Lecture on Myofibrils Structure

Introduction

• Focus on the structure of myofibrils within muscle cells
• Muscle cells are covered by the sarcolemma
• Myofibrils are covered by the SR (sarcoplasmic reticulum) and interact with T tubules

Sarcomeres

• Myofibrils are composed of repeating units called sarcomeres
• Sarcomeres: Functional unit of contraction
  • Run from Z disc to Z disc
  • Contain M line in the middle to secure filaments
• Filaments within Sarcomeres:
  • Thin Filaments: Made of actin, appear as 'beads'
    • Run from Z disc towards M line
    • Areas with only thin filaments are called I bands
  • Thick Filaments: Made of myosin
    • Entire length called A band
    • H zone: Only thick filaments, appears darker
    • Zone of overlap: Both thick and thin filaments are present

Structural Proteins

• Types:
  • Titan: Large, connects thick filaments to Z disc; provides elasticity and extensibility
  • Myomesin: Forms M line, secures thick filaments
  • Dystrophin: Connects myofibrils to sarcolemma, transmits tension
  • Nebulin: Runs along thin filaments, regulates actin length
  • Alpha Actin: Secures thin filaments to the Z disc

Contractile Proteins

• Responsible for muscle contraction
• Myosin:
  • Thick filament, resembles a double-headed golf club
  • Contains ATPase for ATP hydrolysis
  • ATP hydrolysis provides energy for myosin to swivel, pulling thin filaments
• Actin:
  • Part of thin filament
  • Contains myosin binding sites

Regulatory Proteins

• Control interaction between myosin and actin
• Tropomyosin: Covers myosin binding sites on actin
• Troponin: Controls tropomyosin, requires calcium to change shape and expose binding sites

Muscle Contraction Process

1. Calcium is released from the SR
2. Calcium binds to troponin, changing its shape
3. Troponin moves tropomyosin, exposing myosin binding sites
4. Myosin binds to actin after ATP is hydrolyzed, and energy is stored
5. Myosin head swivels, pulling actin towards the M line, shortening the sarcomere

Summary

• Understanding the structure of myofibrils and the role of different proteins is crucial for comprehending muscle contraction
• The interaction of structural, contractile, and regulatory proteins allows for efficient muscle function