Muscle Contraction Mechanism: Sliding Filament Theory Explained
1. Muscle Contraction and Movement
Muscles
Musclesare attachedto bonesbytendons
Musclesworkin antagonisticpairs
Ex. Bicepsandtriceps
One muscle contractswhile the otherrelaxes
Contractile apparatus
Skeletal muscle
Muscle cell = muscle fiber
Muscle fibersare made of myofibrils(striated)
Myofibrilsare made of unitscalledsarcomeres
Sarcomeresare made of thickand thinfilaments
Z line isthe endof the sarcomere
Thickand thinfilamentsslide overone anothertoshortenthe muscle
duringcontraction
The Thick Filament(Myosin)
Consistsof the proteincalledmyosin. A myosin
molecule isshapedabitlike agolf club,but
with2 heads. The headsstickoutto form the
cross bridge.Manyof these myosinmolecules
sticktogethertoform a thickfilament
Thin Filament(Actin)
The thinfilamentconsistsof a
proteincalledactin. The thin
filamentalsocontains tropomyosin.Thisproteinisinvolvedinthe
control of muscle contraction
Slidingfilamenttheory
Linksthe structure of a sarcomere to itsfunction
Duringcontractionthinfilamentsslide overthickfilaments
Thickfilaments=myosinandhave “heads”
Thinfilaments=actin,these slide
Ca and ATPrequiredforslidingandattachment
ATP bindstoa myosinhead,whichisreleasedfroman actin
filament
Hydrolysisof ATPcocksthe myosinhead
The myosinheadattachesto an actin bindingsite withthe
helpof Calcium
The powerstroke slidesthe thinfilamentwhenADPandPi
are releasedfromit
350 myosinheadsperthickfilament
Can bind andunbindto thinfilamentupto5 timesper
second
2. Motor neurons and muscle contraction
Motor neuronsstimulate musclecontraction
Motor neuronsare branchedand can stimulate more thanone muscle
fiber
Motor unit= motor unitandall the muscle fibersitcontrols
Neuromuscularjunctions=the synapse betweenamotorneuronand a
muscle fiber
The strengthof a muscularcontractioniscontrolledbythe numberof
motor unitsactivated.More motorunits= strongercontractions
Musclesrequiringprecise control have one motorneuronpermuscle
fiber
Mechanismof stimulation:
An actionpotential releasesacetylcholine intothe
neuromuscularjunction
Acetylcholine depolarizesthe muscle cellchannelsinside onthe
sarcoplasmicreticulumreleaseCasoit can reach the
contractile apparatus
Mechanismof relaxation
Motor neuronstopsfiring
Ca pumpedbackintothe SR
Muscle injuries
The Injury: The term'pulledmuscle'comesfromthe descriptionof how the injurytakesplace.Usuallythe muscleis
forciblystretchedbeyonditslimitsandthe muscle tissue becomestorn.Dependinguponitsseverityitisclassifiedas
a first,secondor thirddegree strain:
A firstdegree strainisdamage toa fewmuscle fibers.
A seconddegree strainisdamage toa more extensive numberof muscle fibers.
A thirddegree strainisa complete rupture of the muscle itself.
Signs and Symptoms Grade 1
Witha grade 1 the signsmay notbe presentuntil afterthe activityisover.There maybe asensationof cramp or
tightnessanda slightfeelingof painwhenthe musclesare stretchedorcontracted.
Grade 2
Witha grade 2 there isimmediate painwhichismore severe thanthe painof a grade one injury.Itis confirmedby
painon stretchand contractionof the muscle.A grade 2 isusuallysore totouch.
Grade 3
A grade 3 isa catastrophicinjury.There isanimmediate burningorstabbingpainandthe athlete isunable towalk.
The muscle iscompletelytornandthere maybe a large lumpof muscle tissue above adepressionwherethe tearis.