Current source density analysis of the electroretinographic d wave of frog retina.

1. Dark flashes were used to evoke spatiotemporal profiles of the d wave of the electroretinogram in frog retina. The current source density technique was used to compute the extracellular current sinks and sources underlying the d wave. 2. The largest d wave current was a sink at the outer plexiform layer (OPL sink) and a source at the inner plexiform layer (IPL source). Several properties of this sink/source, including its relative insensitivity to Ba2+, suggest that it arises from hyperpolarizing bipolar cells. 3. There was a slower IPL sink, along with a source at the inner limiting membrane (ILM source). Its properties, along with its sensitivity to Ba2+ and enhancement by picrotoxin, suggest it arises from the K+ spatial buffer currents of Müller cells. This IPL sink/ILM source underlies the M wave. 4. A small photoreceptor sink/source also contributes to the d wave. This is most significant during the very beginning of the d wave. 5. Our results indicate that in frog retina and at the stimulus conditions used in this paper, the d wave originates primarily and directly from the hyperpolarizing bipolar cells. Photoreceptors likely contribute a small amount, especially during the initial portion of the d wave. There is possibly a small Müller cell contribution.