Central nervous system
Artwork showing the brain, facial nerves and the top portion of the spinal cord. Above the spinal cord is the brainstem, which controls automatic functions, sleep and arousal and relays messages from the brain to the spinal cord. The striped structure to the right of the brainstem is the cerebellum, which controls muscle coordination and balance
The central nervous system (CNS) represents the largest part of the nervous system, including the brain and the spinal cord. Brain is covered by protective layer of membranes and the skull, while the spinal cord is also protected by the vertebrae. The PNS consists of the large majority of what are commonly called nerves and are actually axonal processes of nerve cells.
The nervous system is made up of neurons which are interconnected to each other in complex arrangements and have the property of conducting, using electrochemical signals, a great variety of stimuli both within the nervous tissue as well as from and towards most of the other tissues.
Neurons are sensors that send electric messages to the Central Nervous System, which send the electric messages back to the neurons telling them how to react and the messages are finally sent back to the brain. Thus, neurons coordinate multiple functions in organisms by sending messages that travel at a usual pace of 100 meters per second. Glia cells provide support and protection for neurons and are thus known as the "glue" of the nervous system.
The four main functions of glia cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons. Glia cells also maintain homeostasis, form myelin and participate in signal transmission in the nervous system.