Each section of the nervous system is composed of nerves and their support cells. Scientists estimate the body has approximately 100 billion nerve cells, or neurons, to perform a variety of functions. These neurons carry nerve signals between the sensory organs (e.g., eyes, skin), information processing structures (e.g., brain, ganglia) and effector tissues (e.g., muscles, glands).

Each nerve performs its functions with the same basic parts. Each neuron contains a central portion called the cell body, which contains the vital parts of the neuron such as its nucleus. One end of the cell body contains the axon, a long extension that carries nerve impulses to other neurons. The nerve cell body also contains one or more projections called dendrites, which receive signals from other cells or other nerves.

Dendrites may project from the cell body as several branches, one main trunk with smaller branches diverting off it, or a combination. The branches can be extensive and some neurons may have approximately 150,000 dendrite contacts with other cells.

While all neurons may have the same general structure, scientists have identified approximately 1,000 types. In general, they can be grouped into three classes. These are:

• Sensory neurons. Transmit information from the sensory tissues to other parts of the nervous system for processing. They allow for the perception of a person’s environment and coordinated movement. Some sensory neurons end with nociceptors in the skin or internal tissues. The nociceptors enable the body to quickly detect pain, transmit the information to the central nervous system, and produce a reaction.
  
  
• Motor neurons. Communicate signals from the brain and spinal cord to the muscles and glands to produce a result. One motor neuron may communicate with multiple muscle fibers, which generates movement with a single nerve message.
  
  
• Interneurons. Neurons that are neither a sensory or motor neuron are classified as an interneuron. They convey signals around the body and are the most common type of neuron.

In addition to the neurons, the nervous system also includes glial cells (also known as glia). Scientists believe there are 10 to 50 times more glial cells than neurons in the body.

Glial cells provide support to keep neurons functioning efficiently. Among their many roles, they provide structural support to the neurons and the brain. Certain types of glia called astrocytes form part of the blood-brain barrier, which regulates materials entering the brain. They allow nutrients to get to the neurons, while blocking toxins and infectious agents. They are also involved in nerve communication by disposing of neurotransmitters after they have been used. Two types of glial cells called oligodendrocytes and Schwann cells produce myelin, which is found on the axons of some nerves.