|MT > 4. Local Field Potential (LFP), EEG and MEG|
Action potentials (spikes) are accompanied by ion fluxes through nerve membrane. Propagation of action potentials also causes ion flow along the axon and dendrites. The ionic processes at dendrites, soma and axon will result in fluctuation of electric fields at the extracellular space. The extracellular electric fields give rise to electroencephalogram (EEG) if recorded from the scalp, and to local field potential (LFP) if recorded by a microelectrode in the brain (Buzsáki et al., 2012). In addition to electric fields, the ion flow can also generate magnetic fields. Magnetoencephalography (MEG) is based on this principle.
Figure 1. An EEG recording setup. [Source: Wikipedia]
EEG is typically noninvasive. In the method, several electrodes are placed along the scalp, measuring voltage fluctuations at various locations (Figure 1). Brain activity is associated with ion flow along axons and dendrites. Displacement of these ions causes fluctuation of the electric field at the scalp, which in turn give rise to voltage fluctuation. After amplification, the voltage fluctuation can be displayed in the EEG. Since the cerebral cortex is near the scalp, the brain activity observed by EEG originates mainly from the cerebral cortex. Furthermore, the axons and dendrites of pyramidal neurons are longer than other types of neurons, and perpendicular to the scalp, their ion flow will result in greater voltage fluctuations on the scalp. Hence, the brain waves observed by EEG are contributed primarily by the pyramidal neurons in the cortex.
Superconducting quantum interference device (SQUID) was commonly used to detect the weak magnetic signals emitted from the brain (Figure 3). Recently, researchers are developing other magnetometers such as SERF (spin exchange relaxation-free). MEG should be performed in a room shielded from external magnetic fields, including the Earth's magnetic field.
The voltage fluctuations revealed by EEG are known as brain waves. They may oscillate at various frequencies. Depending on functional roles, the oscillation frequencies can be divided into several bands.
Brain waves are generated by a large population of neurons (> 10,000) which oscillate synchronously (Murakami and Okada, 2006). The oscillation frequency of a neuron is dictated by its intrinsic biophysical properties (next chapter). Therefore, as long as a population of neurons possess the same biophysical properties, they can oscillate at the same frequency. However, how can widely separated neurons oscillate synchronously -- that is, without any time delay? This book proposes that the long-range synchronization is governed by the extracellular electric fields as well as the microtubules at the axon initial segment (AIS).
Author: Frank Lee