Mechanics of the Sarcomere
Passive compression of the sarcomere showed two inflections in the force length curve; one was at the length where opposing thin filaments meet at the center of the sarcomere, and the other occurred where the Z-discs butt against the thick filaments, but the former inflection showed an unexpected direction.
When all sarcomeres in the preparation were uniform, at constant levels of free calcium concentration, the active forces increased with increasing sarcomere length up to 3.6µm, at the point where the overlap between the thick and thin filaments nearly ends.
At length perturbation frequencies higher than 50 Hz, protein structures in the sarcomere showed increasing stiffness with increasing frequency. However, the stiffness arising from the active force had no frequency dependence in the same frequency range.
Single sarcomeres are expected to produce force fluctuations, arising from cross-bridge cycling, whose rms value is on the order of 1% of the mean force. Fluctuation measurements at a sensitivty at least 100 times greater than the expected level revealed nothing.
KeywordsActive Force Thin Filament Sarcomere Length Thick Filament Passive Force
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