What is the actual molecular mechanism for muscle relaxation?

Question

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?

Answer 1

You have it pictured correctly. Myosin unbinds spontaneously from actin when it binds ATP which is maintained at near-constant levels regardless of the contractile state. It's the sudden drop of Ca2+ (from pumping into the sarcoplasmic reticulum) that causes troponin to change conformation and (with tropomyosin) occlude the myosin head from rebinding. This is the "steric blocking hypothesis" for how troponin-tropomyosin inhibits contraction. A good reference for this is Ebashi from circa 1960 (mentioned in the Lehman review below).

You have a related and more subtle question about the coordination of this process at the scale of a sarcomere. Because the Ca2+ concentration is globally coordinated between sarcomeres, and because tropomyosin is an elongated protein that connects multiple regulatory regions, the thin filament (actin plus troponin-tropomyosin) switches its regulatory states rather cooperatively, so that the whole muscle fiber switches on and off rather uniformly. This is more subtle and recently characterized structural biology.The most direct evidence for cooperative switching comes from a series of papers from McKillup&Geeves (also mentioned in the review below).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479050/