The A bands within each muscle fiber are composed of thick filaments and the I bands contain thin filaments. Movement of cross bridges that extend from the thick to the thin filaments causes sliding of the filaments, and thus muscle tension and shortening.The activity of the cross bridges is regulated by the availability of Ca2+, which is increased by electrical stimulation of the muscle fiber. Electrical stimulation produces contraction of the muscle through the binding of Ca2+ to regulatory proteins within the thin filaments.
When muscle cells are viewed in the electron microscope, which can produce images at several thousand times the magnification possible in an ordinary light microscope, each cell is seen to be composed of many subunits known as myofibrils (fibrils = little fibers) (fig. 12.5). These myofibrils are approximately 1 micrometer (1 |im) in diameter and extend in parallel rows from one end of the muscle fiber to the other. The myofibrils are so densely packed that other organelles, such as mitochondria and intracellular membranes, are restricted to the narrow cytoplasmic spaces that remain between adjacent myofibrils.
With the electron microscope, it can be seen that the muscle fiber does not have striations that extend from one side of the fiber to the other. It is the myofibrils that are striated with dark A bands and light I bands (fig. 12.6). The striated appearance of the entire muscle fiber when seen with a light microscope is an illusion created by the alignment of the dark and light bands of the myofibrils from one side of the fiber to the other. Since the separate myofibrils are not clearly seen at low magnification, the dark and light bands appear to be continuous across the width of the fiber.
Each myofibril contains even smaller structures called myofilaments, or simply filaments. When a myofibril is observed at high magnification in longitudinal section (side view), the A bands are seen to contain thick filaments. These are about 110 angstroms thick (110 A, where 1 A = 10-10 m) and are stacked in register. It is these thick filaments that give the A band its dark appearance. The lighter I band, by contrast, contains thin filaments (from 50 to 60 A thick). The thick filaments are primarily composed of the protein myosin, and the thin filaments are primarily composed of the protein actin.
The I bands within a myofibril are the lighter areas that extend from the edge of one stack of thick filaments to the edge of the next stack of thick filaments. They are light in appearance because they contain only thin filaments. The thin filaments, however, do not end at the edges of the I bands. Instead, each thin filament extends partway into the A bands on each side (between the stack of thick filaments on each side of an I band). Since thick and thin filaments overlap at the edges of each A band, the edges of the A band are darker in appearance than the central region. These central lighter regions of the A bands are called the H bands (for helle, a German word meaning "bright"). The central H bands thus contain only thick filaments that are not overlapped by thin filaments.
In the center of each I band is a thin dark Z line. The arrangement of thick and thin filaments between a pair of Z lines forms a repeating pattern that serves as the basic subunit of striated muscle contraction. These subunits, from Z to Z, are known as sarcomeres (fig. 12.7a). A longitudinal section of a myofibril thus presents a side view of successive sarcomeres.
This side view is, in a sense, misleading; there are numerous sarcomeres within each myofibril that are out of the plane of the section (and out of the picture). A better appreciation of the three-dimensional structure of a myofibril can be obtained by viewing the myofibril in cross section. In this view, it can be seen that the Z lines are actually Z discs, and that the thin filaments that penetrate these Z discs surround the thick filaments
■ Figure 12.5 The components of a skeletal muscle fiber. A skeletal muscle fiber is composed of numerous myofibrils that contain myofilaments of actin and myosin. Overlapping of the myofilaments produces a striated appearance. Each skeletal muscle fiber is multinucleated.
in a hexagonal arrangement (fig. 12.7ft, right). If we concentrate on a single row of dark thick filaments in this cross section, the alternating pattern of thick and thin filaments seen in longitudinal section becomes apparent.
Figure 12.8 indicates two structures not shown in the previous sarcomere figures. The M lines are produced by protein filaments located at the center of the thick filaments (and thus the A band) in a sarcomere. These serve to anchor the thick filaments, helping them to stay together during a contraction. Also shown are filaments of titin, a type of elastic protein that runs through the thick filaments from the M lines to the Z discs. Because of its elastic properties, titin is believed to contribute to the elastic recoil of muscles that helps them to return to their resting length.
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