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Abstract
For several decades the physical mechanism underlying discrete dark noise of photoreceptors in the eye has remained highly controversial and poorly understood. It is known that the Arrhenius equation, which is based on the Boltzmann distribution for thermal activation, can model only a part (e.g. half of the activation energy) of the retinal dark noise experimentally observed for vertebrate rod and cone pigments. Using the Hinshelwood distribution instead of the Boltzmann distribution in the Arrhenius equation has been proposed as a solution to the problem. Here, we show that the using the Hinshelwood distribution does not solve the problem completely. As the discrete components of noise are indistinguishable in shape and duration from those produced by real photon induced photo-isomerization, the retinal discrete dark noise is most likely due to ‘internal photons’ inside cells and not due to thermal activation of visual pigments. Indeed, all living cells exhibit spontaneous ultraweak photon emission (UPE), mainly in the optical wavelength range, i.e., 350–700 nm. We show here that the retinal discrete dark noise has a similar rate as UPE and therefore dark noise is most likely due to spontaneous cellular UPE and not due to thermal activation.
Highlights
Photoreceptor cells have two components of the dark noise: a continuously low amplitude component (% 0.2 pA) and a spontaneous discrete component (% 1 pA) [1]
In this paper we have tried to answer to this question that why there is spiking activity of photoreceptors when there is no external photon absorbed by it? We have considered two possible mechanisms for these false alarms in the eye: (1) thermal energy and (2) spontaneous ultraweak photon emission (UPE)
The Arrhenius equation based on the Boltzmann distribution gives the activation energies of discrete dark noise at a level which is around half the energy for activation in vertebrate rod and cone pigments
Summary
Photoreceptor cells have two components of the dark noise: a continuously low amplitude component (% 0.2 pA) and a spontaneous discrete component (% 1 pA) [1]. We analyze in detail the calculations presented in the work by Luo et al [2] (i.e. the most important paper in the literature on this topic) and identify several shortcomings which question the validity of explaining the dark noise of rods and cones by only assuming a thermal activation energy process. The equipartition theorem [13] cannot be applied for these modes; the application of the Hinshelwood distribution to model the dark noise of photoreceptors is questionable. The m value for Bufo red rhodopsin with λmax = 500 nm is obtained based on the above equation where Ea,H is the thermal isomerization activation energy of 48.03 kcal/mol, Ea,B is the apparent thermal activation energy of 21.9 kcal/mol at absolute temperature T = 296 K [2], and R is the universal gas constant. A value of m different from 45 and 58 is obtained for A1 human red cones (see Table 1)
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