subject: Neurology A Deeper Insight Into The Mind 6 [print this page] Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect it to muscles or other effector organs. Central neurons, which in vertebrates greatly outnumber the other types, make all of their input and output connections with other neurons. The interactions of all these types of neurons form neural circuits that generate an organism's perception of the world and determine its behavior. Along with neurons, the system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support.
Nervous systems are found in most multicellular animals, but vary greatly in complexity.
Sponges have no nervous system, although they have homologs of many genes that play crucial roles in the system function, and are capable of several whole-body responses, including a primitive form of locomotion. Placozoans and mesozoans-other simple animals that are not classified as part of the subkingdom Eumetazoa-also have no nervous system.
In Radiata (radially symmetric animals such as jellyfish) their system consists of a simple nerve net. Bilateria, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, one central cord (or two running in parallel), and peripheral nerves. The size of the bilaterian system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans. Neuroscience is the study of the nervous system.
The nervous system derives its name from nerves, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope.
A microscopic examination shows that nerves consist primarily of the axons of neurons, along with a variety of membranes that wrap around them and segregate them into fascicles. The neurons that give rise to nerves do not lie entirely within the nerves themselves-their cell bodies reside within the brain, central cord, or peripheral ganglia.
All animals more advanced than sponges have systems. However, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the system consists of a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the system has a common structure that originated early in the Cambrian period, over 500 million years ago.
