The Muller cell: A functional element of the retina

Muller 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. Muller 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, Muller cells modulate neuronal activity by regulating the extracellular concentration of neuroactive substances, including: (1) K+, which is transported via Muller-cell spatial-buffering currents; (2) glutamate and GABA, which are taken up by Muller-cell high-affinity carriers; and (3) H+, which is controlled by the action of Muller-cell Na+-HCO3- co-transport and carbonic anhydrase. The two-way communication between Muller cells and retinal neurons indicates that Muller cells play an active role in retinal function.