Muscle Structure and Function: Myofibril and Sarcomere

Recap from Part 1

• Covered skeletal muscle anatomy
  • Muscle belly covered by epimysium
  • Fascicles covered by perimysium
  • Muscle fibers (cells) covered by endomysium
  • Sarcoplasmic reticulum for calcium storage
  • Striated muscle fibers with sarcomeres
  • Multinucleated and cylindrical muscle fibers

Myofibril Overview

• Composed of repeating units called sarcomeres
• Sarcomeres contain thick and thin filaments
• Functional and structural unit of muscle fibers

Detailed Structure of Sarcomere

Zones and Bands

• Z disk: Zigzag protein marking boundaries of sarcomere
  • Made of alpha-actinin
• A band: Dark band (anisotropic), length of thick filament
• I band: Light band (isotropic), between thick filaments of adjacent sarcomeres
• H zone: Distance between thin filaments within a sarcomere
• M line: Middle of sarcomere, contains myomesin and C proteins
• Titan (Blue Protein) connects thick filaments to Z disk and M line

Filaments

• Thick Filament
  • Made of myosin
  • Myosin has tail, neck, and head regions
  • Myosin heads bind to actin and have ATPase activity
  • Neck region contains light chains: Regulatory Light Chain and Essential Light Chain
    • Regulatory can undergo phosphorylation
    • Essential stabilizes the neck and head
• Thin Filament
  • Anchored to Z disk by nebulin
  • Composed of actin
    • G actin (globular) polymerizes to form F actin (filamentous)
    • F actin forms a supramolecular helix
  • Tropomyosin: Blocks myosin binding sites on actin during rest
  • Troponin: Has three subunits
    • Troponin C: Binds calcium
    • Troponin T: Binds tropomyosin
    • Troponin I: Binds actin (inhibitory)

Other Important Proteins

• Dystrophin: Links actin to sarcolemma, connecting to extracellular matrix
  • Mutations lead to muscular dystrophy
    • Duchenne Muscular Dystrophy: No dystrophin produced
    • Becker Muscular Dystrophy: Misfolded dystrophin produced