1Motor mechanismsKeywords (reading p. 1080-1086)• Bundle, fiber, myofibril, sarcomere• Z-line, thick filament, thin filament• Actin, myosin, sliding filament model• Molecular basis for filament movement• Troponin, tropomyosin• Sarcoplasmic reticulum• Integration of synaptic signals• neurotransmittersMotor mechanisms• Create movement• Can be cilia, flagella, contractile proteins, muscles• Will focus on skeletal muscle– Muscle of vertebrates that is under voluntary controlStructure of skeletal muscle• Bundle• Fiber• Myofibril• sarcomerePart of Campbell 6th edp. 1081Features of muscle cells• # of nuclei- multinucleate; formed by fusion of embryonic cells• length - this results in very long cellsPart of Campbell 6th edp. 1081Sarcomere• Structure gives muscle a striated appearance• Z line, thick filaments (myosin), thin filaments (actin)Part of Campbell 6th edp. 10812Part of Campbell 6th edp. 1081Sliding-filament model of muscle contractionPart of Campbell 6th edp. 1081Thick and thin filaments slide past each otherPart of Campbell 6th edp. 1081At maximal contraction, there is no space at end of thick filament,thin filaments overlapPart of Campbell 6th edp. 1081Molecular basis for movement of filaments against each otherPart of Campbell 6th edp. 1082http://www.youtube.com/watch?v=InIha7bCTjM&feature=relatedATP bound, head retracted and unattachedPart of Campbell 6th edp. 10823Hydrolysis of ATP cocks headPart of Campbell 6th edp. 1082Myosin head attaches to actin filamentPart of Campbell 6th edp. 1082Release of ADP + Picauses a further conformational change pushing against the actin filamentPart of Campbell 6th edp. 1082Binding of ATP to myosin head causes dissociation from actin filamentPart of Campbell 6th edp. 1082Cycle repeats and sarcomereshortensPart of Campbell 6th edp. 1082Control of muscle contraction by Ca++• Tropomyosin- blocks the myosin binding sites on the actin filament when muscle is at rest• Troponin complex-binds calcium and controls the position of tropomyosin4At rest, myosin cannot bind because sites are covered by tropomyosinPart of Campbell 6th edp. 1083http://www.youtube.com/watch?v=WRxsOMenNQMDuring muscle contraction Ca++levels rise. Ca++binds to troponin which then pulls tropomyosin way from the binding sitesPart of Campbell 6th edp. 1083What triggers the Ca++rise that induces muscle contraction?[Ca++] regulated by the sarcoplasmicreticulumCampbell 6th edp. 1083Structure of the sarcoplasmicreticulum• T tubules - are a network of the fiber plasma membrane that goes deep into the muscle fiber.• This allows transmission of the action potential into the fiberSequence of events leading to muscle contraction• Motor neuron releases acetylcholine• Depolarization of the muscle fiber membrane results in action potentials• Action potentials trigger release of Ca++from the sarcoplasmic reticulum• Increased Ca++allows actin and myosin to slide against each other5Characteristics of other muscle types:• Cardiac muscle - found only in the heart, striated, gap junctions allow direct electrical signaling between cells• Smooth muscle - involuntary muscle, meshwork of actin and myosin, can contract more (greater shortening), but with less