Skeletal Muscle

Skeletal muscle tissue is packaged into skeletal muscles that attach to and cover the body 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, the in­dividual muscle fibers are held together by several differ­ent layers of connective tissue. Skeletal muscles such as the biceps brachii are surrounded by a dense, irregular con­nective tissue covering called the epimysium (Fig. 4-24). 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 cytoplasm of the muscle fiber (i.e., sarcoplasm) 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 (i.e., 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; each of these units is called a sarcomere (see Fig. 4-24). 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) containing mainly myosin filaments, with some overlap with actin filaments. Straddling the Z band, the lighter I band contains only actin filaments; therefore, 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 line, produced by linkages between the myosin filaments. Z bands 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. This network ends in enlarged, saclike regions called the lateral sacs or terminal cisternae. These sacs store calcium to

 

 

4-24 Connective tissue components of a skeletal muscle. The structure of the myofibril and the relationship between actin and myosin myofilaments are also shown.

be 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. 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 extracellular fluid compartment. Action potentials, which are rapidly conducted over the surface of the mus­cle fiber, are in turn propagated by the T tubules and 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 ions 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. The structures that produce the sliding of the filaments are the myosin heads that form cross-bridges with the thin actin filaments (Fig. 4-25). 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.

 

RE 4-25 Molecular structure of the thin actin filament and the thicker myosin filament of striated muscle. The thin filament is a double-stranded helix of actin molecules with tropomyosin and tro­ponin molecules lying along the grooves of the actin strands. Dur­ing muscle contraction, the ATP-activated heads of the thick myosin filament swivel into position, much like the oars on a boat, form a cross-bridge with a reactive site on tropomyosin, and then pull the actin filament forward. During muscle relaxation, the troponin mol­ecules cover the reactive sites on tropomyosin.

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. 4-25). Tropomyosin, which lies in grooves of the actin strand, provides the site for attachment of the globular heads of the myosin filament. In the non-contracted 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 breaking of the linkage between actin and myosin, 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.