A number of my students asked what happens to the sliding filaments when muscles relax. For example, in an individual sarcomere, do all myosin heads release all at once or one/few at a time?
More generally, what is the process/mechanism by which muscle relaxation occurs? What is actually happening to cross bridges themselves at a molecular level step-by-step from the point of full contraction to full relaxation?
Background to prove prior research: I know that AcH will be removed from the synaptic cleft of the NMJ, ceasing action potential generation due to stopping the influx of Na+. And I know that this causes calcium to stop being released by the sarcoplasmic reticulum (and that calcium pumps are even activated to transport Ca2+ back in). Finally, I know that without Ca2+, troponin moves tropomyosin back over the myosin binding sites of actin.
My question is really focused on what happens next. Some myosin heads must still be bound to actin during and following this process. How are they released? One at a time? All at once?.... What triggers this? (I know ATP will bind to each head causing it to release, but does the body intentionally flood/saturate the sarcoplasm with ATP to cause this to happen quickly, or is there some controlled regulatory mechanism?).
Does an individual sarcomere have a controlled slide back, or does it "snap back" into relaxed state all at once?