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Neuromuscular Junction

Neuromuscular Junction

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Neuromuscular Junction

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  1. Neuromuscular Junction Figure 9.7 (a-c)

  2. Skeletal Muscle Figure 9.2 (a)

  3. Myofibrils Figure 9.3 (b)

  4. Sarcomeres Figure 9.3 (c)

  5. Myofilaments: Banding Pattern Figure 9.3 (c, d)

  6. Ultrastructure of Myofilaments: Thick Filaments Figure 9.4 (a)(b)

  7. Ultrastructure of Myofilaments: Thin Filaments Figure 9.4 (c)

  8. Arrangement of the Filaments in a Sarcomere • Longitudinal section within one sarcomere Figure 9.4 (d)

  9. Sarcoplasmic Reticulum (SR) Figure 9.5

  10. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • At low intracellular Ca2+ concentration: • Tropomyosin blocks the binding sites on actin • Myosin cross bridges cannot attach to binding sites on actin • The relaxed state of the muscle is enforced Figure 9.10 (a)

  11. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • At higher intracellular Ca2+ concentrations: • Additional calcium binds to troponin (inactive troponin binds two Ca2+) • Calcium-activated troponin binds an additional two Ca2+ at a separate regulatory site Figure 9.10 (b)

  12. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • Calcium-activated troponin undergoes a conformational change • This change moves tropomyosin away from actin’s binding sites Figure 9.10 (c)

  13. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • Myosin head can now bind and cycle • This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin Figure 9.10 (d)

  14. Sequential Events of Contraction Myosin head (high-energy configuration) Myosin cross bridge attaches to the actin myofilament 1 Thin filament ADP and Pi (inorganic phosphate) released Thick filament Working stroke—the myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line As ATP is split into ADP and Pi, cocking of the myosin head occurs 2 4 Myosin head (low-energy configuration) As new ATP attaches to the myosin head, the cross bridge detaches 3 Figure 9.11

  15. Motor Unit: The Nerve-Muscle Functional Unit Figure 9.12 (a)