|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. For instance, in the awake cat, a sudden change of a visual pattern induces synchronization between areas of the visual and parietal cortex, and between areas of the parietal and motor cortex. Despite the long distance between synchronized areas, the synchronization occurs with zero phase lag (Roelfsema et al., 1997). The zero lag synchronization has also been observed in hippocampal-prefrontal synchrony during working memory, and prefrontal-amygdala synchrony during anxiety (Harris and Gordon, 2015).
Long-Range Synchronization Suggests Wireless Communication
The zero lag synchronization between distant areas is remarkable, considering that synaptic transmission and axon conduction will cause time delay. Its underlying mechanism remains unclear. However, we all know that nothing can propagate faster than electromagnetic (EM) waves. In our daily life, EM waves have been commonly used in wireless communication. The long-range synchronization suggests that they could also be used by the brain.
In the human-invented wireless communication systems, EM waves are radiated from transmitting antennas and absorbed by receiving antennas. The absorbed EM energy is then transduced into sound 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 firing can dramatically increase the radiation power. The role of microtubules at the AIS in receiving EM waves and subsequent modulation of neuronal excitability is 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. According to physical laws, EM waves will be generated whenever a charge is accelerated. In the ion channel, ions will be accelerated by the electrochemical force. Since ions carry charges, the accelerated motion will radiate EM waves.
As a channel opens, a train of ions will pass through the channel one by one. Thus, the frequency of EM waves radiated from 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.
The Modulating Frequencies Revealed by EEG
Telecommunications involve two types of signals: carrier signals with high frequency and modulating signals with low frequency (see this article). When we tune in to a television or radio channel, the channel frequency refers to the carrier frequency, which is fixed for a particular TV or radio station. The high frequency estimated above belongs to the carrier frequency which does not contain information. The content information is encoded in the wave patterns of modulating signals. For the brain, the modulating frequencies have been revealed by electroencephalography (EEG), but the carrier frequency is too high to be resolved by this method.
The modulating signals, known as brain waves, are divided into several bands.
Synchronization for Maximizing Radiation Power
To influence the 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 (1 electron charge = 1.6 x 10-19 coulombs), εo is permittivity (= 8.8 x 10-12 F/m), and c is the speed of light (= 3 x 108 m).
In a free space, the acceleration can be calculated from the formula,
Force = Ma = qE
where E is the electric field (~ 1 x 105 V/cm) and M is the mass of Na+ (~ 10-26 kg). The result is a = 1014 m/s2. Within the channel pore, the acceleration is likely reduced by the friction. Let us assume a ~ 1012 m/s2. Hence,
Psingle ~ 10-29 W
which is approximately the power radiated by a single Na+ ion passing through an open channel. What will the power be if a large number of neurons fire simultaneously? By simple multiplication, the total power is given by
Ptotal = N x Psingle
where N is the total number of accelerating ions. As estimated above, 107 ions can pass through a single channel per second. The number of ion channels in the olfactory system of the locust is at least 14,000 per neuron (Buchholtz et al., 2002). It is reasonable to assume that the average number of ion channels in a human neuron is on the same order of magnitude. A human brain contains roughly 100 billion (1011) neurons. Therefore, if all brain neurons fire within a second, N ~ 1022 and Ptotal ~ 10-7 W.
This value is very small compared with the power (1 W) emitted from a cellphone. Even if the skull and outer brain tissues can absorb 99% of radiation, the cellphone radiation inside the brain is still five orders of magnitude stronger. As shown in Chapter 1, the electric field generated by the cellphone at the distance of 5 cm is comparable to that used by TTFields which can have significant impact on microtubule dynamics. For microtubules to act as the receiving antennas (Chapter 4), the radiation power from neuronal firing in the transmitting area should be several orders of magnitudes higher.
According to the Larmor formula, the radiation power is proportional to the square of the charge. Thus, if a group of charges are accelerated synchronously so that they can be treated as a single charge, we should have
Ptotal = N2 x Psingle
instead of Ptotal = N x Psingle. In this case, the radiation power can be dramatically increased. This explains why synchronization is essential for wireless communication in the brain. Suppose the transmitting area contains 104 synchronizing neurons, N ~ 1015. Then
Ptotal = N2 x Psingle ~ 10 W
This power is sufficient to influence microtubules in the receiving areas, thereby modulating neuronal excitability (Chapter 4).
Author: Frank Lee