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Muscle
- -highly specialized tissue with the ability to contract in response to stimuli
- -a group of cells termed muscle fibers bound together by connective tissue
- -three types of muscle, based on structure, function, and control mechanisms
- -cardiac muscle-striated(involuntary muscle)
- -smooth muscle-smooth(involuntary muscle)
- -skeletal muscle-striated(voluntary muscle)
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Myofibers
- -long cylindrical multinucleated cells
- -composed of actin and myosin myofibrils repeated as a sarcomere(the basic unit of contraction, gives skeletal muscle its striated appearance)
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Tendons
-connective tissue that link bones to skeletal muscle
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Myofibrils
- -cylindrical organelles within muscle cells
- -composed of two types of filaments
- -thin filaments=actin
- -thick filaments=myosin
- -protein complex composed of actin and myosin is called actomyosin
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Titin
- -AKA connectin
- -large protein that acts as a molecular spring responsible for muscle elasticity connecting the Z line to the M
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A-Band
- -wide band of myosin
- -contains the H zone, a narrow region in the center of the A band between the sets of thin filaments on either side
- -contains the M line which is in the center of the H zone and contains proteins that link central regions of adjacent thick filaments
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Z Line
- -two sets of thin filaments anchored to network of proteins at this point
- -two successive Z lines make a sarcomere
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I Band
-contains portions of thin filaments that do not overlap thick filaments
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Cross Bridges
-extend from surface of myosin toward thin filaments
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Contraction of a Sarcomere
- •Each sarcomere shortens as the thin
- filaments slide closer together between the thick filaments;
- •Z lines are pulled closer together;
- •The A bands does not change as a
- muscle fiber shortens,
- •The I bands and H zones become
- shorter.
- •Sarcomeres shorten as thin filaments slide past stationary thick filaments;
- •Myosin cross-bridges attach
- to thin filament and force thin filament toward center of sarcomere;
- •Cross-bridge repeats motion as long as stimulation to contract continues.
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Myosin
-contains an actin binding site
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Actin
- -form two intertwined helical chains(thin filament) using
- --Troponin-a complex of three regulatory proteins
- --Tropomyosin-a binding protein that regulates actin
- -helical actin chain is intertwined with Tropomyosin(which goes in the groove of the helix) and physically cover the binding sites on the actin molecules for attachment with myosin cross bridges
- -thin filament 2-3 times smaller than thick filament
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Cross Bridge Formation
- -two heads of myosin form the cross bridge
- -6 protein subunits of Myosin combine to form a protein with 2 heads and a long tail
- -tail lies along axis of thick filament
- -two heads form cross bridges
- -each head contains a binding site for actin and ATP
- -the heads of two subunits act in opposing directions with their tails intertwined
- -when the heads contract they pull on the thin filament and shorten the sarcomere
- -the globular heads which protrude at regular intervals from the thick filament form the cross bridges
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ATPase
- -catalyze the decomposition of ATP into ADP and Pi
- -releases energy which is harnessed by the enzyme to drive other chemical reaction
- -provides energy for muscle contraction
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Step 1-Cross Bridge Cycle
- -cross bridge binds to actin because
- -Ca2+ levels are high
- -ADP and Pi complex bound to actin that become energized
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Step 2-Cross Bridge Cycle
- -power stroke
- -release of Pi cause the cross bridge to change conformation, rotating forward and moving toward the H zone
- -filament slides
- -ADP is released
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Step 3-Cross Bridge Cycle
-ATP binds to myosin, causing cross bridge to detach from Actin
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Step 4-Cross Bridge Cycle
- -hydrolysis of ATP
- -reenergizes the cross bridge
- -ADP and Pi remain bound to the cross bridge
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Contraction Regulation
- -regulated by the neuromuscular joint
- -junction of motor neuron's axon and muscle fiber
- -axon divides into terminals containing vesicles of ACh
- -the region of muscle fiber under the axon terminal is folded into junctional area to increase surface area
- -the nicotinic ACh receptor is a ligand gated ion channel(when ACh binds it opens)
- -allows Na to flow into the muscle leading to depolarization and an action potential
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T-Tubules
- -AKA transverse tubules
- -invaginations of plasma membrane that conduct an AP from the outer surface to inner regions of a muscle
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Excitation Contraction
- -motor neurons from somatic NS generate AP that liberate ACh
- -ACh is secreted into neuromuscular junction and binds to nicotinic receptors in the muscle cell membrane
- -Ca2+ is liberated from the sarcoplasmic reticulum
- -Ca2+ binds to troponin which changes shape and forces the tropomyosin off of the actin active site(this allows myosin to form cross-bridges with active actin)
- -allows 4 steps of cross bridge to complete muscle contraction
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Slow Fibers
- -have myosin with a lower ATPase activity(changes how fast it hydrolyzes ATP)
- -maximal force produced by each(fast/slow) only speed varies
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Fast Fibers
- -contain myosin with high ATPase activity(changes how fast it hydrolyzes ATP)
- -maximal force produced by each(fast/slow) only speed varies
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Oxidative Fibers
- -contain large number of mitochondria
- -high capacity for oxidative phosphorylation
- -surrounded by many small blood vessels
- -contains large amounts of myoglobin(oxygen binding protein) for intracellular reservoir of oxygen
- -ATP production depends on blood flow to deliver oxygen and nutrients
- -"dark" meat on a turkey
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Glycolytic Fibers
- -few mitochondria but high concentration of glycolytic enzymes and large stores of glycogen
- -limited use of oxygen
- -few blood vessels
- -little myoglobin=pale color
- -"white meat" on turkey
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Slow Oxidative Fibers
- -have low rates of myosin ATPase, but have ability to make large amounts of ATP
- -do not fatigue easily
- -used for prolonged, regular activity(flight, long distance swimming)
- -gives red color to breast meat on ducks, which use the muscles for flight
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Fast Oxidative Fibers
- -high myosin ATPase activity and can make large amounts of ATP
- -do not fatigue quickly and can be used for long-term activity
- -particularly suited for rapid actions(rapid sounds made by throat muscles on birds, clicking of rattlesnakes tail)
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Fast Glycolytic Fibers
- -high myosin ATPase activity but cannot make as much ATP as oxidative fibers, because source of ATP is glycolysis
- -best suited for rapid, intense actions(short sprint, cat pouncing on prey)
- -fatigue quickly compared to oxidative fibers(breast meat on chicken is white because unlike ducks chickens only fly short distances and do not have oxidative pectoral muscles)
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Exercise
- -can produce increase in size of muscle fibers
- -increase capacity for ATP production
- -number of cells remain the same
- -individual fibers grow which account for the size increase
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Aerobic Exercise
- -low intensity, long duration
- -increases number of mitochondria in slow oxidative fibers
- -number of blood vessels around the fibers increases to supply the greater energy demands
- -increase endurance capability
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High Intensity Exercise
- -short duration
- -affect fast glycolytic fibers
- -fibers increase in diameter becasue new actin and myosin filaments are made, creating more myofibrils
- -glycolytic activity is enhanced by elevated synthesis of glycolytic enzymes
- -high strength/low endurance training
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Atrophy
- -loss of muscle due to cessation of muscular activity
- -denervation atrophy occurs when the neurons to skeletal muscle are destroyed and muscle fibers will progressively shrink in diameter
- -can occur in long periods of disuse, cast of broken limb
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