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:
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:
The brain and spinal cord consist of gray and white matter. Gray matter consists of nerve cell bodies, dendrites and axons, glial cells, and capillaries (the smallest of the body's blood vessels). White matter contains relatively very few cell bodies and consists mainly of axons that are wrapped with many layers of myelin. Myelin is what makes the white matter white. (Myelin speeds the conduction of nerve impulses—see Nerves.)
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, as in the following examples:
Last full review/revision December 2014 by Steven A. Goldman, MD, PhD