Unit V: Movement Muscle Contraction - Part I

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Presentation transcript:

Unit V: Movement Muscle Contraction - Part I Chapter 9 – pg 286-291

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 Motor end plate Neuromuscular junction Myofibril Muscle fiber

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

Excitation of a Muscle Fiber Steps 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).

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

Excitation-Contraction Coupling Steps 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

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

Contraction Steps 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.

Contraction Steps 12 & 13 Power stroke = myosin head releases ADP 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

Contraction Sliding 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 Sarcomere at rest Z line H band I band A band Sarcomere contraction and filament sliding http://www.youtube.com/watch?v=WRxsOMenNQM

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

Relaxation Steps 16 16. Active transport needed to pump calcium back into SR to bind to calsequestrin.

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

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

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