Tripartite synapses: glia, the unacknowledged partner

According to the classical view of the nervous system, the numerically supe rior glial cells have inferior roles in that they provide an ideal environm ent for neuronal-cell function. However, there is a wave of new information suggesting that glia are intimately involved in the active control of neur onal activity and synaptic neurotransmission. Recent evidence shows that gl ia respond to neuronal activity with an elevation of their internal Ca2+ co ncentration, which triggers the release of chemical transmitters from glia themselves and, in turn, causes feedback regulation of neuronal activity an d synaptic strength. In view of these new insights, this article suggests t hat perisynaptic Schwann cells and synaptically associated astrocytes shoul d be viewed as integral modulatory elements of tripartite synapses.