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Z disc

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noun, plural: Z discs

The region or line formed in the sarcomere into which the thin filaments are inserted


The sarcomere, the contractile unit of a myofibril, consists of a highly organized assembly of filaments. It is the smallest functional structures and molecules essential for muscle contraction. The two main proteins in a sarcomere are actin and myosin. The actin forms the thin filament whereas the myosin forms the thick filament. The thin and the thick filaments are arranged in a way that they overlap partially enabling them to slide past each other during contraction. The sarcomere is delimited by two Z discs (also called Z lines, Z bands, or intermediate discs). The Z in Z disc stands for the German Zwischenscheibe.

When viewed under the microscope, the Z disc appears as the darkly staining band that bisects the I (isotropic) band of a striated muscle (i.e. the region surrounding the Z disc and the zone of actin filaments not superimposed by myosin filaments). The Z disc is where the actin filaments attach to and thereby serves as the anchoring point of actin filaments in a sarcomere.


  • intermediate disk (disc)
  • Z line
  • Z band

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