Skeletal Muscle Histology

Muscle fiber structure

Long, cylindrical, multinucleated cell containing usual intracellular organelles but in a unique arrangement

1. Sarcolemma - plasma membrane of muscle cell
   1. Attached to collagen fibers of tendons
2. Sarcoplasm - cytoplasm of muscle cell including mitochondria and glycogen granules
3. Sarcoplasmic reticulum - modified ER of muscle cell
   1. Longitudinal system of membranous tubules and cisternae surrounding myofibrils
   2. Functions in calcium storage (Ca⁺² release channels) and muscle metabolism
4. T-tubule system - infoldings of sarcolemma at A-I junction
   1. Produces extension of extracellular space into fiber, encircle myofibrils and filled with ECF
   2. Provides for rapid conduction of nerve impulses into and throughout the fiber
5. Myofibrils - long, cylindrical contractile fibril composed of filaments (striations)
   1. Attached at ends of sarcolemma and runs the length of the cell
   2. Sarcomere - repeating functional unit of myofibril, 10,000 sarcomeres/myofibril

Myofilament structure

• Myosin - contractile muscle protein
  • Helical rod (light meromyosin), points toward M-line
  • Hinge capable of swiveling
  • Globular head (heavy meromyosin) can form crossbridges with actin filaments, projects outward
• Actin - contractile muscle protein
  • F-Actin in the form of two twisted strands
    • Composed of G-actin monomers
    • Each monomer has a myosin binding site
    • Central strand of nebulin holds monomers in place
  • Tropomyosin strands encircle the F-actin strands covering myosin binding sites
  • Troponin molecules are scattered at regular intervals along the tropomyosin
    • Contains three subunits, binding to tropomyosin, G-actin, and calcium

Myofibril structure

• Z-line - forms boundaries of sarcomere
  • Perforated disc holds thin filaments in place
  • Desmin intermediate filaments connect adjacent myofibrils
• I-band - composed of thin actin filaments only
• A-band - composed of thick myosin filaments including overlapping actin
  • Elastic filament -  composed of titin, anchors thick filaments to Z-lines, acts like spring and is responsible for extensibility and elasticity
• H-zone - in center of A-band, contains only thick filaments
• M-line - in center of H-zone, cross-connections between adjacent thick filaments

Relaxed condition:  (figure in class)

Contracted condition:  A-band remains constant length, I-band shortens.  What happened?