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The concept that the activity of neurons is both passively and actively modified by the surrounding glial and other non-neuronal cells is found in this review to be supported, but not conclusively demonstrated, by recent and previously unreported experiments. The evidence makes such non-neuronal control seem highly likely in the vertebrate retina. In particular, the non-neuronal horizontal cells of the retina were found to sunumate, and apparently to transmit, changes in their membrane potentials (the L-response type of S-potential) over distances many times the span of a single such cell. Available evidence is consistent with the idea that these potential changes can affect the neuronal transmission of excitation from the photoreceptors to the ganglion cells. This is proposed as the basis for a mechanism accounting for the low-luminance portion of light adaptation and for certain retinal functions of spatial summation and movement detection.