|Astrocytes, a form of glial cell (source)| In a sense these
cells are 'filler'. When the brain is damaged, it is glia not
new neurons which grow into the void. (This can sometimes turn
cancerous and lead to glioma) A recent review paper poetically
summarizes the traditional role of glia: "Astroglial cells were
long considered to serve merely as the structural and metabolic
supporting cast and scenery against which the shining neurones
perform their illustrious duties." (Lalo et al., 2011)This
lovely summary is an obvious set up for a paper showing that
"actually glia are quite important." And indeed they are. Even
though they don't fire action potentials, glial cells have
electrical activity and are involved in information processing.
Glial cells have receptors for neurotransmitters (such as
glutamate and GABA). These are the very same types of receptor
that neurons use receive signals from other neurons at the
synapse. Lalo et al. point out four different ways that these
receptors might work on glial cells: