Muscular Tissue

• Muscular tissue


  Produces body movements<|>Stabilizes body positions<|>Stores and moves substances within the body<|>Generates heat
• Properties of muscular tissue

  • Electrical excitability
  • Contractility
  • Extensibility
  • Elasticity
• Three types of muscular tissue

  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle

  
• Sarcoplasm


  Cytoplasm of muscle cells and fibers, contains glycogen and myoglobin

  
• Sarcolemma


  Cell membrane complex
• Sarcoplasmic reticulum


  Specialized endoplasmic reticulum that stores and sequesters calcium
• Sarcomere


  Contractile unit in a striated muscle fiber, extends from one Z disc to the next

  
• Organization of skeletal muscle

  1. Epimysium

  2. Perimysium

  3. Endomysium

  
• Components of skeletal muscle

  • Muscle (organ)
  • Fascicle
  • Muscle fiber (cell)
  • Myofibril
  • Filaments (myofilaments)

  
• Myosin filaments (thick filaments)


  • Bundled together with heads projecting outwards, have a bare zone
• Thin filaments


  • Made up of actin, tropomyosin, and troponin
• Sarcomere


  The functional unit of the myofibril
• Filaments (myofilaments)

  • Thick filaments (myosin)
  • Thin filaments (actin, tropomyosin, & troponin)
• Myosin filaments (thick filaments)


  • Bundled together with heads on the outer part
  • Tails are bundled together with globular heads projecting from the rod shaped tail
  • Have a bare zone (H Bond) where no myosin is projecting
• Thin filaments


  • Made up of three molecules
  • Actin filament consists of G-actin (globular) which form F-actin (filament)
  • Tropomyosin lies along the actin filament
  • Troponin is a tri-molecular complex associated with tropomyosin
• Parts of the sarcomere

  • A bands (dark, anisotropic, mixture of thick and thin filaments)
  • I bands (light, isotropic, thin filaments only)
  • Z disc (dark transverse line bisecting I band)
  • H zone (narrow zone in center of A band, contains thick but not thin filaments)
  • M line (middle of sarcomere, contains myomesin to hold thick filaments together)
• Muscle proteins

  • Contractile proteins (myosin, actin)
  • Regulatory proteins (troponin, tropomyosin)
  • Structural proteins (titin, α-actinin, myomesin, nebulin, dystrophin)
• Titin


  • Connects Z disc to M line, helps stabilize thick filament position
  • Spiral-like structure that can stretch and spring back unharmed, accounts for muscle elasticity
• Myomesin


  • Structural protein that forms M line, binds to titin and connects adjacent thick filaments
• Nebulin


  • Structural inelastic protein that wraps around thin filaments, anchors them to Z discs and regulates their length
• Dystrophin


  • Structural protein that links thin filaments to integral membrane proteins in sarcolemma, which are attached to proteins in connective tissue matrix
• ATPase stain used to differentiate slow oxidative, fast oxidative-glycolytic, and fast glycolytic muscle fibers
• Cardiac muscle


  • Most limited distribution, found lining heart and blood vessels
  • Striated, branching, each fiber has 1-2 nuclei at center
  • Presence of intercalated discs
  • Involuntary, not capable of cell division

  
• Cardiac muscle fiber organization

  • Same arrangement of actin and myosin as skeletal muscle
  • Intercalated discs are unique to cardiac muscle
  • Lacks epimysium, has larger T-tubules than skeletal muscle
  • Has diad structure of terminal cisternae and T-tubules

  
• Smooth muscle


  • Thickest in middle, tapers at ends
  • Single, centrally located nucleus
  • No sarcomere structure, lacks troponin but has caldesmon and calponin
  • Contains intermediate filaments like desmin and vimentin
  • No striations, little sarcoplasmic reticulum, has caveolae invaginations
  • Involuntary, greatest potential for regeneration

  
• Cell adaptations

  • Atrophy (decrease in cell size)
  • Hypertrophy (increase in cell size)
  • Hyperplasia (increase in cell number)
• Hyperplasia does not occur in cardiac muscle