  In this work, we study in detail the distribution of stochastic forces
generated by the molecular motors activity, in the actin cortex of pre-muscular
cells. By combining active and passive rheology experiments, performed on the
same micro-bead bound to the actin network through membrane adhesive receptors,
we measure the auto-correlation function Cff(t) of the average force pulling on
the bead. Like for any out-of-equilibrium system, the force distribution
differs from the thermodynamical equilibrium one, especially at long time scale
t>1sec where the bead motion becomes partially directed. Thus the
fluctuation-dissipation theorem does not apply and one can measure the distance
from equilibrium through its violation. This work focuses on the influence of
various parameters (ligand density, temperature, ATP depletion, molecular
motors activity) on the force distribution. In particular, it is shown that the
amplitude of active forces increases when the bead is more tighly attached to
the cortex: this is interpreted through a model which takes into account the
number of bonds between the bead and the cytoskeleton and the viscoelastic
properties of the medium. It also increases with temperature, consistently with
a description of the cell metabolism in terms of thermally activated reactions.
Last, but not least, ATP depletion in the cell, or partial inhibitition of the
actomyosin activity, leads to a decrease of the amplitude of the force
distribution. Altogether, we propose a consistent and quantitative description
for the motion of a micrometric probe interacting with the actin network, and
for the amplitude of the stochastic forces generated by molecular motors in the
cortex surrounding this probe.
