The Müller cell: a functional element of the retina.

Müller cells are the principal glial cells of the retina, assuming many of the functions carried out by astrocytes, oligodendrocytes and ependymal cells in other CNS regions. Müller cells express numerous voltage-gated channels and neurotransmitter receptors, which recognize a variety of neuronal signals and trigger cell depolarization and intracellular Ca2+ waves. In turn, Müller cells modulate neuronal activity by regulating the extracellular concentration of neuroactive substances, including: (1) K+, which is transported via Müller-cell spatial-buffering currents; (2) glutamate and GABA, which are taken up by Müller-cell high-affinity carriers; and (3) H+, which is controlled by the action of Müller-cell Na(+)-HCO3- co-transport and carbonic anhydrase. The two-way communication between Müller cells and retinal neurons indicates that Müller cells play an active role in retinal function.