Neurons are the principal cellular elements that underlie the function of the nervous system including the brain, spinal cord, peripheral sensory systems and enteric (gut) nervous system. The anatomical variation of these neurons is large, but the general morphology and their electrical and ligand dependent responsiveness allows these cells to be classed as neurons (coined in 1891 by Wilhelm von Waldeyer). Other cellular types are also present in the central nervous system (CNS), most particularly several types of glial elements, initially considered a glue type cell serving as a support matrix for the neuronal circuits. It is now known that these cell types are essential in the maintenance of the neuronal network, in neuronal migration during development and in the generation of myelin. Their electrical response dynamics, being slow compared with that of neurons place them mostly in a modulatory role. Their ability to regulate extracellular background synaptic transmitter levels makes them, however, essential for CNS function.