PSIO 201 5th Edition Lecture 18 Outline of Last Lecture Muscle Introduction Outline of Current Lecture I II III IV V VI VII Sarcomere Thin Filaments Thick Filaments Proteins Sliding Model Crossbridge Cycle Rigor Complex Current Lecture Sarcomere functional unit of contraction contains different regions 1 Thin filaments attached to Z line two twisted strands of pearls composed of two helically arranged strands of actin Myosin binding site on actin covered by regulatory proteins troponin and tropomyosin Tropomyosin is controlled by troponin which is controlled by calcium regulate contraction by regulating binding sites 2 Thick Filament spans the distance and overlaps thin filaments composed of a highly organized array of myosin molecules Contains myosin heads that interact with actin both together form crossbridges Myosin head also has ATPase activity when bound to actin releases ATP to power contraction Protein Overview a Actin found in thin filaments has myosin binding sites for crossbridges formation with myosin b Myosin found in thick filaments has myosin head that binds to the actin and forms cross bridge during contraction Regulatory Proteins a Tropomyosin found in thin filaments b covers the myosin binding sites on actin when muscle is relaxed Troponin found in thin filaments holds tropomyosin in place when muscle is relaxed during contraction calcium binds to troponin and causes a conformational change that shifts tropomyosin away from the myosin binding sites of actin exposing the binding sites for crossbridge formation Sliding Filament Model of Muscle Contraction 1 Myosin heads bind to actin to form a crossbridge 2 Conformational change energized by ATP hydrolysis causes think filaments to sline along thick filaments 3 Myosin head groups release form new crossbridges and the sliding cycle repeats Result Z line move toward one another Sacromere length decreases Myofibril shortens Muscle fiber shortens As Think Thin filaments overlap increases 1 I Band length decreases 2 A band length remains constant 3 H Zone length decreases 4 Zone of overlap increases Crossbridge Cycling cycle is entered following exposure of myosin binding sites on actin filaments regulatory role for CA2 Prior to entering cycle at the end of the contraction myosin heads prepares in the cocked positions to bind to actin 1 ATP has to bind to myosin head 2 ATP is hydrolyzed by the unbound head and releases energy resulting in the cocked Position of the head groups Once Cocked the Crossbridge Cycling can occur 1 Crossbridge formation 2 Power stroke ADP is released and head pivots toward center 3 Crossbrige detachment link between head and actin detaches 4 Reactivation of Myosin head ATP to ADP and inorganic phosphate Ends with calcium ions are transported back to Reticulum Detailed Version in Lecture 1 Myosin heads bind to actin forming crossbridges only when Ca2 is released from SR binding to troponin exposing the binding sites crossbridge formation 2 Myosin heads pivot towards center of sarcomere power stroke 3 As myosin heads bind to ATP the crossbridges detach from actin crossbridge detachment 4 Myosin heads hydrolyze ATP and become reoriented and re energized recocked Rigor Complex the attached head group after the power stroke Rigor Mortis the rigor of death because of lack of ATP to detach the crossbridge 1 Myosin heads rotate back towards center of sarcomere 2 Myosin heads bind to ATP and crossbridges detach
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