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Z Disk

Thick fi|aments Thin filaments Z

Disk

Z Disk

Thick fi|aments Thin filaments Z

Disk

A band

I band

Sarcomere

FIGURE 2 Longitudinal view of the organization of contractile proteins into myofilaments and of myofilaments into a sarcomere. The proteins actin and myosin comprise the major portions of the thin and thick filaments, respectively. Thin filaments project outward from both sides of the Z disks. Thick filaments lie between and overlap with thin filaments from two adjacent Z disks. Regions of overlap appear darker when viewed microscopically; thus, this region is referred to as the anisotropic or A band. Regions that contain only thin filaments appear lighter and are referred to as isotropic or I bands. The structure from one Z disk to another is a sarcomere and is the basic contractile unit of striated muscle. G, globular; F, filamentous.

pearls that are twisted in an alpha helix to form F-actin (filamentous). Each myosin molecule comprises six monomers: two proteins, each with a globular head and a filamentous tail (the myosin heavy chain) and each with a molecular weight of approximately 200,000 Da, and four smaller, globular proteins (the myosin light chains), each with molecular weights of 17,000 to 24,000 Da. The light chains associate with the globular heads of the heavy chains by ionic bonds and Van der Waals forces. Large portions of the tail segments from many heavy chains interact with one another to form the backbone of the thick filament. Part of their tails and the globular heads protrude from the backbone to form structures called crossbridges.

Actin and myosin are found in all muscle cells. However, each protein monomer exists in multiple isoforms that are present in different proportions, depending on the particular type of muscle cell (e.g., striated or smooth, skeletal or cardiac, fast twitch or slow twitch) and on its stage of development (e.g., embryonic or adult). The functional consequences of some of the isoforms are discussed below.

The way in which the thick and thin filaments are arranged within the cells defines in large part the two main types of muscle: striated and smooth. In striated muscle, the thick and thin filaments are very ordered in their anatomic arrangement within the cell. Thin filaments extend in opposite directions from protein structures called Z disks. In relaxed muscle, the thin filaments from two opposing Z disks extend toward each other but do not touch or overlap. Bridging the gap between the thin filaments, and overlapping with them, are the thick filaments. This arrangement—Z disk, thin filament, thick filament, thin filament, Z disk—defines the functional unit called a sarcomere (see Fig. 2). In striated muscle, sarcomeres are arranged in transverse registry, thus accounting for the characteristic banding pattern or striations. As shown in Fig. 2, the sarcomere can be characterized in minute detail. This fine structure was discovered long before the mechanisms responsible for the shortening were elucidated, thus accounting for the descriptive terminology used to describe the sarcomere.

Sarcomeres not only have ordered structure when viewed in the longitudinal axis, but they also exhibit symmetry when viewed in cross section (Fig. 3). Each thick filament is surrounded by six thin filaments in a pattern that is repeated such that there are two thin filaments for every thick filament (each thin filament interacts with three thick filaments). Also, the cross-bridges are arranged around the thick filament, separated by angles of 60°, in such a way that they can associate with the actin monomers in the thin filaments. In smooth muscle, thick and thin filaments are present,

Thick filaments

FIGURE 3 Longitudinal and cross-sectional views of the thick and thin filaments. The cross section was made in a region of overlap of the filaments. The crossbridges extend from the thick filaments toward the thin filament. Each thick filament is surrounded by six thin filaments, and each thin filament interacts with crossbridges from three thick filaments.

but they have no such structured arrangement and, thus, give a homogeneous appearance under the microscope (see Chapter 8).

The specific way in which the proteins actin and myosin are arranged gives muscle cells the ability to shorten. When striated muscles contract, crossbridges from the thick filaments attach to specific regions on the actin molecules (Fig. 4). The crossbridge heads then change angles, causing the thick and the thin filaments to slide over one another. The crossbridges then release, and their angles assume the resting positions. They now are ready to attach to a different actin molecule, thus repeating the cycle until the stimulus to contract ceases. Because two opposing sets of thin filaments are associated with a single set of thick filaments, filament sliding results in movement of the Z disks toward one another without either the thick or the thin filaments changing length. Also, because the Z disks and the thin filaments are linked with other cytoskeletal elements, movement of the Z disks toward one another results in shortening of the muscle cell.

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