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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
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What is endomysium
- Connective tissue which ensheaths individual muscle fibers
- Contains capillaries, nerves, lymphatics
- Overlies the sarcolemma (muscle cell membrane)
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What is the perimysium?
Connective tissue that groups muscle fibers together into bundles (fascicles).
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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
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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
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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
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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
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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
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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
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State function of C-protein (Clamp Proteins)
Bind to myosin tail; maintains myosin in bundles
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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
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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
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State the structure and function of Z-lines
- Structure: made of a-actinin
- Function: anchors actin filaments
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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
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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
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