The nervous system has 2 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 2 types of nerve fibers:
Axon: A long, slender nerve fiber that projects from a nerve cell and can send messages as electrical impulses to other nerve cells and muscles
Dendrites: Branches of nerve cells that receive electrical 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 dendrites of the next nerve cell 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. Others inhibit it.
The brain and spinal cord also contain support cells called glial cells. These cells are different from nerve cells and do not produce electrical impulses. 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 can regulate the neurotransmitters and the external chemical environment around nerve cells to influence how often nerve cells send impulses and thus regulate how active groups of nerve cells may be.
Ependymal cells: These cells form along open areas in the brain and spinal cord to create and release cerebrospinal fluid, The cerebrospinal fluid helps to cushion the brain and spinal cord against sudden jarring and minor injury and remove waste products from the brain.
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 injury and help remove debris from dead cells. Microglia can move around in the nervous system and can multiply to protect the brain during an injury.
Oligodendrocytes: These cells form a coating around nerve cell axons and make a specialized membrane called myelin, a fatty substance that insulates nerve axons and speeds the conduction of impulses along nerve fibers.
Schwann cells are also glial cells. However, these cells are in the peripheral nervous system rather than in the brain and spinal cord. These cells are similar to oligodendrocytes and make myelin to insulate axons in the peripheral nervous system.
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. (The myelin coating around the axon 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:
Nerve cells can degenerate, causing Alzheimer disease, Huntington disease, or Parkinson disease.
Oligodendrocytes (involved in the conduction of nerve impulses) may become inflamed and lost (disrupting communication between nerve cells), 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.
