The nervous system is a highly precise and complex system of cells that allows animals to sense, process, and react to cues from the physical environment. The fundamental duty of the nervous system is to transfer information at relatively high speed from one part of the animal to another. Every animal has at least a rudimentary nervous system. Although plants and fungi are able to sense and respond to aspects of their environment, they do this based solely on chemical physiological responses and not because of the combined activity of specialized cells. The means by which a nervous system transfers information is through electrochemical signal transmission. Single nerve cells, neurons, can receive information in the form of a chemical and electrical signal, and transfer this information to other neurons, as well as to somatic cells, non-neurons.

The reason and the means by which animals originally developed...