The nervous system is composed of trillions of nerve cells, as well as other cells which maintain and support the nerve cells. The scientific name for a nerve cell is a neuron. Neurons function electrically to create thought and action in our bodies. Without nerve cells, human life would not, and could not be. Most of the cells which maintain and support neurons are glial cells. The word "glial" means glue. Glial cells not only maintain and support neurons, they serve as glue to keep them in place.
Neurons have a cell body, which like all cell bodies, is called a soma. Protruding from the soma are many nerve fibers. There are two kinds of nerve fibers, axons and dendrites. Axons are very long nerve fibers which send electro-chemical messages to other neurons or to glands and muscles to create action in them. Dendrites are short nerve fibers which receive electro-chemical information from the axon of other nerve cells. A typical nerve cell has several dendrites, but only one axon. When neurons are linked and bundled together to form and electrical line of conduction, they are called nerves. When neurons have specialized in detecting environmental events, they are called sensory receptors. The rods and cones of the eye and taste buds, for example, are specialized neurons.
Since neurons and their nerve fibers communicate with each other electrically, many of the fibers require insulation to function properly. Those nerve fibers which are insulated are insulated with fatty substances called myelin sheath and neurolemma, and are white in appearance. Those which are not insulated are gray in appearance. Insulation permits the nervous system to exercise fine control over muscles. The reason that babies cannot smile or move precisely at birth is that the insulation for their nerve fibers is not completely developed. The reason that a baby horse can run well shortly after birth is that their insulation is more complete at birth. As the insulation does develop in a child, they can smile and move with greater coordination and precision.
Multiple Sclerosis is a disease in which certain white blood cells think that the myelin sheath does not belong in the body. These white blood cells attack the myelin and destroy it. This causes the nerve to short-circuit, affecting muscle control. Such diseases in which the immune system attacks a body part are called auto-immune diseases.
Where the axon of one neuron meets the dendrite of an adjacent neuron, and where nerve fibers connect to muscles and glands, these connections occur at special junctions called a synapse. The synapse is a tiny gap which must be bridged by a chemical called a neurotransmitter. Some well known neurotransmitters are GABA, histamine, norepinephrine, serotonin, dopamine and acetylcholine. Without neurotransmitters neurons could not communicate with each other or with muscles and glands.
Many diseases are caused by an imbalance of neurotransmitters. Some of these are: