Download Chapter 8 Skeletal Muscle: Structure and Function

ExercisePhysiology:TheoryandApplicationtoFitnessandPerformance
ByScottPowers&EdwardHowley
Chapter8SkeletalMuscle:StructureandFunction
SummaryCreatedbyDanHechler
ClassLectureExercisePhysiologyIDr.LaurieMilliken,UMassBoston,Fall2015
SkeletalMuscle
1.HumanBodycontainsover400skeletalmuscles
-40-50%oftotalbodyweight
-75%water
-20%protein
-5%salts,phosphates,ions,macronutrients
2.Epimysium
-Surroundsallthebundlestoformentiremuscles
3.Perimysium
-Srroundsseveralfibersandformsbundlescalledfasciculi
4.Endomysium
-Wrapseachfiber
5.Tendons:tissueconnectingmuscletoperiosteumofbone
6.Sacrolemma
-Musclecellmembrane
7.Satellitecells:
-Playroleinmusclegrowthandrepair(increasenumberofnuclei)
-Morenucleiallowforgreaterproteinsynthesis
-Importantforstrengthtrainingadaptations
1.Myofibrils
-Containcontractileproteins
• Actin(thinfilament)
• Myosin(thickfilament)
2.Sacromere
-IncludesZline,Mline,Hzone,Aband,Iband
3.Sacroplasmicreticulum
-Storagesitesforcalcium
NeuromuscularJunction
1.Junctionbetweenmotorneuronandmusclefiber
-Motorunit
• Motorneuronandallfibersitinnervates
2.MotorEndPlate
-Pocketformedaroundmotorneuronbysarcolemma
3.Neuromuscularcleft
-Shortgapbetweenneuronandmusclefiber
4.Acetylcholinereleasedfrommotorneuron
-Causesend-platepotential(EPP)
• Depolarizationofmusclefiber
• AcHhydrolysisbycholinesterase
SlidingFilamentModel
1.Muscleshorteningoccursduetomovementofactinfilamentovermyosinfilament
2.Formationofcrossbridges(PowerStroke)
3.ReductionindistancebetweenZlinesofthesarcomere
1.Tropomyosin
-Liesalongactininthegrooveformedbythedoublehelix
• Inhibitsactin-myosininteraction
2.Troponin
-Embeddedatregularintervalsalongactin
• InteractswithCa+2
• Movestropomyosinuncoveringactiveactinsites
EnergyForContraction
1.ATPrequiredformusclecontraction
• MyosinATpasebreaksdownATPfibercontract
• ATPàADP+Pi
• ATPaseislocatedonmyosinhead
• RelaxationaftermusclecontractionalsorequiredATP
• CalciumpumpsrequireATPtopumpcalciumbackintosarcoplasmicreticulum
2.Relaxation
• Ca+2activelypumpedinSacroplasmicReticulum
• Troponinallowstropomyosintointerferewithactin-myosininteraction
Excitation-ContractionCoupling
1.Excitation
• Actionpotentialinmotorneuroncausesreleaseofacetylcholineintosynapticcleft.
• Acetylcholinebindstoreceptorsonmotorendplatewhichleadstodepolarizationthat
isconducteddowntransversetubules,whichcausesreleaseofCa+2fromSR
2.Contraction
• Atrest,myosincross-bridgesinweakbindingstate
• Ca+2bindstotroponin,causesshiftintropomyosintouncoveractivesites,andcrossbridgeformsstrongbindingstate.
• Pireleasedfrommyosin,cross-bridgemovementoccurs
• ADPreleasedfrommyosin
• ATPattachestomyosin,breakscross-bridgeandformingweakbindingstate.ATPthen
bindstomyosin,brokendowntoADP+Pi,whichenergizesmyosin.
• ThiscontinuesaslongasCa+andATParepresent
CharacteristicsofMuscleFiberTypes
BiochemicalProperties
-OxidativeCapacity
• Numberofcapillaries,mitochondria,andamountofmyoglobin
-TypeofmyosinATPase
• SpeedofATPdegradation
-IsoformofLDH(LactateDehydrogenase)
ContractileProperties
-Maximalforceproduction
• Forceperunitofcross-sectionalarea
-Speedofcontraction(Vmax)
• MyosinATpaseactivity
-Musclefiberefficiency
WhatDeterminesFiberType
1.Geneticsdeterminewhichtypeofmotorneuronsinnervatesindividualmusclefibers.
2.Musclefibersbecomespecializedaccordingtotypeofneuronthatstimulatesthem
3.Enduranceandstrengthtraining,andmuscularinactivitymayresultinsmallchanges(less
than10%)in%ofFastTwitchandSlowTwitchFibers.
4.Endurancetrainingreduce%ofFTxfiberswhileincreasingthe%ofFTafibers.
