How actin filament polarity affects crossbridge force in doubly-overlapped insect muscle.

We wished to find out why the internal resistance to shortening, which is negligible above rest length, becomes progressively more important with shortening below rest length. For this reason the movement of detached actin filaments in isometric sarcomeres treated with activating solution has been studied in insect flight muscle after breaking the filaments from the Z lines by stretching fibres in rigor. Evidence of sliding motion of such filaments has been produced by experimentally inducing double overlap zones in activating solution. It was deduced that the forces generated by individual crossbridges are comparable to the internal resistance to relative filament motion. Furthermore the final position of broken actin filaments indicated that wrongly polarized actin slides freely past activated crossbridges, but prevents these same bridges from exerting force on adjacent correctly polarized actin. Apparently only those bridges which are located in the normal overlap zones can generate effective force. It is therefore probable that the isometric tension is directly proportional to the number of bridges overlapped in the normal overlap zones for any sarcomere length equal to or greater than the thick filament length.