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Merck Manual

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Overview of the Nervous System


Steven A. Goldman

, MD, PhD, University of Rochester Medical Center

Last full review/revision Mar 2018| Content last modified Apr 2018
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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 (also called a neuron) 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 dendrites to produce a new electrical current. Different types of nerves use different neurotransmitters to convey impulses across the synapses. Some of the impulses stimulate the next nerve cell, whereas others inhibit it.

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

  • Astrocytes: These cells provide nutrients to nerve cells and control the chemical composition of fluids around nerve cells, enabling them to thrive. They also influence how often nerve cells send impulses and thus regulate how active groups of nerve cells may be.

  • Oligodendrocytes: These cells make myelin, a fatty substance that insulates nerve axons and speeds the conduction of impulses along nerve fibers.

  • Glial progenitor cells: These cells can produce new astrocytes and oligodendrocytes to replace those destroyed by injuries or disorders. Glial progenitor cells are present throughout the brain in adults.

  • Microglia: These cells help protect the brain against infection and help remove debris from dead cells.

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 neurons and consists mainly of axons that are wrapped with many layers of myelin and of the oligodendrocytes that make the 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:

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