Lecture Notes: Muscle Fiber Contraction

Introduction to Muscle Fiber Contraction

• Discuss the microanatomy of skeletal muscle cells prior to contraction mechanisms.
• Skeletal muscle fibers are long, cylindrical cells with multiple nuclei.
• Special terms for components:
  • Sarcolemma: Muscle fiber plasma membrane.
  • Sarcoplasm: Muscle fiber cytoplasm.

Energy and Oxygen Storage

• Glycans: Involved in glycogen storage for energy.
• Myoglobin: Stores excess O2 for sustaining contraction force.

Modified Organelles in Skeletal Muscles

• Myofibrils:
  • Long protein cylinders filling most of the muscle cell cytoplasm.
  • Account for about 80% of muscle cell volume.
  • Provide striation appearance of muscle cells.
• Sarcoplasmic Reticulum:
  • Modified endoplasmic reticulum storing calcium.
• T Tubules:
  • Tubes of sarcolemma for transmitting electrical impulses.

Structure of Myofibrils and Sarcomere

• Myofibrils: Densely packed protein elements with thousands per cell.
• Sarcomeres:
  • Functional unit of muscle contraction.
  • Composed of thick (myosin) and thin (actin) myofilaments.
  • Striations form from repeating dark and light bands.

Sarcomere Anatomy

• Smallest contractile unit from Z-disk to Z-disk.
• Contains A bands (thick filaments) and I bands (thin filaments).
• H Zone: Space between thin filaments.
• M Line: Center of sarcomere.

Myofilaments

• Actin (Thin) Myofilaments:
  • Extend across I band and part of A band, anchored to Z disks.
• Myosin (Thick) Myofilaments:
  • Extend length of A band, connected at M line.
  • Myosin heads form cross-bridges with actin.

Molecular Composition

• Myosin heads use ATP to produce a ratcheting motion.
• Actin is made of G actin subunits that form F actin strands.

Regulatory Proteins

• Tropomyosin: Blocks active sites on actin during rest.
• Troponin: Binds calcium, shifts tropomyosin to expose active sites.

Supporting Proteins

• Titan: Elastic filament providing elasticity and stability.
• Dystrophin: Links thin filaments to the sarcolemma.
• Other proteins like nebulin and myomesin help with alignment.

Sarcoplasmic Reticulum and T Tubules

• Network surrounding myofibrils storing calcium.
• T Tubules increase surface area, facilitate electrical impulses.
• Triads help control calcium channel opening and muscle contraction.

Contraction Mechanism

• Sliding Filament Model:
  • Thin filaments slide past thick filaments; filaments overlap but don't change length.
  • Myosin heads bind to actin; cross-bridges form and break.
  • Z discs pulled closer, I bands shorten, H zones disappear.
  • Elastic protein Titan holds filaments in place.