Thin Filaments in Skeletal Muscle Fibers • Definition, Composition & Function

Author: Scott A. Sheffield MS

Last update:

From the Z discs inside a muscle fiber, 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).

1
2
3

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.

1
2

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.

Learn this topic from scratch or revise what you’ve already learned with these interactive exam-style anatomy quizzes.

1
2
3
4
5

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.

1
2
3

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

1
2