Muscle Fiber Anatomy: Basic Sarcomere Subdivisions

Each muscle fiber is also surrounded by a basal lamina, or basement membrane. Lying under the basement membrane are specialized cells referred to as satellite cells. Satellite cells are derived from embryonic cells called myoblasts. They are likely important in muscle fiber regeneration and are thought to fuse with the muscle fiber during this process. The muscle fiber itself is surrounded by its cell membrane, the sarcolemma. It forms the membrane under which the multiple muscle fiber nuclei reside. Within the boundaries of the sarcolemma, the contractile myofibrils are contained. They are surrounded by the cytoplasm of the muscle fiber, called the sarcoplasm.

Just as a muscle contraction is dependent upon numerous muscle fiber contractions, muscle fiber contraction is dependent upon the action of the numerous muscle fiber subunits, the myofibrils that run longitudinally along the length of the muscle fiber. They are collections of thin and thick filaments—actin and myosin. The contractile unit of the muscle is the sarcomere. Each sarcomere is bound on either end by the Z disk, a proteinaceous structure that is oriented across the myofibril perpendicular to the filaments, and when the fibril is viewed longitudinally, it is apparent as the Z band. Z bands are seen with regular periodicity along the myofibril, defining the several sarcomeres lined up at their ends. The thin filaments, actin, anchor into the Z disk and do not extend along the length of the sarcomere but reside only at the ends.

In contrast, the thick filaments, myosin, are situated at the middle of the sarcomere at the area seen as the A band. A slight enlargement at the middle of the thick filaments, in the midline of the sarcomere, leads to the appearance of the M band, or M line. The thick filaments overlap the ends of the thin filaments not anchored into the Z disk. The thick filaments, however, also do not run the length of sarcomere.

Thus there are two additional areas seen within the sarcomere. On either side of the Z band, there are only thin filaments, and this region straddling the Z disk is the I band. Likewise, in the middle of the sarcomere at rest, there are only thick filaments, which appear as the H zone. Therefore, traveling from the Z disks to the midsarcomere, one sees the Z band, the I band, the A band, the H zone, and the M band.

When seen in cross section, the thick filaments are regularly dispersed throughout the myofibril. Hexagonally arranged around them are the thin filaments. Therefore each thin filament is equally near to three thick filaments. The thick filaments have outwardly oriented heads that are directed toward the thin filaments and that run along the length of the thick filaments, with the exception of the midline. At rest, the heads of the thick filaments are tilted slightly toward their nearest Z disk. During contraction, these thick filaments bind neighboring thin filaments, pulling these toward the sarcomeric midline. The alternating thick and thin filaments develop the contractile force as these thick filaments pull the thin filaments past it. Therefore, when a sarcomere contracts, the Z disks are drawn toward each other, shortening the sarcomere. When all the sarcomeres in a muscle shorten, the muscle contracts.

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