Process of Muscle Contraction and Relaxation 1. At the neuromuscular junction, Acetylcholine (ACh) is released by the synaptic terminal 2. ACh binds to receptors on the sarcolemma 3. The change in the transmembrane potential of the muscle fiber leads to the production of an action potential 4. The action potential spreads across the entire surface of the muscle fiber and down the Ttubules 5. As a result of the action potential, the sarcoplasmic reticulum releases stored calcium ions. 6. The calcium concentration of the sarcoplasmic reticulum increases in and around the sarcomere 7. Calcium ions bind to troponin. 8. Tropomyosin changes its orientation exposing the active sites on the actin myofilaments 9. Cross bridges form when myosin heads bind to active sites on actin myofilaments 10. Phosphate is ejected from the myosin head 11. The power stroke: Myosin heads bend, pulling the actin toward center of sarcomere 12. ADP is ejected 13. ATP binds to the myosin heads 14. Return stroke: ATP molecules break down into ADP and P (stay bonded to myosin heads) 15. Energy is released 16. Process repeats all over again 17. ACh is broken down by Acetylcholinesterase (ACh-ase) 18. Action potential generation in the sarcolemma ceases 19. Concentration of calcium ion in the sarcoplasm declines 20. The sarcoplasmic reticulum reabsorbs calcium ions 21. Calcium concentration in the sarcoplasmic reticulum increases 22. Myosin heads break bone with active sites on the actin 23. Tropomyosin releases from actin active site 24. No cross bridge interaction can occur; contraction ends 25. Muscle relaxation occurs and muscle returns passively to its resting length
