4 steps to contraction cycle atp hydrolysis

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4 steps to contraction cycle ATP hydrolysis attachment of myosin to actin to form crossbridges power stroke detachment of myosin from actin Cycle keeps repeating as long as there is ATP available & high Ca 2+ level near thin filament
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Steps in the Contraction Cycle Notice how the myosin head attaches and pulls on the thin filament with the energy released from ATP
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ATP and Myosin Myosin heads are activated by ATP Activated heads attach to actin & pull (power stroke) ADP is released. (ATP released P & ADP & energy) Thin filaments slide past the thick filaments ATP binds to myosin head & detaches it from actin All of these steps repeat over and over if ATP is available & Ca+ level near the troponin-tropomyosin complex is high
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ATP MOLECULE
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RECREATING ATP WITH PCr
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OXIDATIVE PHOSPHORYLATION ¯
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Overview: From Start to Finish Nerve ending Neurotransmittor Muscle membrane Stored Ca+2 ATP Muscle proteins
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Relaxation Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft Muscle action potential ceases Ca 2+ release channels close Active transport pumps Ca 2+ back into storage in the sarcoplasmic reticulum Calcium-binding protein (calsequestrin) helps hold Ca 2+ in SR (Ca 2+ concentration 10,000 times higher than in sarcoplasm) Tropomyosin-troponin complex recovers binding site on the actin
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The Motor Unit Motor unit - one somatic motor neuron & all the skeletal muscle cells (fibers) it stimulates muscle fibers normally scattered throughout belly of muscle One nerve cell supplies on average 150 muscle cells that all contract in unison. Total strength of a contraction depends on how many motor units are activated & how large the motor units are.
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Optimal overlap of thick & thin filaments produces greatest number of crossbridges and the greatest amount of tension As stretch muscle (past optimal length) fewer cross bridges exist & less force is produced If muscle is overly shortened (less than optimal) fewer cross bridges exist & less force is produced thick filaments crumpled by Z discs Normally resting muscle length remains between 70 to 130% of the optimum Length of Muscle Fibers
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Length - Tension Curve Graph of Force of contraction (Tension) versus Length of sarcomere Optimal overlap at the top of the graph When the fiber is too stretched little force is produced When the fiber is too short, again little force is produced
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Events Occurring After a Nerve Signal Arrival of nerve impulse at nerve terminal causes release of ACh from synaptic vesicles ACh binds to receptors on muscle motor end plate opening the gated ion channels so that Na+ can rush into the muscle cell Inside of muscle cell becomes more positive, triggering a muscle action potential that travels over the cell and down the T tubules The release of Ca 2+ from the SR is triggered and the muscle cell will shorten & generate force
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