1. Muscle fiberMotor neuron Nucleus

2. SarcolemmaMyofibrils Sarcoplasmic reticulum (SR) T-tubule SR Cistern

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) Actin (thin filaments) Myosin (thick filaments)

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

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

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

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

12. ActinMyosin head ADP and phosphate

13. ActinMyosin head PiPi ADP 1.Release of P i initiates the power stroke. 2.At the end of the power stroke, ADP is released. 3.A new ATP is required for myosin to release its hold on actin.

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

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

16. ActinMyosin 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. Actin Myosin heads Protein complex 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. Actin MyosinCalcium Sarcomere (Ca +2 in green)