**2. Applications of PBM to neurodegenerative disorders**

Utilization of tPBM in AD populations aims to increase reactive oxygen species (ROS) [17]. Alterations of ROS are associated with improved adenosine triphosphate (ATP) synthesis as this transcription factor is implicated in mitochondrial electron transport, gene expression, and inactivation of pro-apoptotic proteins and nucleic acids [17, 18]. Because the clinicopathology of AD is associated with dysfunction of the mitochondria, clinical implications of hypometabolism and hypoxia are integral to the understanding the mechanisms that underlie tPBM interventions [17, 18]. As the photons from tPBM are absorbed by CCO and hemoproteins within the brain, this suggests that this modality may alter electrochemical reactions and intracellular signaling as a function of improvements in the mitochondrial electron transport chain [19].

Applications of monochromatic wavelengths across the NIR and IR spectra via tPBM interventions affect complexes within functional mitochondrial pathways differentially [20–22]. NIR is considered as light that is emitted at a frequency between 700 nm and 1 mm whereas IR is photic energy emitted at 1 mm or 300 GHz and above [6]. Examination of homeostatic processes that are mediated by CCO, or complex IV within the electron transport chain, suggests that this target of PBM interventions can *Pharmacodynamic Implications of Transcranial Photobiomodulation and Quantum Physics… DOI: http://dx.doi.org/10.5772/intechopen.106553*

be stimulated to increase ROS production, oxygen consumption, and ATP synthesis when activated by red light and NIR frequencies between 633 nm and 808 nm [20–22]. Evaluations of alterations within complexes I, II, or III were not significant when exposed to frequencies between 633 and 808 nm [20–22].

Examination of higher irradiances within the NIR spectra indicate that exposure to 980 nm induced significant alterations within complexes III and IV [20]. Similar results were examined for exposure to IR at 1064 nm despite the effects being robust across complexes I, III, and IV. These results suggest that complex II within the electron transport chain may require stimulation within a different spectrum that is not within the wavelength frequencies associated with NIR or IR irradiances to induce physiological modulation for processes related to homeostasis [20–22].

Dysregulation of mitochondrial signaling negatively affects homeostatic processes and ATP availability as these cells are significantly implicated in metabolic regulation [5, 20–22]. As the mitochondria influences cellular signaling, proliferation, DNA and RNA synthesis, and gene expression, implications of ATP availability and the mitochondrial membrane potential must be considered. The mitochondrial membrane potential can be modeled as ΔΨm. As changes in mitochondrial signaling are derived as a function of ΔΨm, which can be altered according to electrochemical changes in the photon gradient, this indicates that photic energy can alter the mitochondrial respiratory chain and ATP synthesis [5].

The utilization of animal models in PBM research has allowed for the elucidation that applications of laser light across the 450, 620–680, and 760–895 nm frequency bands may alter complexes within the mitochondrial respiratory chain [5]. These models have allowed for mechanistic evaluations of responsivity to PBM as the elucidation of photoacceptors and photoreceptors are hypothesized to underlie the cellular pathways that are modulated by LLLT therapies [20–24]. Photoreceptors are specialized neuroepithelial cells that respond to photic stimulation within the range of 660 to 1000 nm whereas photoacceptors are non-specialized cells which alter transcription signaling after the photons have been absorbed [20, 23–26].

Laws of photobiology indicate that photons must be absorbed within the tissue prior to transformation into chemical, heat, or kinetic energy [27, 28]. Chromophores are molecular compounds that can convert photons into sources of energy [29]. The activation of chromophores by a specific wavelength of light emission is associated with alterations of biological signaling within the tissue by which it is located [29].
