Muscular System: The Sarcomere

SARCOMERE OVERVIEW

  • The sarcomere is the functional unit of skeletal muscle tissue, specifically involved in muscle contraction and movement.
  • Many sarcomeres align end-to-end within myofibrils, the protein structures found inside muscle cells.
  • Thick (myosin) and thin (actin) myofilaments slide past each other to facilitate muscle contraction and movement.

STRUCTURAL CHANGES IN A SARCOMERE

  • Enlargement of a Sarcomere:
    • H Band: Area of the sarcomere with only thick filaments.
    • M Line: Middle of the sarcomere where thick filaments attach.
    • Z-Disk: Boundary of sarcomeres where actin filaments are anchored.
    • CapZ: Protein that stabilizes the plus end of actin filaments.
    • Titin: A large elastic protein that helps assembly and limits stretching of the muscle.

KEY MUSCLE FIBER VOCABULARY

  • Fibers: Long, cylindrical muscle cells with multiple nuclei.
  • Sarcolemma: The plasma membrane surrounding a muscle fiber.
  • Sarcoplasm: The cytoplasm of muscle fibers, containing organelles and various substances.
  • Myofibrils: Dense rod-like structures within muscle fibers containing sarcomeres and their myofilaments.
Sarcomere Composition
  • Thick Filaments (Myosin):
    • Comprised of myosin protein; responsible for contraction.
  • Thin Filaments (Actin):
    • Comprised of actin protein; interacts with myosin during contraction.
  • Sarcoplasmic Reticulum: A modified endoplasmic reticulum for calcium storage in muscle cells.
  • T-tubules: Invaginations of the sarcolemma that help transmit electrical signals into the muscle cell interior.
  • Triad: Structural arrangement of a T-tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.

MYOFIBRIL STRUCTURE

  • Muscle cells consist mainly of myofibrils, which occupy about 80% of the cell volume.
    • Each myofibril contains thousands of sarcomeres lined end to end.
    • The organization of thick and thin filaments gives striations to skeletal muscle due to their alternating arrangement.

MYOFILAMENTS AND THEIR ARRANGEMENT

  • Myofilament Arrangement:
    • Sarcomeres are arranged in a hexagonal pattern: each thick filament is surrounded by six thin filaments.
    • Actin (Thin Filaments): Provide the necessary framework for muscle contractions.
    • Myosin (Thick Filaments): The motor protein that interacts with actin during contraction.

REGULATORY PROTEINS OF MYOFILAMENTS

  • These proteins control when actin and myosin can interact, acting as regulators:
    • Tropomyosin:
    • Spiral-shaped protein that blocks binding sites on actin, preventing interaction with myosin.
    • Troponin:
    • Changes shape upon binding with calcium, leading to the uncovering of binding sites on actin.
    • Titan:
    • Functions to resist excessive stretching within the sarcomere.
    • Dystrophin:
    • Connects thin filaments of actin to the sarcolemma, helping maintain muscle integrity during contraction.