Abstract
The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.
Highlights
The rods of the several mammals studied here all show the phenomenon of light adaptation
In our work on cat rods we pointed out that the ability of these cells to adapt to light is quite important, because of a relatively high cone threshold in the cat retina due to the paucity of cones
Without adaptation, the rods in the cat retina would saturate with light before the cones were able to fully take over vision, producing a sensitivity discontinuity in mesopic vision
Summary
The retinal rods of almost every cold-blooded vertebrate species that has been studied are found to adapt to light (Dowling and Ripps, 1972; Baylor and Hodgkin, 1974; Coles and Yamane, 1975; Kleinschmidt and Dowling, 1975; Fain, 1976; Hemil~i, 1977; Baylor et al, 1979a, 1980; Lamb et al, 1981; Copenhagen and Green, 1985; Leibovic et al, 1987) This adaptation property can be recognized in one or Address reprint requests to Dr K.-W.
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