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17 Cards in this Set
- Front
- Back
smooth muscles
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- cells are surrounded by a net reticular fibers
- various diameters - arranged in sheets - lack highly structured neuromuscular junctions - numerous bulbous swellings called varicosities - less developed SR - T tubules are absent |
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single-unit smooth muscle
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- in walls of hollow visceral organs
- except heart - have pacemaker cells allowing cells to contract rhythmically - cell connected by gap junction - cells have synchronous contraction |
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multi-unit smooth muscle
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- airways of lungs
- large arrector pili muscles - internal eye muscles that adjust pupil size - muscle fibers are structurally independent - cells work independently of each other - no gap junctions |
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ratio of thick and thin filaments
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- much lower in smooth than skeletal muscle
- thick filaments of smooth muscle have actin heads along entire length - not connected at Z line |
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smooth muscle cytoskeleton
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- bundles of thick and thin filaments criss-cross
- anchored to cell membrane at attachment junctions or plaques and within cell at dense bodies - intermediate filaments form mesh with PM - twist during contraction - thicker and shortened |
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lacks troponin
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- dense body acts like Z line of striated muscle
- contains alpha actin that anchors actin - contains vimentin and desmin IF - contains vinculin that binds integral membrane proteins |
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caveolae
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- numerous invaginations of plasma membrance
- vesicular - help bring in calcium needed for contraction - equivalent to T-tubule system in striated muscle - work with SR to sequester calcium when isn't needed for contraction - major source of Ca - regulate Ca = open and close |
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regulatory scheme 1
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- Ca2 may enter from the extracellular fluid through channels in plasmalemma or caveolae
- open when muscle is electrically stimulated or plasmalemma is depolarized by excess k |
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calcium is released from SR
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- receptor activation causes Ca to be released from SR
- activated receptor interacts with a G protein - G protein activates phospholipase C (PLC) - PLC hydrolyzes phosphatidyl inositol biphosphate - one product of hydrolysis is inositol 1,4,5-triphosphate (IP3) - IP3 binds to its receptor on surface of SR - opens Ca channels and Ca from SR enters the myoplasm |
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regulatory schemes
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- controlled by myosin binding site on actin and actin binding site on myosin
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smooth muscle contraction
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- Ca combines with calmodulin (CaM)
- Ca-CaM complex activates MLCK - MCLK phosphorylates the myosin head - hydrolyze ATP to ADP an Pi both of which bind to head - myosin forms a cross bridge with actin |
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regulatory scheme 2
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- Ca/calmodulin (CaCM) binds a protein = caldesmon
- caldesmon normally bound to thin filaments but removed when CaCM bind - tropomyosin changes it location in helical grooves of F-actin - exposes myosin binding site on actin - Ca depleted the CaCM dissociates, caldesmon released and reassociates with thin filament |
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caldesmon
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- prevents myosin binding to actin
- blocks myosin binding site on actin - replaces troponin as calcium dependent regulator of tropomyosin's location - Ca present = released - phosporylation by several kinases (MAO kinase) and dephosphorylation by phosphatases regulate caldesmon actin-binding activity |
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low Ca concentrations
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- caldesmon binds to troponin and actin
- reduce binding of myosin to actin - keep muscle in relaxed state |
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higher Ca concentrations
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- Ca-calmodulin complex binds to caldesmon releasing it from actin
- myosin can interact with actin - muscle can contract |
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long contractions
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- muscle has cross bridges that remain in a latch state even after Ca is removed
- ATP replaces ADP slowly - MLCK and MLC phosphates may be involved |
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regulation of contraction
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- neural regulation by autonomic nerves
- hormones like gastrin - stretch provokes contraction - local factors: low oxygen, histamine, excess CO2 |