|11. Wireless Communication in the Brain||MT|
A brain consists of many functionally specialized areas. Synchronization among relevant areas is critical for efficient performance of a specific task. The mechanism of synchronization between remote brain areas remains elusive. The previous chapter proposes that it could be mediated by electromagnetic (EM) coupling. According to this mechanism, long-range communication in the brain resembles the wireless communication used by televisions and mobile phones.
Wireless Communication in the Brain
In the human-invented wireless communication system, EM waves are radiated from transmitting antennas and absorbed by receiving antennas (Figure 1). The metal rod in an antenna is a conductor, in which electrons can flow in either direction, giving rise to electric currents. The EM wave consists of electric and magnetic fields, both can exert a combined force (Lorentz force) on the electrons, causing them to flow along the rod. Figure 1 shows only the effects of the electric field. The absorbed EM energy is then transduced into sound and/or images. The wireless communication in the brain may work in a similar manner:
The following sections will explain how EM waves are radiated from ion channels and how synchronized neuronal spiking can dramatically increase the radiation power. The roles of AIS microtubules as receiving antennas and energy transducer are discussed in the next chapter.
Ion Channels as the Transmitting Antennas
Neural activity is associated with the motion of ions passing through ion channels in the nerve membrane. From the viewpoint of wireless communication, the ionic current through a channel is equivalent to the electric current in a metal antenna. According to physical laws, EM waves will be radiated whenever an electric charge is accelerated, either the electrons in metal antennas or ions in ion channels. In the metal antenna, the acceleration of electrons is caused by the incoming EM fields. In the ion channel, ions are accelerated by the electrochemical force resulting from the differences in the electric potentials and ion concentrations on both sides of the membrane. Since ions carry charges, their accelerated motion will emit EM waves.
During the opening of a channel, a train of ions will pass through the channel one by one. Each ion will generate a pulse of EM wave. Thus, the frequency of EM waves emitted by these accelerated ions can be estimated from single channel currents. For voltage-gated sodium and potassium channels, the single channel current is about 1 - 4 pA (Aldrich and Stevens, 1987; Zagotta et al., 1988), where 1 pA = 10-12 ampere and 1 ampere = 6 x 1018 electron charge per second. Hence, about 107 ions can pass through a single channel within a second. The interval between two consecutive ions is then equal to 10-7 second, corresponding to a frequency of 10 MHz.
By using the dielectric-barrier discharge (DBD) method, it has been possible to detect the EM waves emitted from fingers which contain high density of neurons. Remarkably, the observed frequencies were found to be in the range 0.3-200 MHz (Zheng et al., 2016). Their median value happens to be about 10 MHz!
Synchronization for Maximizing Radiation Power
To influence a receiving brain region, the transmitting signals must be sufficiently strong. The power radiated from an accelerating charge can be obtained from the Larmor formula. In SI units, it is given by
where a is the acceleration, q is the charge, εo is the vacuum permittivity (= 8.8 x 10-12 F/m), and c is the speed of light (= 3 x 108 m).
In free space, the acceleration can be calculated from the formula,
Force = Ma = eE
where E is the electric field (~ 1 x 107 V/m), e and M are the charge and mass of an ion respectively. For Na+, e = 1.6 x 10-19 coulombs and M ~ 10-26 kg. The above equation gives a = 1014 m/s2. Within the channel pore, the acceleration could be reduced by friction. Let us assume a ~ 1013 m/s2. Hence,
Psingle ~ 10-28 W
which is approximately the power radiated by a single Na+ ion passing through an open channel. What will the power be for a group of accelerating ions? By simple multiplication, the total power radiated by N ions is given by
Ptotal = N x Psingle
However, if in a small region a large number of neurons fire synchronously, a group of ions may be accelerated simultaneously so that N ions can be treated as a single charge. In this case, instead of using the simple multiplication, we should substitute q = Ne into the Larmor formula. The result is
Ptotal = N2 x Psingle
Note that the total power is now proportional to the square of N. Thus, synchronization can dramatically increase the radiation power. This explains why synchronization is crucial for information transfer within the brain.
As estimated above, 107 ions can pass through a single channel per second. The open duration of a channel is about 1 ms. Therefore, during channel opening, roughly 104 ions can pass through a channel. These ions may be treated as a single charge in the Larmor formula. Suppose a network of 106 neurons fire synchronously, each having 1000 open channels, then N ~ 1013. Finally, the synchronized network may radiate
Ptotal = N2 x Psingle ~ 0.01 W
This radiation power could be sufficient to influence brain activity based on two different experimental findings. First, mobile phones have been demonstrated to affect brain activites (Chapter 10). The 3G phones radiate only 0.25 W, which will be attenuated further by the skull. Second, some neurons are very sensitive to electric field, even as small as 1 V/m (Francis et al., 2003; Reato et al., 2010). The EM wave consists of oscillating electric and magnetic fields. Given the radiation power, the electric field strength at certain distance from the source can be obtained from the formula available on this website. For power = 0.01 W and distance = 5 cm, the formula gives E = 10 V/m, which is sufficient to influence brain activity.
Author: Frank Lee