  Living cells adapt and respond actively to the mechanical properties of their
environment. In addition to biochemical mechanotransduction, evidence exists
for a myosin-dependent, purely mechanical sensitivity to the stiffness of the
surroundings at the scale of the whole cell. Using a minimal model of the
dynamics of actomyosin cortex, we show that the interplay of myosin power
strokes with the rapidly remodelling actin network results in a regulation of
force and cell shape that adapts to the stiffness of the environment.
Instantaneous changes of the environment stiffness are found to trigger an
intrinsic mechanical response of the actomyosin cortex. Cortical retrograde
flow resulting from actin polymerisation at the edges is shown to be modulated
by the stress resulting from myosin contractility, which in turn regulates the
cell size in a force-dependent manner. The model describes the maximum force
that cells can exert and the maximum speed at which they can contract, which
are measured experimentally. These limiting cases are found to be associated
with energy dissipation phenomena which are of the same nature as those taking
place during the contraction of a whole muscle. This explains the fact that
single nonmuscle cell and whole muscle contraction both follow a Hill-like
force-velocity relationship.
