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The outer segments comprise dense stacks of polyunsaturated lipid membranes in rods, these membranes are densely packed with the light-absorbing G-protein coupled receptor rhodopsin and the rest of the molecular machinery of phototransduction. (A) Schematic of a portion of the photoreceptor layer of the Leopard frog ( Rana pipiens) showing four rods and three cones. Photoreceptor outer segments are filled with membranes with polyunsaturated lipids hypothesized to produce ultraweak photon emission.
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(2019) makes it clear that this floor of noise is not set by the UPE, it nonetheless focuses attention on an interesting, ubiquitous photonic feature of biological systems. Amazingly, this greatly reduced thermal isomerization rate still dictates the absolute sensitivity of night vision by setting a floor of noise, the Eigengrau, of the entire visual system ( Hecht et al., 1942 Barlow, 1956 Naarendorp et al., 2010). (2019) indirectly lend support to the long-standing idea that the spontaneous activation of rhodopsin arises from an intrinsic susceptibility of the rhodopsin chromophore in situ to thermal activation ( Luo et al., 2011), a susceptibility that the evolution of rhodopsin was unable to eliminate, despite greatly lowering its rate relative to the rate of thermal isomerization in vitro ( Kim et al., 2003). No doubt this was achieved by evolutionary selection pressure on the rhodopsin molecule itself, on the biochemistry of rod phototransduction, and on retinal cell types and circuitry ( Pugh, 2018). The ability of rods to respond reliably to single photons is an extraordinary capability-one that clearly expanded the photic environment in which vertebrates could survive. provide evidence very important for resolving this debate. In this issue of the Journal of General Physiology, Govardovskii et al. Specifically, recent publications have proposed the hypothesis that spontaneous ultraweak photon emission (UPE) in the eye is the mechanism underlying the photon-like CNG current fluctuations of dark-adapted rods ( Bókkon and Vimal, 2009 Wang et al., 2011 Salari et al., 2015, 2016 Li and Dai, 2016). While the evidence is compelling that these spontaneous “photon-like” events arise from activation of rhodopsin and consequently phototransduction, the physical mechanism of rhodopsin activation is somewhat controversial.
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Some of the fluctuations, occurring about once per 100 s in mammalian rods, have a time course and amplitude that makes them indistinguishable from the single-photon response (SPR)-the electrical response of the rod to the isomerization of a single rhodopsin molecule ( Baylor et al., 1980, 1984 Pugh, 2018). In absolute darkness, the CNG channel current of rod photoreceptor outer segments exhibits fluctuations.