MusclePhysiology
PhysiologyUnit2
InPhysiologyToday
SkeletalMuscle
• Characteristics– Striated–Multinucleated–Voluntary
• Organization–Myofiber• Myofibril–Myofilament
Sarcomere
• Functionalunitofskeletalmuscle
• Composedof3filaments– Thickfilament
• Myosin
– Thinfilament• Actin• Troponin• Tropomyosin
– Elasticfilament• Titin
ThickandThinFilaments
SlidingFilamentTheoryofContraction
• Crossbridgesformbetweenthethickandthinfilaments
• Thinfilamentsslideacrossthethickfilaments– Thinfilamentswillmove
closertogether– DistancebetweenZlines
decreases– IbandandHbandsshorten
duringcontraction– Abandstaysthesame
MusclePhysiology
• 2distincteventshappenwhichleadtomusclecontraction
1.Electricalevents– ActionPotential– Receptoractivation– EPP– AP– Electricaleventstriggerthemechanicalevents
2.Mechanicalevents– Developingtensioninthemuscle– IncreaseCa2+ levels– contractileproteinsmoving– musclefibershortens
StimulustoContraction• Stimulus
– Receptoractivation– Skeletalmuscle:AChbindingtoN-Achr– ResultsinanEPP
• Latentperiod– Excitation-Contractioncoupling
• Contractionperiod– Cross-BridgeCycling– Generatestensioninthemuscle
• Relaxationperiod– Stimulusendsorcellfatigues– Musclereturnstoitsrestingstate
MuscleTwitch
Stimulus
• Somaticmotorneuronsinnervateskeletalmuscle• Largestdiameterneurons• Myelinated• HighvelocityAP
• Uponreachingmuscle,axondividesintomanybranches
• Eachbranchformingasinglejunctionwithamusclefiber=motorunit
EPPtoAP
ActionPotentialtoContraction• APlasts1-2ms• Completedbeforeany
mechanicalactivitybegins• Mechanicalactivity
(contraction)maylast>100ms
• Electricalactivity(actionpotential)doesnotactoncontractileproteins
• Producesastateofincreasedcytolsolic[Ca2+]– Resting[Ca2+]=0.1mMol/L– AfterAP[Ca2+]=1mMol/L
LatentPeriod
• Excitation-ContractionCoupling• SequenceofeventsfromthegenerationofanAPacrossthesarcolemmatoCa2+ releaseinsideofthemyofiber
• Sarcolemmaisanexcitablemembrane– GeneratinganAP(fromEPP)– PropagatinganAP– Similarmechanismsasneurons
ContractionPeriod
• IncreaseinintracellularCa2+ levelstriggerthemechanicalevents
• Ca2+ activatesCross-BridgeCycling
SarcoplasmicReticulum• LateralsacsstoreCa2+
• T-tubulehasDHPreceptors– DHPreceptorsarenormally
voltagegatedCa2+channels– Inskeletalmusclet-tubules,
actsasavoltagesensor
• SRhasryanodinereceptors– IntracellularCa2+channels
– WhenCa2+channelsopen,Ca2+ movesintocytoplasm
CalciumRelease
• DHPreceptorstriggercalciumrelease
• Ryanodinecalciumchannelsopen
• InfluxofcalciumfromSRintocytoplasm
• Calciuminfluxtriggerscrossbridgecycling
ActivationbyCa2+• Tropomyosincoversthe
myosinbindingsitesonactin• Troponinholdstropomyosin
inplace- has3sub-units1. TroponinI
• inhibitory2. TroponinT
• Tropomyosinbinding3. TroponinC
• Calciumbinding
• IncreaseinintracellularCa2+levelscauseTroponinCtobindtoCa2+ whichexposesbindingsitesonactin
Troponin
CrossBridgeCycling
1. Attachmentofthemyosincross-bridgetoactinofathinfilament
2. Movementofthecross-bridge,pullingonthethinfilament– Eachcross-bridgemovesindependentlyofallothercross-
bridges– Asynchronouspullingaction
3. Detachmentofcross-bridgefromthethinfilament4. Energizingthecross-bridgesoitcanagainattachto
athinfilamentandrepeatthecycle
Cross-BridgeCycling
ATPinMuscleMetabolismUsesofATPinMuscle
Contraction
• Activationofmyosin– High-energymyosin
• Releaseofmyosinheadfromactinmolecule
• ActivetransportofCa2+intoSRfromthesarcoplasm
MuscleContractionRequiresALotofATP!
• NoATP“storage”• 3pathwaysforregeneration– Phosphagensystem– Glycolysis– Aerobicrespiration
EnergyATP+H20 à ADP+pi +H+ +
SourcesofATP
1.Phosphagen System2.Glycolysis3.Oxidativephosphorylation
PhosphagenSystem• Creatine
– Naturalproducednythebody– Madefromaminoacids
• CreatinePhosphate– Astoreofhighenergy
phosphate
• CreatineKinase– Transfersphosphategroup
fromCPtoADP– Presentat3xhigher
concentrationinskeletalmuscle CrP+ADP+H+---->Cr+ATP
PhosphagenSystem
• AdenylateKinase– AwaytoquicklymakeATP– ADP+ADP---- >ATP+AMP
AerobicRespiration
• PrimarysourceofATPproductionformuscleduringrestorlightexercise
• Fuelutilizationbyskeletalmuscle– fattyacids– muscleglycogen– bloodborneglucose
AnaerobicMechanisms• Oxygenconsumption– ExercisingmusclecanconsumemoreATPthancanbeproducedbyaerobicrespiration
– Musclecellswillutilizeavailableglucoseandglycogenreserves
– Glycolysis willthenproduceATPtokeepupwiththedemandoftheactivemuscle• Lacticacidaccumulates
– OxygenDebt• Theamountofoxygenconsumedtogetthemusclecellsandplasmabacktonormalconditions– Glucoselevels– Glycogenreserves– Convertinglacticacidbacktopyruvic acid
SmoothMuscle• singlecells,nostriations• circularlayer
arrangement• nosarcomeresor
troponin• actin:myosinratio=13:1• utilizesCa2+/calmodulin
mechanism• gradeddepolarizations• singleunitvsmulti-unit• autonomicinnervation
SmoothMuscle