Dynamical Instability in Rigor Fibers?

When muscle fibers are in rigor, most of the crossbridges are thought to exist in a "rigid" type of mobility state. This is due to strong attachment to binding sites of actin ("thin") filaments. Using EPR spectroscopy, we have studied changes in crossbridge mobility state as a function of time. We observed both small and large oscillatory changes over timescales ranging from minutes to hours. The large changes would correspond to long-range correlations in space. They could be taken to arise from slow dynamics of rigor cluster rearrangements in space. The mixed mobility state of the entire fiber could then be viewed as a type of structural phase coexistence. Finally, the slow dynamic rearrangements could be viewed as heterophase fluctuations that undergo avalanche-like excursions - in both directions. These spontaneous directional switching events may then correspond to some type of extreme dynamical instability existing within the rigor-state fiber.