Lévy distributed fluctuations and cytoquakes in the actomyosin cortex*

The mechanics of the actomyosin cortex, a thin sheet of active material that provides animal cells with a strong and flexible exterior, have long defied satisfactory explanation. Here we report low noise, high statistical power measurements of lateral fluctuations in the cortex of multiple cell types, using arrays of flexible microposts. Such fluctuations are found to be highly non-Gaussian and have previously been observed to contain occasional large rearrangements, termed cytoquakes. Analysis of the structure of the largest post displacements shows that they are indistinguishable from chance fluctuations of the superdiffusive random process describing the rest of the fluctuations. The full distribution of micropost displacements is well-described by an exponentially truncated Lévy alpha-stable distribution, further reinforcing the notion that cells' non-Gaussian fluctuations and cytoquakes are manifestations of a single phenomenon. The lag time dependence of these distributions is not captured by existing models, and we conjecture that the Lévy distributed fluctuations are caused by a heavy-tailed distribution of microscopic stresses on cytoskeletal elements. These findings will be used to inform and constrain future physical modeling of the cortex.