Photoreceptor coupling in retina of the toad, Bufo marinus. II. Physiology.

1. Intracellular recordings were made from red rods in the isolated toad retina in order to measure the extent of the rod coupling. 2. The network space constant, lambda, was determined from edge-response profile measurements on single cells (lambda = 19 +/- 7 micrometer). 3. The network input resistance was estimated to be between 45 and 100 M omega. 4. Both discrete and continuous resistance networks, which were based on the anatomical observations, were analyzed. For lambda/D greater than 1, a continuous network provides an accurate description of the discrete network (D, cell spacing for a square grid). 5. The network analysis was used to estimate the specific resistance of junction membrane (0.75--1.5 omega.cm2) and the conductance provided by a single junctional channel (2.0--3.9 X 10(-12) mho). 6. The anatomical and physiological results provide strong evidence that the rods are electrically coupled. It is concluded that photoreceptor coupling provides averaging of the voltages in adjacent cells. The consequences of this averaging are discussed.