Mechanism of Contraction: According to the sliding-filament hypothesis, skeletal muscle contraction is initiated by and includes the following chain

1. The nerve impulse is carried along the axon of the motor neuron and causes

2. depolarization of the presynaptic membrane (Na+ influx), which causes

3. fusion of the synaptic vesicles with the presynaptic membrane and exocytosis of acetylcholine into the synaptic cleft.

4. Acetylcholine crosses the synaptic cleft and binds to receptors in the postsynaptlc mem- brane, causing

5. depolarization of the sarcolemma (influx of Na+), causing

6. depolarization of the T tubules (sarcolemmal invaginations), causing

7. depolarization of the terminal cisternae, causing

8. depolarization of the rest of the sarcoplasmic reticulum, causing

9. release of sequestered Ca2+ from the sarcoplasmic reticulum into the sarcoplasm surrounding the myofibrils.

10. Ca binds to the TnCs of the troponin complexes, causing

11. a conformational change of each troponin complex, causing

12. the TnIs of the troponin complexes to move away from the myosin head-binding sites on the actin filaments, allowing the

13. myosin heads to bind to actin, causing

14. activation of the ATPase in the myosin heads, causing

15. the production of energy and ADP from ATP and movement of the myosin heads, which

16. pull the actin filaments toward the center of the sarcomere, resulting in

17. simultaneous shortening of the sarcomeres by shortening of the I bands (A bands do not narrow), resulting in

18. shortening of the myofibrils, resulting in

19. shortening of the entire muscle fiber.