Glial cells were non-neuronal cell that maintained homeostasis, formed myelin, and provided support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomic nervous system.
In 2369, Rao Vantika transmitted his neural patterns which he had encoded into bioelectrical pulses along Doctor Julian Bashir's glial cells, and "injected" his mind into Bashir's body. Jadzia Dax was later able to disrupt Vantika's control over Bashir by using an electromagnetic pulse, allowing the glial cells affected by Vantika to be beamed out. (DS9: "The Passenger")
In 2373, The Doctor tested Ensign Michael Parsons' glial cells with a DNA probe when he developed a microbial infection. (VOY: "Real Life")