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Muscle fiber

Muscle fiber. Motor neuron. Nucleus. Sarcoplasmic reticulum (SR). Sarcolemma. Myofibrils. SR Cistern. T-tubule. Action potential reaches synaptic terminal of motor neuron. New AP generated at motor end-plate and propogated down T-tubules. Myofibrils (relaxed). Sarcomere.

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Muscle fiber

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  1. Muscle fiber Motor neuron Nucleus

  2. Sarcoplasmic reticulum (SR) Sarcolemma Myofibrils SR Cistern T-tubule

  3. Action potential reaches synaptic terminal of motor neuron

  4. New AP generated at motor end-plate and propogated down T-tubules

  5. Myofibrils (relaxed) Sarcomere (Sarcolemma and other structures not shown)

  6. Myofibrils (contracted) Sarcomere

  7. Sarcomere (relaxed) Myosin (thick filaments) Actin (thin filaments)

  8. Sarcomere (contracted) Myosin (thick filaments) Actin (thin filaments)

  9. Myosin heads Actin (thin filament) Myosin (thick filaments)

  10. Protein complex Actin Myosin head ADP and phosphate Note that ATP is hydrolyzed at this point, but still attached to myosin head.

  11. Calcium ion Protein complex Myosin-binding sites (exposed) ADP and phosphate

  12. Actin Myosin head ADP and phosphate

  13. Actin Myosin head ADP Pi • Release of Pi initiates the power stroke. • At the end of the power stroke, ADP is released. • A new ATP is required for myosin to release its hold on actin.

  14. Actin Myosin head ATP ATP binds to the myosin head, causing it to detach from actin.

  15. Actin Myosin head ADP and phosphate When the ATP is broken down to ADP and phosphate, the myosin head extends.

  16. Actin Myosin heads The sequence repeats as long as calcium ions are present. The combined work of many myosin heads causes the actin filaments to slide past the myosin filaments.

  17. Protein complex Actin Myosin heads When the action potentials stop, calcium ions are pumped back into the ER, the myosin-binding sites on actin are again blocked, and the muscle relaxes.

  18. Calcium Myosin (Ca+2 in green) Actin Sarcomere

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