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Sliding Filament Mechanism
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Sliding Filament Mechanism
Describes the cause and mechanism of muscle contraction Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere The thin filaments are pulled toward the M line and meet at the center of the sarcomere The Z discs come closer together and the sarcomere shortens. This shortening shortens the entire muscle
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Sliding Filament Mechanism
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The Contraction Cycle Contraction
SR releases Ca2+ ions into the cytoplasm Ca2+ binds to troponin to move tropomyosin out of the way of the myosin-binding sites on actin Contraction cycle – repeating sequence of events that causes the filaments to slide 4 steps
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4 Steps to Contraction Cycle
1. ATP hydrolysis Breaking ATP into ADP + P adds energy to the myosin head so it can turn 2. Attachment of myosin to actin to form cross-bridges Myosin head attaches to binding site on actin Releases P Crossbridge forms
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4 Steps to Contraction Cycle
3. Power Stroke Myosin crossbridges rotate 4. Detachment of myosin from actin Myosin remains attached until another ATP binds causes it to detach * rigor mortis Contraction cycle continues - with ATP - with Ca2+ levels high enough
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Contraction Cycle
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Contraction The myosin heads “walk” down the actin
They can attach and detach about 5 times per second
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Neuromuscular Junction (NMJ)
Somatic motor neurons – neurons that stimulate skeletal muscle fibers Place where action potentials arise Synapse – region where 2 neurons or 1 neuron and a target cell (muscle fiber) meet Synaptic cleft – gap that separates two cells Neurotransmitter – chemical released for the 2 cells to communicate
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NMJ
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NMJ
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NMJ
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NMJ Parts At the end of each axon are synaptic vesicles filled with acetylcholine (ACh) ACh is the neurotransmitter Motor end plate – sarcolemma opposite the synapse Contains acetylcholine receptors Action potential starts here with stimulation from neurons
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Action Potential 1. Release of ACh 2. Activation of ACh receptors
Vesicles do exocytosis Diffuse to motor end plate 2. Activation of ACh receptors 2 Ach bind to motor end plate and opens the receptor Na+ flows across membrane through channel
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Action Potential 3. Production of muscle action potential
Na+ makes fiber more + charged This causes release of Ca+ Contraction cycle! 4. Termination of ACh activity Acetylcholinesterase (AChE) – breaks down ACh so further muscle contractions don’t occur Unless more ACh is released
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Overview
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