This is also the range of intensities relative to the mean background level typical of natural scenes ( Mante et al., 2005). This feature allows cones to respond over about two log units of light intensity centered on the background level, regardless of the absolute background intensity ( Burkhardt and Gottesman, 1987 Perlman and Normann, 1998). That is, flashes of a given intensity measured as a percentage of the background intensity generate the same amplitude response regardless of the absolute magnitude of the background light ( Normann and Werblin, 1974 Normann and Perlman, 1979 Burkhardt and Gottesman, 1987 Burkhardt, 1994). Over the first six log units of light intensity above threshold, cones respond with constant contrast. In human cones, for example, when over 90% of the visual pigment (VP) is bleached, the dark current amplitude is only half that measured in the dark ( Kenkre et al., 2005) and the same is observed in cones of non-mammalian species ( Jones et al., 1993). In cones, extremely intense steady light suppresses the circulating current for only a brief moment and it then recovers to a new steady value, reflecting reopening of the CNG channels. Indeed, under bright steady illumination the outer segment dark current can be fully suppressed in rods (response saturation), but not in cones (response cannot be saturated) ( Jones et al., 1993 Burkhardt, 1994 Kenkre et al., 2005). Rods, however, adapt over a smaller range of light intensities than do cones ( Baylor et al., 1984 Fain et al., 1989 Matthews et al., 1990 Schnapf et al., 1990). Cones adjust their photosensitivity as a function of mean background intensity, and thus can respond to changes over 9 log units of light intensity, the range of illuminance from a clear night sky (2 × 10 −3 lux) to that by direct sunlight (1.3 × 10 5 lux) (). Thoroughly dark adapted rods yield a detectable signal, a signal larger than their intrinsic noise, when only a single visual pigment molecule is excited by light ( Baylor et al., 1979b) while cones yield a detectable signal only when light flashes excite 4 to 10 visual pigment (VP) molecules per cell ( Naarendorp et al., 2010 Koenig and Hofer, 2011 Korenbrot, 2012b). The functional features of the light response of rods and cones are well suited to the ecological needs of vertebrate behavior. Light suppresses this current by closing the outer segment CNG channels and, as a consequence, the cell membrane potential hyperpolarizes ( Baylor and Fuortes, 1970 Tomita, 1971 Baylor and Hodgkin, 1973 Schwartz, 1973) initiating the process of vision. The circulating current is an outward K + ion flux across the inner segment membrane mediated by voltage-gated K + channels ( Bader et al., 1982 Hestrin, 1987 Barnes and Hille, 1989 Maricq and Korenbrot, 1990a, b) and an inward Na + and Ca 2+ ion flux across the outer segment membrane mediated by cyclic nucleotide-gated ion channels (CNG channels) ( Fesenko et al., 1985 Yau and Nakatani, 1985a). In the dark, rod and cone photoreceptors of the vertebrate retina sustain a circulating ionic current that flows along the extracellular space from the inner to the outer segment ( Hagins et al., 1970). Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized.
These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane.
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