Sensitivity of toad rods: Dependence on wave‐length and background illumination.

Abstract

SU3MMARY1. There are five morphological types of photoreceptors in the retina of the toad, Bufo marines: red and green rods, single cones, and the principal and accessory members of double cones. The largest and most abundant of these is the red rod.2. Intracellular recordings were used to investigate the dependence of the sensitivity of red rod responses on wave-length and background light.3. The spectral sensitivity of dark-adapted and moderately lightadapted red rods can be satisfactorily fitted with the absorbance spectrum of the red rod photopigment. There are no significant contributions to red rod responses from cones or green rods.4. In contrast, L-type horizontal cells, whose responses are dominated by input from the red rods near threshold, can be shown also to receive input from cones.5. Steady background light produces a response in the red rods consisting of an initial hyperpolarization, followed by a decay of potential to a steady-state plateau level. The slow decay of response amplitude is accompanied by an increase in sensitivity to increment test flashes.6. The increment sensitivity at steady-state decreases with increasing background intensity according to a modified Weber-Fechner relation.The dependence of increment sensitivity on the wave-length of the background light can be predicted by the red rod spectral sensitivity, showing that cones do not influence the light adaptation of rods. 7. At a background intensity of 11-5 log equivalent quanta cm-2 sec', sensitivity begins to deviate from the Weber-Fechner relation. In background light one log unit brighter, the rods are completely saturated. 8. Small responses having the spectral sensitivity of cones can be recorded from saturated rods. These potentials have a prominent off response whose wave form resembles the d-wave of the e.r.g. 9. A comparison of the increment-sensitivity curves of single receptors shows that rods are light-adapted by backgrounds one thousand times dimmer than those which affect cones. The increment-sensitivity curves of rods and cones cross, so that single cones become more sensitive than single rods even before the rods begin to saturate. INTRODUCTIONThere is now abundant evidence that much of visual behaviour has its basis in the physiology of photoreceptors. Psychophysical phenomena such as dark adaptation, light adaptation, wave-length discrimination, contrast sensitivity, directional sensitivity, and lateral summation all have been observed to some extent in the signals of single rods and cones (Grabowski & Pak, 1975; Kleinschmidt, 1973; Fuortes, Schwartz & Simon, 1973; Baylor, Fuortes & O'Bryan, 1971; Baylor & Fettiplace, 1975). For this reason, the investigation of receptor responses should lead to a clearer understanding of the neural basis for visual perception. Although much has been learned over the last few years about the physiology of vertebrate cones (Baylor & Fuortes, 1970;Baylor et al. 1971;Fuortes et al. 1973;Baylor & Hodgkin, 1973 Baylor, Hodgkin & Lamb, 1974; Baylor & Fettiplace, 1975), les...

Light-evoked oxygen responses in the isolated toad retina

“…The light-evoked PO2 changes, therefore, are believed to reflect primarily changes in photoreceptor metabolism, and are expected to be rod dominated since the sensitivity was similar to that measured from toad rods (Fain, 1976), The illumination required for a half-maximal response from single toad rods, measured by Fain (1976), was in the upper end of the range of illuminations which produced half-maximal PO2 (and c-wave) responses in this study. Amphibian retinas also contain cones (Liebman and Entine, 1968) which absorb maximally at 575 nm, and the cone population in the toad retina is substantial (Zhang and Straznicky, 1991), but their size and their percentage of total photopigment is small compared to the rods.…”

Adaptation of the odour‐induced response in frog olfactory receptor cells

“…This contrasts with the situation in other sensory receptors, which appear to adapt in a more graded manner during continuous stimulation, exhibiting a less steep decline in sensitivity as the adapting stimulus is increased. For example, toad rod photoreceptors adapt by decreasing their sensitivity inversely with steady light intensity according to Weber's law (Fain, 1976;Baylor et al 1980), while in turtle hair cells adaptation simply displaces the stimulus-response relation to higher stimulus levels without a change in form (Crawford et al 1989). Even more profound were the decreases in sensitivity estimated from the peak spike firing frequency evoked by the test pulse following adaptation to the pre-pulse.…”

The initiation of excitation and light-adaptation in Limulus ventral photoreceptors

“…To find the action spectrum of light adaptation, we measured, at several wavelengths, the intensity of a steady adapting light needed to produce a criterion desensitization. (This strategy is similar to that used by Stiles, 1939;Fuortes et al, 1961;and Fain, 1976. ) The details of the procedure for determining the criterion action spectrum of excitation were as follows.…”