Summary

The sequence of events leading to contraction of a skeletal muscle fiber following stimulation of its motor neuron can be summarized as follows:

1. Acetylcholine released from the presynaptic terminal depolarizes the end-plate region of the muscle fiber.

2. The depolarization initiates an all-or-none action potential in the muscle fiber, and the action potential propagates along the entire length of the fiber.

3. Depolarization produced by the action potential spreads to the interior of the fiber along the transverse tubule system.

4. Depolarization of the transverse tubules causes release of calcium ions by the sarcoplasmic reticulum.

5. Released calcium ions bind to troponin molecules on the thin filaments.

6. When calcium combines with troponin, tropomyosin uncovers the myosin-binding site of actin.

7. Globular heads of myosin molecules, which have been energized by splitting a high-energy phosphate bond of ATP, are then free to bind to actin.

8. The stored energy of the activated myosin is released to propel the thick and thin filaments past each other. The spent ADP is released from myosin at this point.

9. A new ATP binds to myosin, releasing its attachment to the actin molecule.

10. The new ATP is split to re-energize myosin and return the contraction cycle to step 7 above.

11. Contraction is maintained as long as internal calcium concentration is elevated. The calcium concentration falls as calcium ions are taken back into the sarcoplasmic reticulum via an ATP-dependent calcium pump.

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