contraction
? Mostacceptedtheoryatpresent"
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Slidingfilamenttheory"
? Proposesthatamuscleshortensorlengthens
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becausethemyofibrillarfilamentsslidepasteachotherwithoutactuallychangingtheirlength.
? Themolecularmotortodrivethisshortening
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processistheactionoftheoftheMyosincrossbridges,whichcyclicallybindorattach,rotateand
detachfromtheactinfilamentswithenergy
providedbyATPhydrolysis.
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Sliding Filament Theory
I Sliding Filament Theory
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CalciumActin Filament
Myosin
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ATP
Filament
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ADP+ P
Slidingfilamenttheory
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Whenmuscleisintherelaxedstate? Caionconc.inthecytosolislow.
? Atthispointactinandmyosinfilamentsliealong
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eachotherinthesarcomere.? TheMyosinheadatthispointisinahighenergy
condition"cockedup"withADPandinorganic
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phosphateboundtoit? ActivesitesontheGactinmoleculesarecovered
bythetroponintropomyosincomplex.
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II Sliding Filament Theory
Actin Filament
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MyosinADP
Filament
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+ P
Myosin head
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cocked upSlidingfilamenttheory
? ActionpotentialsintheTtubulecausetherelease
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ofCaionsfromSRintothemusclecytosol? CabindsTroponinC.Aconformationalchangeis
inducedintheTroponinweakensthethebond
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betweenitandActin.? Thisallowstropomyosintomovelaterallyand
exposetheactivesitesonGActin.
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? Thecockedupmyosinmoleculerapidlybindstothe
Actin:thislinkisa"crossbridge"
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III Sliding Filament TheoryCalcium binds to Actin
ADP
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+ P
IV Sliding Filament Theory
Calcium opens binding sites
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V. Sliding Filament Theory
Cross bridge forms,
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Connecting Myosin to ActinSliding Filament Theory
? Myosin head then undergoes a
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conformational change causing a "rachetaction" and pulls the actin filament to the
centre of the sarcomere.
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? ADP and Pi are released by this process
? This is called the "power stroke" which
causes the sliding action
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VI. Sliding Filament Theory
Conformational Changes
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Myosin head: Actin Moves~"power stroke"
Release of ADP+P
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Slidingfilamenttheory? AnATPbindstotheActomyosincomplex
? Thiscausestheaffinityofmyosinforactin
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todecrease? Themyosinheadchangesitspositionto
closearoundtheATPandhydrolyzeit.
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? ThischangeinconformationoftheMyosin
headreleasesthemyosinfromtheactin.
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VII. Sliding Filament TheoryNew ATP binds
IX. Sliding Filament Theory
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ATP hydrolysedADP
Myosin returns to cocked
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+ P
up position
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Fresh cycle startsSlidingfilamenttheory
? CyclingcontinuesuntilcytosolicCalevels
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remainhigh
? OneCaionreleasesoneTroponinwhich
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covers7activesites.? Allmyosinmoleculesdonotmove
simultaneouslybutsequentiallylikeoarsona
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boatandcausethemyosinslidealongtheActinfilament
Muscle relaxed
Calcium pumped out of cytosol: active sites covered
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ADP
+ P
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Muscleenergetics?
EnergycurrencyformusclecontractionisATP
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?
HydrolysisofATPbyMyosinATPaseenergizescross
bridgespriortocycling.
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?
BindingofATPtomyosindissociatescrossbridgesbound
toactinallowingthebridgestorepeattheircycleof
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activity.?
HydrolysisofATPbyCa?ATPaseprovidesenergyfor
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activetransportofCaintosarcoplasmicreticulumthusendingthecontractionandallowingthemusclefiberto
relax.
Rigor mortis
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? A condition of the muscles seen afterdeath
? ATP not available
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? Cycle stops at the point of formation of
actomyosin complexes
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? "permanent actomyosin" complexesformed
? Leads to a state of rigor
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? Ends with denaturation of protiens