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Thin Filaments in Skeletal Muscle Fibers • Definition & Function

From the Z discs, the thin filaments extend inward toward the center of a sarcomere, where they partially overlap with the thick filaments.

A sacromere with thin filaments magnified

Approximately 300-400 G actin (= globular actin) proteins make up most of a thin filament. The image descriptionG actins attach end to end (= polymerize) to form two twisted strands. Together, the two strands are referred to as image descriptionF actin (= fibrous actin).

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Large image descriptionnebulin proteins span the length of the F actins and help direct their assembly. The nebulins seem to function as “measuring sticks” that make sure the F-actins extend to the proper length.

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After the F actins are constructed, capping proteins prevent them from depolymerizing or unraveling. The medial end of an F actin is presumably capped by image descriptiontropomodulin and the lateral end by image descriptionCap Z. The Cap Z proteins are attached to the image descriptionZ disc by image descriptionactinin proteins.

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Each of the G actins has a binding site for a myosin cross-bridge or head. When a muscle fiber is at rest, the myosin image descriptionbinding sites are covered by a series of image descriptiontropomyosin proteins.

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A troponin molecule is associated with each image descriptiontropomyosin. When bound to calcium, the troponins move the tropomyosins so contraction can take place.

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