  Many eukaryotic cells are able to perform directional mechanosensing by
directly measuring minute spatial differences in the mechanical stress on their
membranes. Here, we explore the limits of a single mechanosensitive channel
activation using a two-state double-well model for the gating mechanism. We
then focus on the physical limits of directional mechanosensing by a single
cell having multiple mechanosensors and subjected to a shear flow inducing a
nonuniform membrane tension. Our results demonstrate that the accuracy in
sensing the mechanostimulus direction not only increases with cell size and
exposure to a signal, but also grows for cells with a near-critical membrane
prestress. Finally, the existence of a nonlinear threshold effect,
fundamentally limiting the cell's ability to effectively perform directional
mechanosensing at a low signal-to-noise ratio, is uncovered.
