Gap junctions connect the cytoplasms of neighboring cells, allowing small second messengers to diffuse between cells. Astrocytes are connected via gap junctions and can respond to changes in the environment through the propagation of calcium waves. Astrocytes have also been implicated in contributing to neuronal excitability, which may relate to the astrocytic calcium wave phenomenon. Rouach et al. demonstrated that the effect of culturing neurons with astrocytes was increased gap junctional communication (measured by dye diffusion) among the astrocytes in a manner that depended on the age and ratio of neurons to glia of the coculture and correlated with the activity of the neurons. Astrocytes cultured with neurons exhibited increased recruitment of glial cells participating in mechanically induced calcium waves. One mechanism for the enhanced glial gap junctional communication is that neurons stimulated the expression of connexin 43 in the glial cells.