Sliding Filament Mechanism Ch. 10-3 notes

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

Sliding Filament Mechanism

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

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

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

Contraction Cycle

Contraction The myosin heads “walk” down the actin They can attach and detach about 5 times per second http://www.dnatube.com/video/4154/Muscle-Contraction http://www.dnatube.com/video/1305/Muscle-Contraction http://www.dnatube.com/video/1306/Muscle-contraction http://www.dnatube.com/video/1308/Muscle-Contraction http://www.sci.sdsu.edu/movies/actin_myosin_gif.html

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

NMJ

NMJ

NMJ

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

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

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

Overview