The tissue of the central nervous system is made up of two classes of cells that can be classified into two main categories: neurons and glia. Neurons are the anatomical, functional and trophic units of the brain. Neurons are also called nerve cells and are excitable cells. Despite a great variability in size and shape, all neurons share common morphological characteristics, key elements of a highly complex communication network. Neurons transmit a bioelectric signal called nerve impulses. They have two physiological properties: excitability, i.e. the ability to respond to stimuli and convert them into nerve impulses, and conductivity, i.e. the ability to transmit impulses. Neurons are dynamically polarized cells that constitute the main signaling unit of the nervous system. The human brain contains about 1 x 10^11 neurons, each of which is capable of making contact with at least 10,000 other neurons.