Electromagnetic characteristics of in vivo nerve fibers

IntroductionAs a complex physical system, the human brain has often been a popular topic in life science research, especially in the study of neural information generation and transmission in neurobiology (Adolphs, 2015; Liu, 2018). Electrochemical theory played a leading role in the early studies, although it could not perfectly explain many underlying problems, such as the generation and origin of consciousness (Tononi, 2004; Tegmark, 2015). Meanwhile, action potential signaling mainly relies on the transmembrane transport of ions to cause the potential difference between the excitatory site and the non-excited site, thereby forming a local current. However, traditional electrochemical theories cannot explain the transmission mechanism of high-frequency signals in living organisms, such as how THz and FIR signals that are generated by various redox reactions in biological cells are transmitted. In this context, research of the physical mechanisms that coexist with existing electrochemical theories may reveal new prospects for explaining high-level signal transduction mechanisms, including whether there are more efficient physical ways to transmit and store information in the nervous system. Neuro-electromagnetics provides a new way to study the mechanism of the high-level communication and rapid response of the nervous system. However, it also has many problems, such as how to generate and transmit signals efficiently.Recently, studies on THz-FIR spectroscopy have confirmed that many oxidative metabolic processes in biological cells produce a large number of biological photons, covering the spectrum range of THz-FIR. Many of the biological macromolecules that play an important role in life activities, such as proteins and phospholipids, also have vibration frequencies in the THz-FIR range (Wetzel and LeVine, 1999; Laman et al., 2008; Miller and Dumas, 2010). The ion channels on the surface of the nerve membrane are not only related to action potential activity but also show that the process of ion transport across the membrane generates THz-FIR radiation (Li et al., 2021; Liu et al., 2021; Guo et al., 2022; Hu et al., 2022; Wang et al., 2022). These are several potential sources for the THz-FIR signals transmitting on the nerve fibers. The THz-FIR signals can also affect the activity of the proteins in the cells (Chao et al., 2021), DNA molecular unhelix (Wu et al., 2020), and the selective transmission of ion channels (Li et al., 2021). As information carrier candidates, THz biophotons may become high-speed signal carriers that connect intracellular organelles and cells independently of electrical and chemical transmissions (Xiang et al., 2020).Some nerve fibers in the vertebrate nervous system are a kind of EM waveguide structure (i.e., wrapped by high resistance and the low capacitance of nerve myelin sheath). This structure can accelerate the conduction of action potential. Meanwhile, in the FIR range, it supports biological photon signal transmission between cells (Kumar et al., 2016; Zangari et al., 2018; Liu et al., 2019). This natural waveguide structure enables signal transmission at a speed of up to 108 m/s, which is far higher than the conduction speed of the action potential. The nerve myelin sheath with a dense membrane structure can isolate the polar water environment on both sides of the membrane and greatly reduce the attenuation of the THz-FIR wave due to polarization loss in the transmission process.Our current work is focused on how THz-FIR waves behave in nerve fibers, and how the THz-FIR waves hop and exchange energy with periodic nodes of Ranvier structures. In view of this hypothesis, we build a transmission model of nerve fibers and nodes of Ranvier (as shown in Figure 1). The myelinated nerve axon can be regarded as a fiber transmission line that is loaded with layered annular media (i.e., nerve cell fluid, myelin sheath, and extracellular fluid). In contrast to optical fiber, the energies of THz-FIR waves concentrate in the myelin sheath area rather than the fiber core area because of its high dielectric constant; while the physical environment of the nerve fibers are polarity solutions, and there is no obvious boundary. Meanwhile, the node of Ranvier is a periodic structure that is formed by the discontinuity of the myelin sheath, which will inevitably produce energy changes. The main work and innovations of this article are as follows: 1) the field equations and dispersion curves of THz-FIR wave transmission in both myelinated and unmyelinated bare nerve fibers are obtained; 2) the transmission efficiencies of THz-FIR electromagnetic waves in different modes are quantitatively analyzed; and 3) a new mode matching algorithm is proposed that can quickly calculate the scattering parameters and energy exchange of THz-FIR wave transmission at the nodes of Ranvier.FIGURE 1. Schematic diagram of the nerve fiber structure. (A) Cross section of the nerve fiber and (B) 3D perspective graphics of the nerve fiber.MethodsTHz-FIR transmission modes in myelin-sheathed nerve fibersAs shown in Figure 1, the nerve fiber is structurally composed of three regions, and the dielectric constants are represented by ε1, ε2, and ε3, respectively. Compared with the intracellular fluid and extracellular fluid of nerve cells (i.e., water as the solvent and have strong absorption characteristics in the THz-FIR band), the myelin sheath is a tight membrane structure with a high dielectric constant and low loss in the THz-FIR band (Liu et al., 2019). The relationship of the dielectric constant of the three-layer media can be expressed as ε3
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