Muscle tissue, whose primary function is contraction, is responsible for movement of the body and its parts and for changes in the size and shape of internal organs. Muscle tissue contains two types of fibers that are responsible for contraction: thin and thick filaments. The thin filaments are composed primarily of actin, whereas the thick filaments are composed of myosin. The two types of myofilaments occupy the bulk of the cytoplasm, which in muscle cells is called the sarcoplasm.
There are three types of muscle tissues: skeletal, cardiac, and smooth. Skeletal and cardiac muscles are striated muscles, in which the actin and myosin filaments are arranged in large, parallel arrays in bundles, giving the muscle fibers a striped or striated appearance when observed with a microscope. Smooth muscle lacks striations and 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 and common bile duct, 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.
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. Each skeletal muscle is a discrete organ made up of hundreds or thousands of muscle fibers. At the periphery of skeletal muscle fibers, randomly scattered satellite cells are found. They represent a source of undifferentiated myoblast cells that may be involved in the limited regeneration capabilities of skeletal muscle. Although muscle fibers predominate, substantial amounts of connective tissue, blood vessels, and nerve fibers are also present.
Organization and Structure. In an intact muscle, several different layers of connective tissue hold the individual muscle fibers together. Skeletal muscles such as the biceps brachii are surrounded by a dense, irregular connective tissue covering called the epimysium (Fig. 4.22A). Each muscle is subdivided into smaller bundles called fascicles, which are surrounded by a connective tissue covering called the perimysium. The number of fascicles and their size vary among muscles. Fascicles consist of many elongated structures called muscle fibers, each of which is surrounded by connective tissue called the endomysium. Skeletal muscles are syncytial or multinucleated structures, meaning there are no true cell boundaries within a skeletal muscle fiber.
The sarcoplasm of the muscle fiber is contained within the sarcolemma, which represents the cell membrane. Embedded throughout the sarcoplasm are the contractile elements actin and myosin, which are arranged in parallel bundles called myofibrils. The thin, lighter-staining myofilaments are composed of actin, and the thicker, darker-staining myofilaments are composed of myosin. Each myofibril consists of regularly repeating units along the length of the myofibril, called sarcomeres (see Fig. 4.22B).
Sarcomeres are the structural and functional units of cardiac and skeletal muscle. A sarcomere extends from one Z line to another Z line. Within the sarcomere are alternating light and dark bands. The central portion of the sarcomere contains the dark band (A band) consisting mainly of myosin filaments, with some overlap with actin filaments. Straddling the Z line, the lighter I band contains only actin filaments; there- fore, it takes two sarcomeres to complete an I band. An H zone is found in the middle of the A band and represents the region where only myosin filaments are found. In the center of the H zone is a thin, dark band, the M band or M line, produced by linkages between the myosin filaments. Z lines consist of short elements that interconnect and provide the thin actin filaments from two adjoining sarcomeres with an anchoring point.
The sarcoplasmic reticulum, which is comparable to the smooth ER, is composed of longitudinal tubules that run parallel to the muscle fiber and surround each myofibril (see Fig. 4.22D). This network ends in enlarged, saclike regions called the lateral sacs or terminal cisternae. These sacs store calcium that is released during muscle contraction. A binding protein called calsequestrin found in the terminal cisternae enables a high concentration of calcium ions to be sequestered in the cisternae.9 Concentration levels of calcium ions in the cisternae are 10,000 times higher than in the sarcoplasm.
A second system of tubules consists of the transverse or T tubules, which are extensions of the plasma membrane and run perpendicular to the muscle fiber. The hollow portion or lumen of the transverse tubule is continuous with the extra-cellular fluid compartment. Action potentials, which are rap- idly conducted over the surface of the muscle fiber, are in turn propagated by the T tubules into the sarcoplasmic reticulum. As the action potential moves through the lateral sacs, the sacs release calcium, initiating muscle contraction. The membrane of the sarcoplasmic reticulum also has an active transport mechanism for pumping calcium back into the reticulum. This prevents interactions between calcium ions and the actin and myosin myofilaments after cessation of a muscle contraction.
Skeletal Muscle Contraction. During muscle contraction, the thick myosin and thin actin filaments slide over each other, causing shortening of the muscle fiber, although the length of the individual thick and thin filaments remains unchanged (see Fig. 4.22C). The structures that produce the sliding of the filaments are the myosin heads that form cross-bridges with the thin actin filaments (Fig. 4.23). When activated by ATP, the cross-bridges swivel in a fixed arc, much like the oars of a boat, as they become attached to the actin filament. During contraction, each cross-bridge undergoes its own cycle of movement, forming a bridge attachment and releasing it, and moving to another site where the same sequence of movement occurs. This pulls the thin and thick filaments past each other.
Myosin is the chief constituent of the thick filament. It consists of a thin tail, which provides the structural backbone for the filament, and a globular head. Each globular head contains a binding site able to bind to a complementary site on the actin molecule. Besides the binding site for actin, each myosin head has a separate active site that catalyzes the breakdown of ATP to provide the energy needed to activate the myosin head so that it can form a cross-bridge with actin. After contraction, myosin also binds ATP, thus breaking the linkage between actin and myosin. Myosin molecules are bundled together side by side in the thick filaments such that one half have their heads toward one end of the filament and their tails toward the other end; the other half are arranged in the opposite manner.
