Below are steps of skeletal muscle excitation and contraction (i. e., excitation-contraction coupling). The statements need to be rearranged in the order that they occur from 1 - 19.

Arrange the statements in the correct order in which they occur.

1. very low calcium in sarcoplasm, actin and myosin unbound, troponin and tropomyosin in plasma

2. myosin heads firmly bound to actin

3. sodium rushes into muscle fiber, causing local depolarization, and the end plate potential (type of graded potential)

4. activation of L-type voltage-gated calcium channels

5. calcium binds troponin on sarcomeres of myofibril

6. muscle action potential travels down sarcolemma and t-tubules

7. motor neuron action potential travels down axon, stimulating ACh release from terminals

8. threshold is reached and muscle action potential is generated

9. ACh diffuses across synapse and binds to ligand-gated receptors at muscle end plate

10. calcium diffuses out of sarcoplasmic reticulum, raising concentration in sarcoplasm (End of Excitation Steps)

11. myosin quickly breaks phosphate bond of ATP

12. myosin heads weakly bind actin

13. ADP released from the ATP binding site of myosin

14. activation of calcium release channels

15. tropomyosin slides out of position, off myosin binding sites on actin

16. free phosphate group released from the ATP binding site of myosin

17. new ATP molecule binds to open ATP binding site of myosin

18. myosin heads swing forward, towards the M line of the sarcomere (i. e., power stroke)

19. myosin becomes re-energized and bound with ADP+P, ready to contract again and repeat the cycle