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Chapter 12 textbook material after midterm Exam review

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University of Waterloo
BIOL 273
Katherine Wood

Unit 3Muscle Chapter 12 Muscles SKELETAL MUSCLE Muscle Contraction Creates ForceContraction of muscle fibres allows us to move or resist a loadMuscle tensionforce created by contracting muscleLoadweight or force that opposes contraction of muscleContractioncreation of a tension in a muscle o Active process that requires ATPRelaxationrelease of tension created by a contractionMajor steps leading to a muscle contraction o Events are the neuromuscular junctionConvert ACh signal from a somatic motor neuron into an electrical signal in the muscle fibre o Excitationcontraction EC coupling Process in which muscle action potentials initiate calcium signals In turn activate a contractionrelaxation cycle o Molecular levelcontractionrelaxation cycleExplained by sliding filament theoryOne contractionrelaxation cycletwitch Actin and Myosin Slide Past Each Other During Contraction Myosin Crossbridges Move Actin FilamentsRotation of myosin crossbridges provides the force that pushes the actin filament during contraction o Myosin heads bind to actin moleculesPower strokeinitiated by calcium signal o Happens when myosin crossbridges swivel and push the actin filaments toward the center of the sarcomere o At the end the myosin head releases actinThen swivels back and binds a new actinAnother contractile cycleContractionheads do not release all at the same time o If so fibres would slide back to original positionPower stroke repeats many times in a contraction o Bindpushreleasebindpushreleaseetc toward the center of the sarcomereMyosin ATPase o Myosin converts the chemical bond energy of ATP into the mechanical energy to crossbridge motionallows to swivel o Myosin ATPasehydrolyzes ATP to ADP and inorganic phosphateEnergy is trapped by myosin and stored as potential energy in the angleBecomes kinetic energy in power stroke Calcium Signals Initiate ContractionRigour state only last a little while in a living muscle because there is a good supply of ATP and then o ATP binds and myosin detachesATPbinding decreases actinbinding affinity of myosinMyosin releases o ATP hydrolysis provides energy for the myosin head to rotate and reattach to actinATPbinding site on myosin head closes around ATP and hydrolyzes itADPinorganic phosphateADP and Premain bound to myosinenergy rotates head to 90 degree i position with actinBinds a molecule 13 gactin molecules away from where it startedweak and low force because tropomyosin is blocking bond a little o Most resting molecules in this state waiting for calcium to bind and contractstored potential energy o The power strokeBegins after calcium ion binds troponin to uncover the rest of the myosin binding siteCrossbridges transform into highforce strong bonds when Pis released iAllows myosin head to swivel o Swing towards M line sliding attached actin filament with itAlso called cross bridge tiltingtilt from 90 degrees to 45 degreesThe rigor state o After death metabolism stopsATP supplies exhausted myosin stays tightly boundrigor mortisIt now appears the myosin can influence troponincalcium ion binding o Depends on whether myosin is in rigour statebound strongly to actin o Depends if actin and myosin is not bound o Or depends on if they are bound weakly Acetylcholine Initiates ExcitationContraction Coupling Skeletal Muscle Contraction Requires a Steady Supply of ATPMuscles also obtain energy from fatty acids o Process always requires oxygen
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