Lecture Notes: Muscle Contraction and Crossbridge Cycling

Introduction

• Organize a sequence of events to understand complex processes like muscle contraction.
• Detail of the process is sequential and follows a strict order in a healthy individual.
• Key focus: Excitation-contraction coupling, calcium's role, ATP's role in muscle cells.

Excitation-Contraction Coupling

• Motor Endplates & Nicotinic Acetylcholine Receptors:
  • Sodium influx initiates the process.
• Role of Calcium:
  • Released from the sarcoplasmic reticulum (SR).
  • Binds to Troponin C, which is essential for muscle contraction.

Muscle Cell ATP Production

• Mitochondria: Primary site of ATP production.
• Glycolysis: Occurs in the cytoplasm; another ATP production method.
• Creatine Phosphate System: Provides a quick ATP source by donating a phosphate to ADP.
• Aerobic vs Anaerobic Respiration:
  • More ATP with oxygen (aerobic).
  • Anaerobic glycolysis for low oxygen environments, used by fast-twitch fibers.
  • Slow-twitch fibers are more efficient with oxygen due to good blood supply.

Crossbridge Cycling and Sarcomere Contraction

• Initial Conditions:
  • Calcium binds to Troponin C, revealing myosin binding sites on actin.
• Key Proteins:
  • Actin: Composed of G-actin monomers forming F-actin filaments.
  • Troponin Complex: Includes Troponin C (binds calcium), Troponin T (binds to tropomyosin), and Troponin I (inhibitory).
  • Tropomyosin: Blocks myosin binding sites on actin when muscle is relaxed.

Myosin-Actin Interaction

• Myosin Structure:
  • Comprised of long tails and heads with ATP binding sites.
  • Myosin heads bind to actin forming crossbridges.
  • ATP binding causes myosin to release actin.
• ATP Role:
  • ATP is needed for myosin to release actin, not for the binding.
  • Hydrolysis of ATP to ADP + PI provides energy for the power stroke (movement of myosin heads causing contraction).
  • Fast-twitch vs slow-twitch differentiation based on ATP hydrolysis speed.

Sarcomere Structure

• Z-lines: Define boundaries of a sarcomere.
• Structural Proteins:
  • Titin: Stabilizes myosin, prevents wobbling.
  • Nebulin: Stabilizes and guides actin.
  • Actinin: Associates with nebulin and part of Z-line.
• Bands & Zones:
  • I-band: Actin only; shortens during contraction.
  • A-band: Actin and myosin; length remains constant.
  • H-zone: Myosin only; shortens during contraction.

Termination of Contraction

• Calcium Removal:
  • Calcium must be returned to the SR via SERCA pumps (uses ATP).
  • Tropomyosin covers binding sites once calcium is removed, stopping interaction.
• Repolarization:
  • Sarcolemma repolarizes, restoring ion gradients.

Conclusion

• Understanding the precise sequence and role of different molecules in muscle contraction is crucial.
• For further study, focus on the details of molecular interactions and ion management in muscle cells.