It has been known for more than a century that living cells emit ultra-weak light (UWL) or photons [JJC13, DUI97] – a phenomenon also refered to as bio-photons. However, it is still not clear what role these bio-photons play nor how they are generated in the first place [CP14]. Although the exact process responsible for the bio-photons has not been identified, there is a strong indication that metabolic processes are involved [ZTZ73]. It is tacitly assumed that bio-photons originate from radiative decay of electronically excited molecular species, which are generated chemically during oxidative metabolic processes in the mitochondria of microbial, plant and animal cells. These molecules, known as Reactive Oxygen Species (ROS), have a number of variants with multiple mechanisms for their pro- ductions [PPR14]. Among these variants are the triplet excited carbonyls (emitting mostly in the visible part of the spectrum, 350–550 nm), dimolar (emitting at around 634 nm and 703 nm) and monomolar singlet oxygen (emitting at 1270 nm) [PPR14]. The mechanisms underlying the generation of ROS can be classified as occurring either during the normal functioning of the cell (i.e., the electron transport chain) or initiated by various biotic or abiotic stressors and oxidative factors (i.e., chemically induced oxidative stress) [PP11]. It is important to note that other forms of bio-luminescence, such as fluorescence or phosphorescence, also occur in living organisms. What distinguishes bio-photons as described above is the spontaneous nature of their emission.

It is possible that bio-photons are merely a byproduct of such metabolic activity in the cells. However, a number of studies point to a functional role of the bio-photons within organisms, from the control of embryonic development [Gur23, GA28, VB15] to information transmission in the nervous system [BSTA10, SVB+15, ZKT+18, GKK04, RTB+11]. A recent study found that light can trigger a physiologically meaningful function (i.e., thermogenesis) deep in the brain (via the light sensitive protein opsin5) [ZDU+20]. A number of experiments have established a correlation between the intensity of the bio-photons and the firmly accepted electro-chemical signalling of the neurons [TD14]. In particular, the application of glutamate, which is the most abundant neurotransmitter in the brain, has shown to significantly enhance the bio-photon emission in mouse brain slices while certain action potential inhibitors (e.g. Tetrodotoxin) or anesthetics (e.g. procaine) suppressed the bio-photon emission. In other studies, the electroencephalographic (EEG) activity from the brain was found to be strongly correlated with the emission intensity of the bio-photons [KTS+99, DSP12]. Such results manifest a potential connection between brain activity and bio-photons generation, but for an indication of a functional role we look at the capability of neurons to guide light and, thus, their potential for being involved in neuronal communication [SVB+15]. Note that even if the observed bio-photon intensity is weak and, thus, seemingly ineffective as a signal carrier, the intensity may be much larger inside cells [BSTA10]. The light guiding capacity of neurons has been shown both in models and experiments [SWD10, HMB+94, DGB+20, KBT+16]. The first direct experimental evidence obtained using rat spinal nerve roots suggested that light stimulation can generate bio-photons, which are transmitted through the neural fibers [SWD10]. In addition to this, multiple experiments have shown a directional dependence for light propagation in the neural tissue; including the observation of increased transmission along the axes of the white matter tracts [HMB+94] and a recent study in which the scattering coefficients of the white matter in spinal cord of various species were found to be significantly lower in the longitudinal direction i.e., along the axis of the axons [DGB+20]. These observations spurred theoretical modelling of the light guiding properties of axons. A detailed model based on photonic simulation software found that myelinated axons, i.e. axons enclosed in a myelin sheath, are capable of guiding a large range of wavelengths over distances relevant for connecting separate parts of a brain [KBT+16].

Current projects:

References – Bio-Photons

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