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The basic unit of skeletal and cardiac muscle is the sarcomere. The sarcomere consists of thin filaments and thick filaments. The thin filaments themselves consist of globular proteins called actin that join together to form long polymer chains. Two of these polymer chains intertwine in a helical fashion to form the thin filament. The thin filament also contains two other proteins called troponin and tropomyosin, which are involved in muscle contraction. The thick filament consists of a protein called myosin. Many of these myosin twist around one another to form the thick filament. At both ends of the thick filament are extensions called myosin heads, which are involved in binding to the thin filaments to cause the muscle contraction. The H-zone is the region that only contains the thick filaments, the I-band only contains the thin filaments, the A-band contains the thick-filaments in their entirety, which includes a small portion of the thin filaments as well. The Z-lines are the boundary lines for the sarcomere. When the muscle contracts, the distance between the two Z-lines decreases, the I-band and H-zone also decrease in length but the A-band remains unchanged because the thick filament does not change in size.
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