ACh Removal
Once the ACh is no longer available, the sarcolemma stops depolarizing and no more action potentials are created. This means that the channels in the sarcoplasmic reticulum that had originally dumped calcium close and the cell begins using ATP to actively pump the calcium from the sarcoplasm back into the storage facility. Once calcium is removed from troponin, the tropmyosin slides back into place and blocks actin and myosin from attaching - stopping the Sliding Filament Theory in its tracks. Since the myosin heads are no longer holding the actin molecules, the sarcomere then slides apart back to its resting length. Muscle relaxation is essentially our excitation - contraction coupling saga halted at 3 separate steps #1, 6, and 12, all animated below.
Video 8. An Action Potential Starts in the Brain (opens YouTube in new window)
Video 9. ACh Binds to Voltage-Gated Sodium Channel on the Motor End Plate (opens YouTube in new window)
Video 10. Calcium binding to Troponin and the Relaxation of Skeletal Muscle (opens YouTube in new window)
Summary
Video 11. Simplification of the Total Excitation Coupling Process (opens YouTube in new window)
Removal of ACh from synapse and Calcium from the sarcomere allowing contraction to end:
1. ACh can simply diffuse away (See note back in Figure 8).
2. Enzymatic degradation of ACh by the enzyme acetylcholinesterase
3. SR Calcium channels are closed; Calcium actively pumped back into the SR
Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads, and many cross-bridges form and break continuously during muscle contraction. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much energy (ATP) is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the actin-binding sites, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.
