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Unit V: Movement Muscle Contraction - Part I

Chapter 9 – pg 286-291. Unit V: Movement Muscle Contraction - Part I. Electrophysiology of Muscle Tissue. Review Resting, excitable cell – polarized Resting muscle cell: excess Na + outside, K + and anions inside Resting Membrane Potential

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Unit V: Movement Muscle Contraction - Part I

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  1. Chapter 9 – pg 286-291 Unit V: MovementMuscle Contraction - Part I

  2. Electrophysiology of Muscle Tissue • Review • Resting, excitable cell – polarized • Resting muscle cell: excess Na+ outside, K+ and anions inside • Resting Membrane Potential • Stimulation of muscle cell – Na+ enters, then K+ leaves • Action Potential • spreads along the sarcolemma Motor neuron Axon Neuromuscular junction Muscle fiber Myofibril Motor end plate

  3. Excitation of a Muscle FiberSteps 1 & 2 1. Nerve signal opens voltage-gated calcium channels. 2. Calcium stimulates exocytosis of synaptic vesicles containing ACh = ACh release into synaptic cleft.

  4. Excitation of a Muscle FiberSteps 3 & 4 3. Binding of ACh to receptor proteins 4. opens Na+ and K+ channels resulting in reversed polarity forming an end-plate potential (EPP).

  5. Excitation of a Muscle FiberSteps 5 5. Voltage change in end-plate region (EPP) opens nearby voltage-gated ion channels producing an action potential

  6. Excitation-Contraction CouplingSteps 6 & 7 6. Action potential spreading over sarcolemma enters T tubules 7. voltage-gated channels open in T tubules causing calcium gates to open in SR

  7. Excitation-Contraction CouplingSteps 8 & 9 Myosin head Troponin Resting Sarcomere Actin Tropomyosin 8. Calcium released by SR binds to troponin. 9. Troponin-tropomyosin complex changes shape and exposes active sites on actin.

  8. ContractionSteps 10 & 11 10. Myosin ATPase in myosin head hydrolyzes an ATP molecule, activating the head and “cocking” it in an extended position. 11. It binds to actin active site forming a cross-bridge.

  9. ContractionSteps 12 & 13 • Power stroke = myosin head releasesADP and phosphate as it flexes pulling the thin filament past the thick • Recovery stroke = with the binding of more ATP, the myosin head extends to attach to a new active site Contracted Sarcomere

  10. ContractionSliding Filament Theory • Thick and thin filaments slide past one another • They DO NOT become shorter! • Half of myosin heads bound to thin filaments at a time I band A band A band I band H band Z line H band Z line Z line Z line Sarcomere contraction and filament sliding Sarcomere at rest http://www.youtube.com/watch?v=WRxsOMenNQM

  11. RelaxationSteps 14 & 15 14. Nerve stimulation ceases 15. Acetylcholinesterase removes ACh from receptors. Stimulation of the muscle cell ceases.

  12. RelaxationSteps 16 16. Active transport needed to pump calcium back into SR to bind to calsequestrin.

  13. RelaxationSteps 17 & 18 17. Loss of calcium from troponin. 18. Moves troponin-tropomyosin complex over active sites. Muscle fiber returns to its resting length.

  14. Rigor Mortis • Hardening of muscles, stiffening of body • Time period: 3 – 60 hours after death • No ATP being produced!

  15. Myasthenia Gravis • Women between ages 20-40 • Autoimmune disease • Antibodies clump ACh receptors together • Muscle fibers less sensitive to ACh • Treatment • Cholinesterase inhibitors • Immunosuppresive agents • Removal of thymus

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