Electrical properties of the rod syncytium in the retina of the turtle.

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Abstract
Intracellular responses were recorded from rods in isolated eye-cups of the snapping turtle Chelydra serpentina. Responses to flashes of small (less than 100 .mu.m dimeter) and large (1000 .mu.m diameter) spots of 500 nm light were studied. Responses produced by small and large diameter spots which delivered less than 0.3 photons .mu.m-2 had the same shape. The responses produced by large spots were nearly 10 times greater in amplitude. The difference in amplitude is termed enhancement. Perfusing an eye-cup with a Co2+-containing medium blocked synaptic transmission from receptors to horizontal cells but did not affect the responses of rods. The membrane conductance of a single rod, estimated by 3 independent methods, was approximately 1-2 .times. 10-9 mho. Enhancement was predicted by a mathematical model which treated rods as an electrical syncytium. The space coefficient describing the spread of current was approximately 65 .mu.m, indicating that the coupling conductance between rods was relatively high. When the intensity of a small spot was increased from 0.3 photons .mu.m-2-6 photons .mu.m-2, the shape of the response was unchanged. When the intensity of a large spot was increased to more than 0.3 photons .mu.m-2, the voltage during the recovery phase was decreased. This decrease was termed disenhancement. The voltages produced by bright, large and small diameter spots which delivered the same quantity of light to the impaled rod were compared. The voltage produced by a large diameter spot became for a short period during the recovery phase less than the voltage produced by a small diameter spot. The response to a large spot included during recovery an active process which was not apparent in the response to a small spot. The time course of the SD of responses produced by a dim, small spot was similar to the time course of the mean, indicating that only 1 process contributed to the response. The time course of the SD of responses produced by a dim, large spot deviated from the time course of the mean, indicating that more than 1 process contributed to the response. The time course of the conductance change produced by a large spot differed from that of the voltage. The net conductance change is probably the sum of a conductance decrease initiated by the absorption of light and a conductance increase which is voltage and time dependent. A slow conductance increase accounted for the phenomenon of disenhancement.

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