By means of three-dimensional Contact Dynamics simulations, we inves- tigate the microstructural properties of sheared granular packings by varying sys- tematically the inertial number I from the quasi-static to the collisional regime. A detailed analysis of the connectivity of the particles reveals that the transition be- tween the regimes is correlated to profound changes of the nature of the contact net- work. In particular, we show that the increase of internal friction angle with inertial number is mainly a consequence of contact anisotropy. As the inertial number in- creases, the role of force chains decreases and that of friction mobilization increases as long as percolating force chains are present in the system. A detailed analysis of the spatial correlations of floating particles, i.e., particles without bearing con- tacts, reveals the occurrence of “fluidized” volumes in the dense flow regime, which increase in size and coalesce at the approach of the collisional regime.