|
A sarcomere is the basic unit of a cross striated muscle's myofibril. Sarcomeres are multi-protein complexes composed of three different filament systems. The thick filament system composed of the myosin protein, the thin filaments are assembled by actin monomers and the elastic filament system is composed of the giant protein titin (also called connectin).
A muscle cell, from a bicep, may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres.
The sarcomeres are what give skeletal and cardiac muscles their striated appearance. A sarcomere is defined as the segment between two neighbouring Z-lines (or Z-discs). In electron micrographs of cross striated muscle the Z-line is appears as a series of dark lines. Surrounding the Z-disc is the region of the I-band. Following the I-band is the A-band. Within the A-band is a paler region called the H-band. Finally, inside the H-band is a thin M-line (or M-band). A-bands and I-bands were named after anisotropic and isotropic, respectively; their properties under a polarizing microscope.
Actin filaments are the major component of the I-band and extend into the A-band. Myosin filaments extend throughout the A-band and are thought to overlap in the M-band. The giant protein titin (connectin) extends from the Z-disc of the sarcomere, where it binds to the thin filament system, to the M-band, where it is thought to interact with the thick filaments. The Titin protein (and its splice isoforms) is the biggest single protein found in nature. It provides binding sites for numerous proteins and is thought to play an important role as sarcomeric ruler and as blueprint for the assembly of the sarcomere.
Several proteins important for the stability of the sarcomeric structure are found in the Z-disc as well as in the M-band of the sarcomere. Actin filaments and Titin molecules are cross-linked in the Z-disc via the Z-disc protein alpha-Actinin. The M-band proteins Myomesin as well as M-protein crosslink the thick filament system (Myosins) and the M-band part of Titin (the elsatic filaments). The interaction between actin and myosin filaments in the A-band of the sarcomere is responsible for the muscle contraction (sliding filament model).
Upon muscle contraction, the A-bands maintain their length (1.6 micrometer in mammalian skeletal muscle) whereas the I-bands shorten.
The A-band, I-band and Z-line are the only components visible at the light-microscope level.
|