5.Agingchangesin%ofFTandSTfibers
Slow-TwitchFibers:TypeI
1.Highaerobic(oxidative)capacityandfatigueresistance
2.Lowanaerobic(glycolytic)capacityandmotorunitstrength
• Slowcontractilespeed(110mstoreachpeaktension)andmyosinATPase
• LowSRdevelopment
• SlowerCa2+releaseanduptakebySR
• Largeandnumerousmitochondria
• Approx50%ofallfibers
• 10-180fiberspermotorneuron(hasasmallcellbody)
Fast-TwitchFibers:TypeII
1.HighCapacitytotransmitActionPotential
2.HighmyosinATPaseactivity
3.RapidCa2+releaseanduptakebySR
4.Highrateofcrossbridgeturnover
5.Capableofhighforcegeneration
6.Relyonanaerobicmetabolism
Fast-TwitchIIaFibers
1.Fastshorteningspeed
2.Moderateaerobic(oxidative)capacityandfatigueresistance
3.Highanaerobic(glycolytic)capacityandmotorunitstrength
4.Fastcontractilespeed(50mstoreachpeaktension)andmyosinATPase
5.300-800fiberspermotorneuron
• Motorneuronhaslargecellbody
• LargerfibersizecomparedtoSTfibers
6.HighSRdevelopment
Fast-TwitchIIxfibers
1.MostRAPIDshorteningvelocity
2.Lowaerobic(oxidative)capacityandfatigueresistance
3.Highanaerobic(glycolytic)capacityandmotorunitstrength
4.Fastcontractilespeed(50mstoreachpeaktension)andmyosinATPase
5.300-800fiberspermotorneuron
• MotorNeuronhasalargecellbody
• LargerfibersizecomparedtoSTandFTIIa
6.HighSRdevelopment
FiberTypeAndPerformance
MuscleAtrophyDueToInactivity
1.Lossofmusclemassandstrength
-Immobilizedduetoinjury
2.Initialatrophy(2days)
-Duetodecreasedproteinsynthesis
3.Furtheratrophy
-Duetoincreasemuscleproteinbreakdown
4.Atrophyisnotpermanent
-Canbereversedviastrengthtraining
TypesofMuscleAction
1.Isometric
-Muscleexertsforcewithoutchanginglength
-Pullingagainstimmovableobject
-Posturalmuscles
2.Isotonic(dynamic)
-Concentric
• Muscleshortensduringforceproduction
-Eccentric
• Muscleproducesforcebutlengthincreases
FactorsThatRegulateProductionOfForceDuringMuscleContraction
1.Typesandnumberofmotorunitsrecruited
• Moremotorunits=GreaterForce:FastMotorunits=GreaterForce
2.Initialmusclelength
• “Ideal”lengthforforcegeneration
• Increasedcross-bridgeformation
3.Natureofneuralstimulationofmotorunits
-Frequencyofstimulation(SimpleTwitch,Summation,Tetanus)
WhatCausesMuscleCramps?
1.ElectrolyteDepletion
2.AlteredNeuromuscularControlTheory
• AbnormalactivityofbothmusclespindleandGolgitendonorgan
• IncreasedmusclespindleactivityanddecreasedGolgitendonactivity
WhatCausesMuscleFatigue?
HighIntensityActivity
• AccumulationofLactate
• HydrogenIons
• ADP
• InorganicPhosphate
• FreeRadicalswithinActiveMuscleFibers
EnduranceFatigue
• AccumulationofFreeRadicals
• Muscle/Extracellularelectrolyteimbalances
• DepletionofMuscleGlycogen
• CentralFatigue(CentralGoverningTheory)
SizePrincipalOfMuscleFiberRecruitment
-MotorUnitsareRecruitedSmallesttoLargest(SnowballEffect)
LengthTensionRelationship
1. Lessthanoptimallengthwillresultinfewercrossbridgeinteractionsandproducelow
force
2. Optimallengthwillresultinmaximalcrossbridgeinteractionsandproducingmaximal
force
3. GreaterthanoptimallengthwillresultinNOcross-bridgeinteractionandnoforce
MuscleForce-VelocityRelationships
1. Absoluteforcethespeedofmovementisgreaterinfasttwitchfibers
2. Maximumvelocityofshorteningisgreatestatthelowestforce
• Trueforbothslow-andfasttwitchfibers
MuscleForce-PowerRelationships
1. Anygivenvelocityofmovementpowerisgeneratedgreaterinmusclewithhigher
percentoffast-twitchfibers
2. PowerdecreasesbeyondPeakPowerbecauseforcedecreaseswithincreasing
movementspeed
DiseasesThatNegativelyImpactMuscleFunction
1.Agingandmuscleloss
-Sarcopenia
-10%ofmusclemasslostbetween25-50Years
-Additional40%lostbetween50-80Years
-Lossoffastfibersandgaininslowfibers
-Alsoduetoreducedphysicalactivity
-Resistancetrainingcandelayage-relatedatrophy
2.Diabetes
-Associatedwithprogressivelossofmusclemass
-Combinedwithagerelatedloss
-Aerobic/Resistanceareprotective
3.Cancer
-50%ofcancerpatientssufferrapidmusclemassloss(Cachexia)
-Accountsfor20%ofdeathsincancerpatients
4.Musculardystrophy
-Hereditarydefectsmuscleprotein
-Resultsinlossofmusclefibers/weakness