The aim of this manuscript is to determine the influence of proteic factors on cytoskeleton dynamics using a micromanipulation tool. We have determined the microrheological response of the actin network for individual cells. We applied with an optical tweezer oscillating forces to a micrometric bead specifically bound to the actin network of C2 myoblasts, and measured the amplitude and phase shift of the induced cell deformation. For a non-perturbated single cell, we have shown that the elastic and loss moduli G' and G" behave as power laws fΑ et fΒ of the frequency f (0.01 < f < 50 Hz), Α and Β being in the range 0.15 - 0.30. This demonstrates that the dissipation mechanisms in a single cell involve a broad and continuous distribution of relaxation times. After adding blebbistatin, an inhibitor of myosin II activity, the exponent of G' decreases to about 0.10, and G" becomes roughly constant for 0.01 < f <10 Hz. The actin network appears less rigid and less dissipative than in the control experiment. This is consistent with an inhibition of ATPase and reduction of the gliding mobility of myosin II on actin filaments. In this frequency range, the actomyosin activity appears as the principal mechanism allowing the cell to adapt to an external mechanical stress. The goal of this work is to put in agreement both structural and behaviour cell mechanical models by the mean of microrheological measurements on adherent living cells in normal and disturbed conditions.