The thin filaments are composed mainly of actin, a globular protein lined up in two rows that coil around each other to form a long helical strand. Associated with each actin filament are two regulatory proteins, tropomyosin and troponin (see Fig. 4.23A). Tropomyosin, which lies in grooves of the actin strand, provides the site for attachment of the globular heads of the myosin filament. In the noncontracted state, troponin covers the tropomyosin-binding sites and prevents formation of cross-bridges between the actin and myosin. During an action potential, calcium ions released from the sarcoplasmic reticulum diffuse to the adjacent myofibrils, where they bind to troponin. Binding of calcium to troponin uncovers the tropomyosin-binding sites such that the myosin heads can attach and form cross-bridges. Energy from ATP is used to break the actin and myosin cross-bridges, stopping the muscle contraction. After the linkage between actin and myosin is broken, the concentration of calcium around the myofibrils decreases as calcium is actively transported into the sarcoplasmic reticulum by a membrane pump that uses energy derived from ATP.
The basis of rigor mortis can be explained by the binding of actin and myosin. As the muscle begins to degenerate after death, the sarcoplasmic cisternae release their calcium ions, which enable the myosin heads to combine with their sites on the actin molecule. As ATP supplies diminish, no energy source is available to start the normal interaction between actin and myosin, and the muscle is in a state of rigor until further degeneration destroys the cross-bridges between actin and myosin.6
Smooth muscle is often called involuntary muscle because its activity arises spontaneously or through activity of the autonomic nervous system. Smooth muscle contractions are slower and more sustained than skeletal or cardiac muscle contractions.
Organization and Structure. Smooth muscle cells are spindle shaped and smaller than skeletal muscle fibers. Each smooth muscle cell has one centrally positioned nucleus. Z lines and M lines are not present in smooth muscle fibers, and cross-striations are absent because the bundles of filaments are not parallel but crisscross obliquely through the cell. Instead, the actin filaments are attached to structures called dense bodies (Fig. 4.24). Some dense bodies are attached to the cell membrane, and others are dispersed in the cell and linked together by structural proteins.
The lack of Z lines and the regular overlapping of contractile elements provide a greater range of tension development. This is important in hollow organs that undergo changes in volume, with consequent changes in the length of the smooth muscle fibers in their walls. Even with the distention of a hollow organ, the smooth muscle fiber retains some ability to develop tension, whereas such distention would stretch skeletal muscle beyond the area where the thick and thin filaments overlap.
Smooth muscle is usually arranged in sheets or bundles. In hollow organs, such as the intestines, the bundles are organized into the two-layered muscularis externa consisting of an outer, longitudinal layer and an inner, circular layer. A thinner muscularis mucosae often lies between the muscularis externa and the endothelium. In blood vessels, the bundles are arranged circularly or helically around the vessel wall.
Smooth Muscle Contraction. As with cardiac and skeletal muscle, smooth muscle contraction is initiated by an increase in intracellular calcium. However, smooth muscle differs from skeletal muscle in the way its cross-bridges are formed. The sarcoplasmic reticulum of smooth muscle is less developed than in skeletal muscle, and no transverse tubules are present. Smooth muscle relies on the entrance of extracellular calcium and its release from the sarcoplasmic reticulum for muscle contraction. This dependence on movement of extracellular calcium across the cell membrane during muscle contraction is the basis for the action of calcium-blocking drugs used in the treatment of cardiovascular disease.
Smooth muscle also lacks troponin, the calcium-binding regulatory protein found in skeletal and cardiac muscle. Instead, it relies on another calcium-binding protein called calmodulin. The calcium–calmodulin complex binds to and activates the myosin-containing thick filaments, which interact with actin.
Types of Smooth Muscle. Smooth muscle may be divided into two broad categories according to the mode of activation: multiunit and single-unit smooth muscle. In multiunit smooth muscle, each unit operates almost independently of the others and is often enervated by a single nerve, such as occurs in skeletal muscle. It has little or no inherent activity and depends on the autonomic nervous system for its activation. This type of smooth muscle is found in the iris, in the walls of the vas deferens, and attached to hairs in the skin. The fibers in single-unit smooth muscle are in close contact with each other and can contract spontaneously without nerve or hormonal stimulation. Normally, most of the muscle fibers contract synchronously, hence the term single-unit smooth muscle. Some single-unit smooth muscle, such as that found in the gastrointestinal tract, is self-excitable. This is usually associated with a basic slow-wave rhythm transmitted from cell to cell by nexuses (i.e., gap junctions) formed by the fusion of adjacent cell membranes. The cause of this slow-wave activity is unknown. The intensity of contraction increases with the frequency of the action potential. Certain hormones, other agents, and local factors can modify smooth muscle activity by depolarizing or hyperpolarizing the membrane. Smooth muscle cells found in the uterus and small-diameter blood vessels are also singleunit smooth muscle.