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Skeletal muscle contraction

Skeletal muscle contraction

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Skeletal muscle contraction

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  1. Skeletal muscle contraction • Contraction, force and tension • Sliding Filament Theory of Contraction • Contraction cycle • Regulation of the contraction cycle

  2. Muscle contraction • Movement or resist a load (force) • Load is the weight or force that opposes the contraction of a muscle • Tension is the force created by a muscle • Need ATP to generate tension

  3. Observations during muscle contraction Muscle shortens when it moves a load. (When muscle contracts, it does not always shorten)

  4. Observations during muscle contraction: A band does not shorten during contraction.

  5. Sliding filament theory of contraction: movement and force Resting length How about force without movement?

  6. During contraction • Z discs move closer together • Sarcomere shortens • A band same length • I band reduced • H band reduced

  7. What pushes the actin filaments into the myosin? • Cross-bridges link myosin to actin • Power stroke: myosin head binds to actin  myosin head release actin. Repeated many times. • Myosin molecules are flexible • ATP causes movement of myosin molecules

  8. Myosin • A motor protein • Converts chemical bond energy of ATP to mechanical energy of motion • Each myosin as ATPase • Energy from ATP hydrolysis is stored as potential energy in the myosin molecule, and is used to create the power stroke.

  9. Why don’t actin and myosin continuously bind together? • ATP is usually available • Actin’s binding site for myosin is revealed only during cross-bridge (binding). • During relaxation, actin’s binding site for myosin is concealed

  10. Energy for skeletal muscle contraction • ATP sources • The many causes of muscle fatigue • Classification of skeletal muscle fiber types

  11. ATP and muscle contraction • Need ATP for • Cross-bridge formation, power stroke (myosin ATPase) • Ca++ transport to SR (Ca++ ATPase) • Na+/K+ transport across sarcolemma (Na+/K+ ATPase)

  12. Sources of ATP • ATP pool • Phosphocreatine. • At rest, ATP phosphorylates creatine. • During exercise, creatine kinase (creatine phosphokinase) moves phosphate from phosphocreatine to ATP

  13. Sources of ATP • Glucose (glycolysis) to pyruvate citric acid cycle  oxidative phosphorylation (about 30 ATP per glucose molecules) • Anaerobic glycolysis: glucose  lactic acid (2 ATP per glucose molecule)

  14. Sources of ATP • Beta oxidation of fatty acids. Fatty acids are converted to acetyl CoA citric acid cycle in the mitochondria, need oxygen • Slow • During light exercise

  15. Sources of ATP • glucose catabolism during heavy exercise • carbo loading builds up glycogen stores • Protein catabolism during starvation

  16. Fatigue • Muscle is no longer able to generate sustained expected power output • A variety of contributing factors • depends on the degree of muscle activity

  17. Fatigue: contributing factors • Intensity of muscle activity • Duration of muscle activity • Aerobic/anaerobic metabolism • Muscle composition • Fitness level • Ions • Nutrients • Neurotransmitter

  18. Fatigue during extended submaximal exertion • Not ATP shortage • Glycogen depletion may affect Ca++ release from SR

  19. Fatigue during short duration maximal exertion • Lots of inorganic phosphate from ATP hydrolysis •  may slow P release from myosin:ADP:Pi •  slows power stroke • Acidosis may inhibit some enzymes

  20. More factors for muscle fatigue • K+ : intracellular K+ lowered during repetitive action potentials affects Ca++ release channels on SR membrane • Acetyl choline depletion at the myoneural junction low end-plate potential (disease)

  21. More factors for muscle fatigue • CNS: • Subjective feelings preceding physiological fatigue • Acidosis may influence perception of fatigue