The EEG consists of the summed electrical activities of populations of neurons, with a modest contribution from glial cells. The neurons are excitable cells with characteristic intrinsic electrical properties, and their activity produces electrical and magnetic fields. Electroencephalography measures the electrical fields that reach the surface of the scalp and are produced by the joint contribution of several neuron’s populations with similar spatial orientation. One of the main advantages of EEG is its non-invasive nature. Other brain electrical-field recording techniques such as electrocorticography (ECoG) require surgery to implant an electrode grid on the cortex surface.
But, how does the brain generate this electrical activity? The human brain is compound of billions of neurons which are electrically excitable cells that processes and transmits information through electrical and chemical signals. It has been estimated that one neuron can receive contacts from up to 10,000 other cells, and so altogether human brain neurons constitute nowadays most complex network.
Neurons are compound of the cell body, dendrites and axon, as it can be seen in the picture. The cell body (also known as soma) contains all organelles necessary for cellular function, dendrites receive connections from other neurons while the axon is the communication channel by which the information is transmitted to another neuron. The process by which two neurons transmit information is called synapse. The synapse is nothing else but the means by which information from one neuron flows to another.
The synapse process typically occurs between an axon terminal that is connected to another neuron’s dendrite through the synaptic terminal. The synaptic terminal is a gap between the two of them by which neurotransmitters are exchange during the synaptic process. These neurotransmitters lead to a change in the permeability of ion channels in the membrane.
In rest state (when a neuron is not excited) there is a voltage difference between the inner and external part of the of the neuron’s membrane of about -70mV. This voltage difference is due to ions distribution mainly Cl and Na outside the membrane and K inside. Changes in the membrane permeability during the synapsis lead to different ionic concentrations and its consequent voltage differences. When a neuron is electrically excited by another neuron a phenomenon known as Action Potential occurs. An action potential is a rapid, temporary change in electrical potential across a membrane, from negative to positive and then back to negative. The process covers three phases:
- Depolarization Phase: If a threshold voltage is reached a strong depolarization occurs caused by the opening of the Na channels that provokes the injection of Na cations into the cell increasing the membrane potential.
- Repolarization Phase: The rising voltage provokes the opening of K channels while Na channels gradually closes decreasing the membrane potential. These channels do not close right away when the membrane returns to its resting voltage leading to a voltage difference slightly lower than the resting potential.
- Afterhyperpolarization Phase: Phase in which the membrane repolarizes until it reaches the rest potential.
 Da Silva, F. L. (2010). EEG: origin and measurement. In EEG-fMRI (pp. 19-38). Springer Berlin Heidelberg.
 Barlow, J. S. (1993). The electroencephalogram: its patterns and origins. MIT press.
 Buzsaki, G. (2006). Rhythms of the Brain. Oxford University Press.