The nervous system has two distinct parts: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves outside the brain and spinal cord).

The basic unit of the nervous system is the nerve cell (neuron). Nerve cells consist of a large cell body and two types of nerve fibers:

• Axon: One elongated extension for sending messages as electrical impulses
• Dendrites: Usually many branches for receiving impulses

Normally, nerves transmit impulses electrically in one direction—from the impulse-sending axon of one nerve cell to the impulse-receiving dendrites of the next nerve cell. At contact points between nerve cells (synapses), the axon secretes tiny amounts of chemical messengers (neurotransmitters). Neurotransmitters trigger the receptors on the next nerve cell's dendrites to produce a new electrical current. Different types of nerves use different neurotransmitters to convey impulses across the synapses.

The brain and spinal cord also contain support cells called glial cells. There are several types, including the following:

• Astrocytes: These cells help provide nutrients to nerve cells and control the chemical composition of fluids around nerve cells, enabling them to thrive.
• Oligodendrocytes: These cells make myelin, a fatty substance that insulates nerve axons and speeds the conduction of impulses along nerve fibers.
• Microglia: These cells help protect the brain against infection and help remove debris from dead cells.

Nerve cells routinely increase or decrease the number of connections they have with other nerve cells. This process may partly explain how people learn, adapt, and form memories. But the brain and spinal cord rarely produce new nerve cells. An exception is the hippocampus, an area of the brain involved in memory formation.

The nervous system is an extraordinarily complex communication system that can send and receive voluminous amounts of information simultaneously. However, the system is vulnerable to diseases and injuries. For example, nerve cells can degenerate, causing Alzheimer's, Huntington's, or Parkinson's disease. Oligodendrocytes may become inflamed, causing multiple sclerosis. Bacteria or viruses can infect the brain or spinal cord, causing encephalitis or meningitis. A blockage in the blood supply to the brain can cause a stroke. Injuries or tumors can cause structural damage to the brain or spinal cord.

Last full review/revision November 2007 by Steven A. Goldman, MD, PhD