BILD 2: Multicellular LifeLECTURE #7Website: http://www.biology.ucsd.edu/classes/bild2.WI11.1Instructor: Darwin K. Berg ([email protected])ANNOUNCEMENTS:1) First midterm: Monday, Jan 24. Material: up through Lec #8.2) Exam rooms: WLH 2001 for students with last names beginningwith A-P: Pepper Canyon Hall 106 for last names Q-Z.3) Review Session: Sunday, Jan 23, 6-9 pm in Peterson Hall 108.Fig. 50-25:The structureof skeletalmuscleMuscleBundle ofmuscle fibersSingle muscle fiber(cell)Plasma membraneMyofibrilLightbandDark bandZ lineSarcomereTEM0.5 μmI band A bandI bandM lineThickfilaments(myosin)Thinfilaments(actin)H zoneSarcomereZ lineZ lineNucleiZRelaxedmuscleMZFully contractedmuscleContractingmuscleSarcomere0.5 mContractedSarcomereFig. 50-26: The sliding-filament model of muscle contraction(a) Relaxed muscle fiber. In a relaxed muscle fiber,the I bandsand H zone are relatively wide.(b) Contracting muscle fiber. During contraction,the thick and thin filaments slide past eachother, reducing the width of the I bands and Hzone and shortening the sarcomere.(c) Fully contracted muscle fiber. In a fullycontracted muscle fiber, the sarcomere is shorterstill. The thin filaments overlap, eliminating the Hzone. The I bands disappear as the ends of thethick filaments contact the Z lines.Fig. 50-27: Myosin-actin interactions underlying muscle fiber contraction1 The myosin head hydrolyzes ATP to ADP and inorganic phosphate (i) and is in its high-energy configuration.1 The myosin head binds toactin, forming a cross-bridge.Thick filamentThin filamentsThin filamentATPATPADPADPADPPiPiPiCross-bridgeMyosin head (low-energy configuration)Myosin head (high-energy configuration)+Myosin head (low-energy configuration)Thin filament moves toward center of sarcomere.ThickfilamentActinCross-bridgebinding site1 Starting here, the myosin head is boundto ATP and is in its low-energyconfiguration.2 P34Releasing ADP and ( i), myosinrelaxes to its low-energy configuration,sliding the thin filament. P5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.Myosin-binding siteTropomyosin(a) Myosin-binding sites blocked(b) Myosin-binding sites exposedCa2+Ca2+-binding sitesTroponin complexActinFig. 50-28: The role of regulatory proteins and calcium in muscle fiber contractionFig. 50-29a: The roles of the sarcoplasmic reticulum and T tubules inmuscle fiber contractionMotorneuron axonMitochondrionSynapticterminalT tubuleSarcoplasmicreticulumMyofibrilPlasma membraneof muscle fiberSarcomereCa2+ releasedfrom sarcoplasmicreticulumFig. 50-29b: Review of contraction in a skeletal muscle fiberAChSynapticterminalof motorneuronSynaptic cleftT TUBULEPLASMA MEMBRANESRADPCYTOSOLAction potential is propa-gated along plasmamembrane and downT tubules.Action potentialtriggers Ca2+release from sarco-plasmic reticulum(SR).Acetylcholine (ACh) released by synaptic terminal diffuses across synapticcleft and binds to receptor proteins on muscle fiber’s plasma membrane, triggering an action potential in muscle fiber.123Tropomyosin blockage of myosin-binding sites is restored; contractionends, and muscle fiber relaxes. 7Cytosolic Ca2+ is removed by active transport into SR after action potential ends.6Myosin cross-bridges alternately attachto actin and detach, pulling actinfilaments toward center of sarcomere;ATP powers sliding of filaments.5Calcium ions bind to troponin;troponin changes shape,removing blocking actionof tropomyosin; myosin-bindingsites
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