Myosin-generated stresses are responsible for non-equilibrium mechanical behavior of synthesized cytoskeletal networks in vitro. In particular, it is found that myosin stresses can modify the network elasticity. For living cells, it has been suggested that internally generated stress might help cells sense and mimic the stiffness of their environments. However, cellular mechanical responses to intracellular stress are not well understood.;To address these questions, we studied microrheology inside living cells by comparing their mechanical properties to those expected by a statistical analysis of non-thermal fluctuations. We used an experimental method that combines optical tweezers-based active microrheology with particle-tracking passive microrheology. First, we calibrated the trapping force in the linear restoring-force regime with oscillatory optical tweezers. Then, we used optical tweezers to test the response functions against the fluctuation-dissipation theorem in equilibrium systems (i.e., polymer solutions or colloidal crystal gels) and in non-equilibrium systems (i.e., living cells).;In living cells, we employed cellular microrheology using an internal probe as well as an externally attached particle. Whereas extracellular probes attached to the cytoskeleton provide a measure of global cell mechanical properties, intracellular probes provide direct measurements of intracellular mechanical properties. We used an engulfed micro-particle as a probe to study local intracellular stress and stiffness. The relationship between fluctuations in stress and in cell elasticity for living cells under different internal tensions reveals a strong non-linearity between cell elasticity and intracellular stress, which follows a master curve. Our results show that the motors induce an internal tension that forces the network into a non-equilibrium and non-linear state.;These aspects provide a better understanding of the noise in a non-equilibrium system. The relationship between the different sources of noise in living cells helps reveal the inner workings of the highly dynamic cytoskeleton network. Studies of intracellular stress and mechanical properties promote our current understanding of how cells sense and respond to their mechanical environment. Such knowledge could lead to new designs in biomaterials and advance our understanding of diseases related to cellular mechanotransduction. Our studies in active systems contribute to our knowledge of fundamental non-equilibrium statistical physics in biological systems.
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