5. Excitation-Contraction Coupling
1. Impulse arrives from motor neuron
2. Neuron releases acetylcholine (Ach) into synapse with muscle
3. Ach binds to receptors on muscle opening ion channels
4. Muscle membrane depolarizes - wave travels down t-tubules
5. Triggers release of Ca2+ from sarcoplasmic reticulum (SR)
6. Ca2+ binds to troponin which changes conformation
7. Tropomyosin released from active sites on actin
8. Myosin/ATP heads bind to actin forming cross-bridges
9. ATP hydrolyzed to ADP + ~Pi
10. ~Pi released - myosin/ADP head swivels
11. Pulls actin toward center of sarcomere, drawing in Z-discs
12. ADP released from myosin
13. ATP binds to myosin causing it to release from actin
14. If Ca2+ is still present, go to # 8
15. When impulses cease, SR takes Ca2+ back up via Ca-ATPase
16. Tropomyosin moves back over active site