Muscle Structure and Contraction

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Muscle Structure and Contraction
2011-01-03 13:16:49

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  1. Describe the structure of a muscle
    • Muscles made up of muscle fibers
    • Muscle fiber surrounded by endomysium
    • Fibers arranged in fascicles (bundles) surrounded by perimysium
    • Entire muscle surrounded by epimysium
  2. What is endomysium
    • Connective tissue which ensheaths individual muscle fibers
    • Contains capillaries, nerves, lymphatics
    • Overlies the sarcolemma (muscle cell membrane)
  3. What is the perimysium?
    Connective tissue that groups muscle fibers together into bundles (fascicles).
  4. What is epimysium?
    • Connective tissue which surrounds entire muscle
    • Continuous with sndomysium and perimysium
    • also continuous with tendons
    • Prevents friction between muscles and between muscle and bone
  5. Discuss structure of a muscle fiber (muscle cell)
    • Sarcolemma: cell membrane
    • Sarcoplasm: intracellular fluid, rich in glycogen, glycolytic enzymes, creatine phosphate, and mitochondrion
    • Myofibrils: suspended in sarcoplasm
    • Actin and myosin filaments: make up myofibrils
    • Sarcoplasmic reticulum: flattened vesicles surround myofibrils; sequester calcium
    • Transverse tubules: invaginations of cell membrane; carry AP to inside of the muscle fiber
  6. Describe the structure of myosin molecule
    • Molecules made of 6 polypeptides – 2 heavy chains and 4 light chains
    • Heavy chains form a double helix tail
    • Heavy chains fold bilaterally at one end into a pair of heads
    • Light chains part of each head
    • Molecules organized into myosin filaments
  7. What is a sarcomere?
    • Unit of contraction in a muscle fiber
    • Made of light bands (I) and dark bands (A)
    • Dense bands in middle of light bands (Z-disc) separate sarcomeres
  8. Describe the role of titin in muscle contraction
    • Titin is very springy protein
    • Runs through myosin bundles
    • Holds myosin and actin filaments in place during contraction
  9. Discuss components of a sarcomere
    • Thick filaments: myosin, titin, C-protein
    • Thin filaments: actin, troponin, tropomyosin
    • Z-disk: a-actinin, desmin
    • M-line: creatine kinase, M-protein, myomesin
  10. State function of C-protein (Clamp Proteins)
    Bind to myosin tail; maintains myosin in bundles
  11. Discuss structure of the actin filament
    • Made up of 3 molecules
    • Double-stranded F-actin:
    • - made of polymerized G-actin
    • - each G-actin contains one active site (ADP)
    • Tropomyosin:
    • - wraps around F-actin
    • - covers active site in the resting state
    • Troponin:
    • - attached to side of tropomyosin
    • - I subunit: binds actin
    • - T subunit: binds tropomyosin
    • - C subunit: binds calcium
    • - attaches tropomyosin to actin
  12. Discuss function of creatine kinase and clinical significance
    • Phosphorylates creatine to make creatine phosphate
    • Creatine phosphate is energy reservoir for rapid regeneration of ATP in anaerobic respiration
    • Clinical significance:
    • - High levels indicate rhabdomyoysis / MI / renal failure (or side effect of statin drugs)
    • - Low levels indicate RA and alcoholic liver disease
  13. State the structure and function of Z-lines
    • Structure: made of a-actinin
    • Function: anchors actin filaments
  14. Briefly discuss the sliding-filament theory of contraction
    • Relaxed state: actin filaments barely overlap each other
    • Contracted state: myosin filaments pull actin inwards; actin pulls Z-discs up against ends of myosin filaments
  15. Deeply discuss sliding filament theory of contraction
    • 1.
    • - myosin head bind to ATP
    • - intrinsic ATPase activity
    • - ADP + P is stored in head
    • - myosin head perpendicular but not attached to actin filament
    • 2.
    • - calcium binds to troponin-tropomyosin complex
    • - uncovers active site
    • - myosin head binds with active site
    • 3.
    • - head simultaneously tilts towards arm (power stroke)
    • - using stored energy
    • 4.
    • - After tilting, ADP + P is released
    • - new ATP molecule binds
    • - cocks head back to perpendicular position
    • 5.
    • - new power stroke