|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:
- Glia might receive direct signals from neurons. (synapse-like connections)
- Glia might respond to neurotransmitter released for non-synaptic (ectopic) sites.
- Glia might respond to transmitter released from other glia
- The receptors on glia might be activated by 'ambient' neurotransmitter.
No matter how these receptors are stimulated, they can depolarize the glial cells and even induce calcium transients. Lalo et al. explain that these actions might cause the glial cells to release lactate which is taken up by neurons as an energy source.
In short, the role of these glial cells might be mainly metabolism control near synapses, and the ionotropic neurotransmitter receptors might be the mechanism that signals when, where, and how much metabolism control is needed.
Lalo U, Pankratov Y, Parpura V, & Verkhratsky A (2011). Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells. Biochimica et biophysica acta, 1813 (5), 992-1002 PMID: 20869992