The phenomenon of spontaneous firings induced by a low Mg2+ condition in primary cortical neuronal cultures with different amounts of glial cells are used to study the effects of glia on the dynamics of the network. Single cell patch-clamp measurements have shown that the firing patterns during spontaneous firings are different for networks with and without glia. In general, the synchronization of networks with glia is better than that of those without glia. Furthermore, originally glia-free cultures can be made to produce better synchronization and recover the firing patterns similar to those from cultures with glia after glia have been added to the network. Our finding indicates that glia are interacting with neurons in the network to coordinate the firings. Furthermore, our experimental findings are consistent with a mean field model for synchronized bursting.