Functional Anatomy of a Sarcomere

Functional Anatomy of a Sarcomere

Sarcomere Structure

  • Definition: A sarcomere is the region between two Z discs.
  • Role: It is the functional contractile unit of muscle tissue.
  • Composition:
    • Sarcomeres arrange end to end to form a myofibril.

Key Components of the Sarcomere

  • Z Disc:

    • Bisects the I band.
    • Anchors thin (actin) filaments and connects adjacent myofibrils.
    • Alpha-actinin: A protein that forms Z discs and helps anchor thin filaments.

  • A Band:

    • Contains thick (myosin) filaments.
    • The length of the A band corresponds to the thick filaments no matter the contraction length.

  • I Band:

    • Area where thick filaments are not found.
    • Contains only thin (actin) filaments.
    • Thin filaments extend from Z discs into the A band but are absent in the H zone.

  • H Zone:

    • A lighter stripe located in the middle of the A band.
    • Absence of thin (actin) filaments.

  • M Line:

    • Bisects the H zone and A band.
    • Composed of myomesin, which are fine protein strands that keep thick filaments aligned.

Filament Types

  • Thin Filaments (Actin):

    • Composed of actin monomers.
    • Extend from the Z discs into the A band.

  • Thick Filaments (Myosin):

    • Consist of myosin protein.
    • Run the width of the A band.
    • Structure:
    • Myosin has two heads and a long tail made of two twisted heavy polypeptide chains.

  • Elastic Filaments:

    • Composed of giant protein titin.
    • Run from Z disc to the myosin and extend to the M line.
    • Function:
    • Holds thick filaments in place.
    • Extensible during muscle stretching and recoils back to original length after contraction.

Myofilaments

  • Definition: Myofilaments are the fibers within the sarcomere.
  • Types:
    • Thick Filaments:
    • Myosin
    • Thin Filaments:
    • Actin
  • Relationships:
    • Desmin proteins hold adjacent thick filaments together, ensuring A bands line up perfectly during muscle contraction.