High-pass filtering of small signals by the rod network in the retina of the toad, Bufo marinus.

The electrical spread of excitation in the network of rod photoreceptors was studied by intracellular recording in the isolated, perfused retina of the toad, Bufo marinus. Experiments with dim, bar-shaped flashes of light revealed that the rod network behaves as a high-pass filter to laterally propagating small signals. Such a behavior had been found earlier in the turtle (Detwiler et al., 1980). Three electrical equivalent circuit models that can explain this behavior were considered and analytical solutions to the network equations were obtained. By fitting these analytical expressions to linear responses elicited by weak light flashes and to voltage excursions elicited by extrinsic current injections, values for the circuit parameters were determined. Values obtained by independent methods were consistent. The effects of changing each of these parameters in turn upon the high-pass filtering of small signals were then predicted. These predictions provided a framework for an analysis of the ionic basis of the underlying mechanism, which is described in the following paper.