proliferate and undergo mitotic ac­tivity. Some increases in smooth muscle are physiologic, as occurs in the uterus during pregnancy. Other increases, such as the increase in smooth muscle that occurs in the ar­teries of persons with chronic hypertension, are pathologic.

Although the three types of muscle tissue differ sig­nificantly in structure, contractile properties, and control mechanisms, they have many similarities. In the follow­ing section, the structural properties of skeletal muscle are presented as the prototype of striated muscle tissue. Smooth

muscle and the ways in which it differs from skeletal mus­cle are also discussed. Cardiac muscle is described in Chap­ter 23.