|10. Introduction to Electromagnetic Coupling||MT|
A large number of studies have demonstrated that low-intensity, microwave frequency electromagnetic fields (EMFs) can affect neural activity in the brain. The EMFs emitted from mobile phones have been shown to increase the power of brain waves at alpha, beta and theta bands (Valentini et al., 2007; Suhhova et al., 2013; Roggeveen et al., 2015; Henz et al., 2018), and produce neuropsychiatric effects including depression (Pall, 2016), possibly via activation of voltage-activated calcium channels (Pall, 2013).
It was suggested that EMFs may act directly on the voltage sensor within the ion channel (Saunders and Jefferys, 2007). However, detailed calculation indicates that direct activation of ion channels is improbable. 3G phones radiate only 0.25 W which will be attenuated further by the skull. The radiative EMF (EM wave) consists of oscillating electric and magnetic fields (Figure 1). Given the radiation power, it is possible to estimate the electric field strength at certain distance from the source. A formula is available on this website. Although the formula is accurate only for large distance, our purpose is to get a rough estimate. For power = 0.05 W and distance = 5 cm, the formula gives E = 25 V/m. This field strength is negligible compared with the resting membrane potential field of ~ 107 V/m (Chapter 3), but is sufficient to influence neural activities as described above.
Chapter 3 posits that the external electric field could be amplified by the microtubules located at the axon initial segment (AIS) where microtubules are flexible to move. Microtubules are enriched with negatively charged residues which may influence the open probability of voltage-gated ion channels in the membrane. Their direct interaction with the external electric field could cause them to move away from the membrane, thereby increasing the open probability of voltage-gated ion channels. Further details are described in Chapter 12.
If the EM coupling does exist in the brain to mediate long-range synchronization (Chapter 9), a group of locally synchronized neurons must be able to generate endogenous EMFs so that the synchronous spiking of these neurons may immediately induce spiking in distant areas via EMFs. How are the endogenous EMFs generated?
Theoretical studies of neural activities commonly treat the neuronal circuit as an electrical circuit. For instance, to simulate ephaptic coupling, Stacey et al. (2015) employed a popular model system known as NEURON (Hines and Carnevale, 1997), where the nerve membrane was modeled as a combination of resistors and capacitors. The ion movement across the nerve membrane was described simply as the electric currents through these resistors and capacitors. This approach does not take radiative EMFs into account.
According to physical laws, the accelerated motion of any charged particles will radiate EM waves. Ions are charged particles. Before they enter ion channels, they move slowly and randomly in the solution. When a neuron fires, a large number of ions will be passing through the channels, with acceleration driven by the "electrochemical force" which consists of both electric force due to membrane potential difference and the chemical force due to ion concentration difference across the membrane (Figure 2). The accelerated motion will generate EM waves, with a frequency around 10 MHz (next chapter).
The local electric field (non-radiative) that mediates ephaptic coupling decreases rapidly with distance (proportional to 1/r2). By contrast, the ion channel in a polarized membrane resembles a "dipole antenna" where the radiative electric field is proportional to 1/r (Wikipedia). Such slow distance-dependence makes the radiative EMF well suited for long range synchronization.
Author: Frank Lee