Lecture Notes: Sliding Filament Theory of Skeletal Muscle Contraction

Key Terms and Prefixes

• Sarco: Greek for muscle
  • Examples:
    • Sarcoplasmic Reticulum: Endoplasmic reticulum specific to skeletal muscle
    • Sarcolemma: Plasma membrane of skeletal muscle
    • Sarcoplasm: Cytoplasm of skeletal muscle
• Myo: Latin for muscle
  • Examples:
    • Myocyte: Muscle cell
    • Myofibril: Composed of myocytes
    • Myofilaments: Contractile proteins (actin and myosin)

Anatomy of Skeletal Muscle

• Whole Muscle: Comprised of bundles known as fascicles
• Fascicles: Made up of multiple muscle fibers (muscle cells)
• Muscle Fiber (Myocyte):
  • Cylindrical, striated, and multinucleated
  • Composed of myofibrils, which are responsible for the striations
• Myofibrils: Made up of myofilaments
  • Actin: Thin contractile protein
  • Myosin: Thick contractile protein

Muscle Contraction Process

• Sarcomere: Smallest unit of contraction, defined from one Z-disc to another
• Bands:
  • A Band: Dark band with overlapping actin and myosin
  • I Band: Lighter area where actin doesn't overlap with myosin
  • M Line: Center of the sarcomere
  • Z Disc: Boundary of sarcomere

Neuromuscular Junction

• Involves the connection between a lower motor neuron and a muscle fiber
• Acetylcholine: Neurotransmitter that triggers muscle contraction

Action Potential and Depolarization

• T Tubules: Allow depolarization to penetrate deep into muscle fibers
• Sarcoplasmic Reticulum: Releases calcium when depolarized

Sliding Filament Theory

• Calcium's Role:
  • Calcium binds to troponin, which releases tropomyosin from actin
  • Myosin heads can then bind to actin
• ATP's Role:
  • Myosin head requires ATP to bind and cock into position
  • ATP dissociates into ADP and phosphate, allowing the head to bind to actin
  • Power stroke occurs when ADP and phosphate are released
  • New ATP binds to release myosin head from actin
• This cycle repeats, causing muscle contraction by sliding actin over myosin

Summary of Muscle Shortening

• Contraction: Myosin heads pull actin filaments towards the M line
• Sarcomere shortens, leading to the shortening of the entire muscle fiber and muscle