Abstract
This lecture continues the presentation of mechanical instabilities. Leaving purely elastic systems, we now see hydrodynamic flows of active gels such as actin networks set in motion by Myosin-II molecular motors (see 2017-2018 lecture 6 " Cellular tension-cortical tension "). These actomyosin networks display complex dynamics that sometimes form remarkable spatial patterns in the stationary state. Local excitation comes from a mechanical feedback loop (advection) that amplifies local fluctuations by further concentrating actin and Myosin-II. Diffusion flattens these transient irregularities. The advective flow of the network is of active origin (Myosin-II motor). Friction opposes this flow, acting as a remote inhibitor. Competition between local excitation and global inhibition gives rise to spatial patterns.
The lecture then turns to the temporal instabilities characteristic of excitable systems of a mechanochemical nature (bistability, excitability, oscillations), and spatio-temporal instabilities (trigger waves). The combined action of diffusion and advection of molecules that regulate the cytoskeleton generates complex dynamics that are observed almost ubiquitously in the cortex of animal cells. We highlight in particular the morphogenetic manifestations of these dynamic behaviors of actomyosin networks.
