Download Skeletal Muscle Anatomy and Function

ELEC811–
SkeletalMuscleAnatomyandFunction

Theprimaryfunctionofaskeletalmuscleisto
generateforces,bycontracting;theseforcesallow
ustomovethroughandinteractwithour
environment
Skeletalmusclesactonbonesto
producemovementofthelimb
andtomove(liftandcarry)
objects.
Imagefrom:
http://www.botany.uwc.ac.za/sci_ed/grade10/m
anphys/skel_mus.htm
Skeletal muscle anatomy


Amuscleisa
bundleof
individualmuscle
cellsorfibres
Eachfibreis
surroundedby
connectivetissue,
calledthe
endomysium
Skeletal muscle anatomy


Thefibresare
organizedin
bundlescalled
fascicles*
Eachfascicleis
surroundedby
connectivetissue,
calledthe
perimysium
*Thesearealsocalled
compartments.
Skeletal muscle anatomy


Anumberof
fasciclesor
compartmentsare
bundledtoforma
skeletalmuscle
Themuscleis
surroundedbya
connectivetissue
sheathcalledthe
epimysium
Skeletal muscle anatomy

Attheendsofthe
muscle,the
endomysium,
perimysiumand
epimysiummerge
toformthetendon,
whichconnectsthe
muscletobone
Skeletal muscle anatomy – gross anatomy


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Thereare5differentshapesofmusclesinthehumanbody:circular,
convergent,parallel,pennateandfusiform.
Weareprimarilyinterestedinpennateandfusiformmuscles.
Inpennatemuscles,themusclefibresareatanangletothetendons
Infusiformmuscles,themusclefibresareinlinewiththetendons
tendon
tendon
UnipennateandbipennatearrangementFusiformarrangement
Agonist and antagonist muscles


Musclesgenerateforceby
contractingtopullthe
tendonofinsertion
(usuallyfurtherawayfrom
thecentreofthebody)
closertothetendonor
origin(usuallyclosertothe
centreofthebody.
Inordertomovealimbin
opposingdirections,
musclesareoften
organizedinpairs– called
agonist‐antagonistpairs.
The biceps brachii contracts to
move the forearm closer to the
upper arm or flex the elbow.
The triceps brachii contracts to
move the forearm away from the
upper arm or extend the elbow.
Motor control – the brain
• Motorprogrammingis
doneinthepre‐motor
cortex,supplementary
motorareaandother
associatedareas.
• Theprimarymotor
cortexreceives
informationfromthe
motorprogramming
areas,thecerebellum
andbasalganglia.
Areas of the brain
Themotorcortexisattherearportionof
thefrontallobe.
• area6– thepre‐motorcortex
• area4– theprimarymotorcortex
Thecerebellumisthestructuretucked
underthebackpartofthebrain.The
cerebellumpreciselyregulatesthe
sequenceanddurationofmovement.
Thebasalgangliaarelocated
deepinthecerebrum.They
functiontoinitiateand
regulatemotorcommands.
See:TheBrainfromToptoBottom:BodyMovementandtheBrain
http://thebrain.mcgill.ca/index.php
Motor control – the motor nerves
• Motorcommandsare
relayedfromthemotor
cortex,alongthe
corticospinaltracttothe
muscles.
• Thecorticospinaltractis
comprisedof‐
motoneurons,whichare
thenerveswhichmake
contactwiththemuscle
cells.
The motor unit (MU)

Each‐motoneuronconnectswithseveralmusclefibres
– amotoneuronandallmusclefibresinnervatedbythe
motoneuronarecalledamotorunit.
‐motoneuronwithterminalbranches
1
2
.
.
n
musclefibres
Motoneuron and muscle cell characteristics

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Motoneurons varyintheirmorphology(sizeandshape)
andelectrophysiologicalcharacteristics(actionpotential
conductionvelocity).
Musclecellsvaryintheirmorphologyandcontractile
characteristics– contractiontimeandsensitivitytofatigue.
Themusclecellswithinamotorunitwillhaveuniform
contractilecharacteristics.
Musclefatigueisaphysiologicalprocessresultinginthe
declineofmuscleoutputforceorpoweroverthetime
courseofasustainedmusclecontraction.
Motor unit types
Threetypesofmusclefibrehavebeenidentified– typeI,IIaand
IIb.Eachmotorunitwillincludeonlyonetypeofmusclefibre.
MotorUnit
Type
Mechanical
Properties
Electrical
Properties
Other
Slowtwitch
orTypeI
Slow, smallforce
twitch;smallerfibre
diamterandMUsize
Slowernerve
conductionvelocity
Recruitedat
lowforcelevels
Fast,fatigue‐
resistantor
TypeIIa
Fastforcetwitch
Intermediatenerve
conduction velocity
Recruitedat
intermediate
forcelevels
Fast,
fatiguingor
TypeIIb
Fast,largeforcetwitch;
largefibrediameter
andMUsize
Fasternerve
conductionvelocity
Recruitedat
highforce
levels
From:MerlettiandParker,Electromyography,
Motor unit contraction
u(t)
Activation:excitation‐
contractioncoupling
a(t)
Musclecontraction
dynamics
Fm(t)
• Aneuralexcitationsignal,u(t),isreceivedfromthe‐
motoneuron.
• Amuscleactivationstate,a(t),isgeneratedviatheexcitation‐
contractioncouplingmechanism– thisactivatesthemuscle
contractileprocess.
• Thecontractilemechanisminthemusclecellsturnsonandforce
isgeneratedviamusclecontractiondyanmics.Allmusclecellsin
themotorunitcontracttodevelopanoutputforceFm(t).
AdaptedfromZajac,1989.
The force twitch
FasttwitchmotorunitSlowtwitchmotorunit
Potential
(mV)
Potential
(mV)
t(ms)
t(ms)
Force
(mN)
Force
(mN)
t(ms)
‐motoneuron
potentials
Motorunit
forcetwitches
t(ms)
EMDEMD
Anactionpotentialinan‐motoneuronresultsincontractionofallmuscle
fibresinthemotorunitandgenerationofaforcetwitch.
EMD=electromechanicaldelay.
Adaptedfrom:MerlettiandParker,2004
Smooth force production


Whenamotorunitisexcitedbyits‐motoneuron,itwill
produceashortdurationforcetwitch
Inordertogetlonger,smoothforcegeneration,themotor
unitmustbecontinuouslyexcited.
Excitation
pulses
Force
t(ms)
t(ms)
References




R.MerlettiandP.A.Parker,Electromyography,IEEEPress,
Piscataway,NJ,2004.
F.Zajac,Muscleandtendon:properties,models,scaling
andapplicationtobiomechanicsandmotorcontrol,CRC
Crit.Rev.Biomed.Eng.,17(4):359‐411,1989.
TheBrainfromToptoBottom:BodyMovementandthe
Brain,http://thebrain.mcgill.ca/index.php
Skeletalmuscles,
http://www.botany.uwc.ac.za/sci_ed/grade10/manphys/s
kel_mus.htm