Impulse to activate action potential in skeletal muscle, My notes.... VIII.Muscle Fibers a.specific structures in muscle cells (muscle fibers)(skeletal) allow the cells to contract and relax i.myofibrils (fills nearly all cytoplasm) 1.cylindrical structures that make up bulk of cytoplasm 2.consist of a chain of small, contractile units (sarcomeres) a.give muscle fiber striped appearance b.composed of myosin-II and actin i.myosin-II = thick center ii.actin = thin filaments that overlap mysoin-II 1.attach plus end to Z disc a.intersection of 2 sarcomeres b.Contraction i.when sarcomeres shorten, muscle fibers contract 1.heads of myosin filaments start walking along respective actin filaments a.pulls actin and myosin past each other b.occurs very quickly (less than 1/10 of a second) i.each myosin filament has 300 heads pulling on the actin ii.muscle fibers relax when myosin heads release the actin filaments iii.steps 1.attached (myosin attached to actin)(post stroke) a.rigor state b.ATP binds causes release 2.released a.ATP hydrolysis to ADP Pi allows cocked state 3.cocked a.Pi release is coupled with binding of myosin head to actin 4.force-generating(POWER STROKE) a.actin moves b.then ADP is released c.ATP binds 5.detached a.caused by ATP binding 6.ATP is hydrolysed to ADP Pi a.Cause cocked state iv.Steps (alternate) 1.Myosin starts attached to actin (just after the last stroke)("rigor" state). 2.ATP binds to myosin head causing a release from actin. 3.Then ATP hydrolysis ...

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