The ionic and electrophysiological properties of Müller cells, the principal glial element of the vertebrate retina, were investigated. The membrane potential of enzymatically dissociated and in situ Müller cells was about ‐80 mV and depended on external K+ concentration in a manner that was described by the Goldman‐Hodgkin‐Katz equation with a Na+‐K+ permeability ratio of 0.037. The current‐voltage relation showed marked inward rectification, with the input resistance at the resting potential being about 30 M omega for dissociated cells and about 3 M omega for in situ cells. In situ Müller cells were found to be electrically coupled to each other which could explain their lower resistance. We conclude that Müller cells are similar to other types of glia. In spite of a finite Na+ permeability their membrane potential is determined mainly by K+, they are electrically inexcitable and form an electrically coupled network in the retina.
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