sarcomere: thick, thin filaments B

Overview of Skeletal Muscle Structure

  • Focus on microscopic anatomy of skeletal muscle, particularly myofibrils and sarcomeres.

  • Myofibrils: Contractile elements of skeletal muscle consisting of thick and thin filaments.

  • Importance of understanding these components for muscle contraction.

  • Reference material: Saladin, pages 397 to 402.

Myofibril Structure

  • Myofibrils are collections of proteinaceous fibers, specifically:

    • Thick Filaments: Appear thicker in structure.

    • Thin Filaments: Thinner than thick filaments.

  • Overlap between thick and thin filaments results in visible striations (dark staining) in skeletal muscle tissue.

  • Thick and thin filaments do not extend the full length of myofibrils or muscle cells.

Functional Units: Sarcomeres

  • Sarcomeres: The functional unit of each myofibril.

    • Defined as the segment between two Z-discs.

    • Z-Disc: Zigzag structure that anchors thin filaments.

    • M Line: Central anchoring point for thick filaments.

  • Z-discs and M lines ensure proper filament orientation for contraction.

Parts of the Sarcomere

  • The following regions were described:

    • A Band:

      • Contains the entire length of thick filaments.

      • Appears dark due to overlap with thin filaments.

    • I Band (I Zone):

      • Region on either side of the A band.

      • Contains only thin filaments and elastic filaments.

    • H Zone:

      • Partial region within the A band without thin filament overlap.

      • Named from "helis," meaning clear.

Detailed View of Thick and Thin Filaments

  • Thick Filaments: Composed primarily of the protein myosin.

    • Myosin structure includes a globular head and a long fibrous tail.

    • Approximately 250-300 myosin molecules per thick filament.

    • Arranged in a spiraling pattern around which thin filaments surround.

  • Thin Filaments: Composed of three proteins:

    • Actin:

      • Globular protein forms F-actin (filamentous actin) in a bead-like structure.

    • Tropomyosin:

      • Fibrous protein that blocks active sites on actin.

    • Troponin:

      • Calcium-binding protein that regulates tropomyosin.

      • Interaction with calcium alters tropomyosin's position, uncovering actin's active sites.

3D Structure of Filaments

  • Thin and thick filaments are oriented in three dimensions:

    • Each thick filament surrounded by six thin filaments.

    • Thick filaments appear as dark spots from a transverse section view.

Myofibril Composition Recap

  • Thick Filaments

  • Composed of myosin with distinct structural regions (Hinge region, globular heads).

  • Thin Filaments

  • Comprised of actin (G-actin and F-actin), tropomyosin, and troponin.

Connection to Muscle Contraction

  • Structural content lays foundation for understanding muscle contraction.

  • Next discussion: Sliding Filament Theory explaining the interaction in contraction.