Mode of origin of large extracellular photovoltages from the retinal rods of the frog.

Arrays of extracellular microelectrodes were advanced into the receptor layer of isolated frog retinae to measure interstitial voltage and resistance. In addition, current was locally injected through one of the micro-electrodes and spatial profiles of local resistance were recorded. A large increase of local resistance in the proximal half of the receptor layer was frequently associated with the occurrence of large extracellular voltages arising from rods. The large responses presumably occur at sites where the micropipette tip makes tight contact with membrane process of the rod inner segment and a quasi-intracellular mode of recording results.