TABLE OF CONTENTS The main building blocks of the brain are the neurons. As is the case with the myocardial cells in the heart, and the smooth- and skeletal muscle cells, electrical current sources are generated by the cell membrane in a non-steady state. Neurons are excitable cells as well, and can generate action potentials. In fact, these action potentials are the carriers of information within the brain and between the brain and the rest of the body. However, it is not the action potentials that are the generators of the electric potentials and magnetic fields that can be measured on and over the scalp surface as the Electro- and Magneto Encephalogram (EEG and MEG).
The brain contains about 1010 neurons. They are found in various aggregates in both cortex and subcortical areas and in the cerebellum. The neuron consists of a cell-body called soma with tree-like extensions called dendrites that receive information and a single long trunk-like extension called axon that outputs information through action potentials. Connections between different neurons are through synapses that provide a one-way electro-chemical connection between the output axon of one neuron and the input dendrites of another.
If an action potential arrives at a synapse (through a process of diffusion of certain chemical agents called neuro-transrnitters), the dendritic membrane can become depolarized, i.e. the resting membrane potential becomes less negative and more excitable, or hyperpolarized more negative and less excitable.
