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