The nervous system has two distinct parts: the central nervous system (the brain Brain The brain’s functions are both mysterious and remarkable, relying on billions of nerve cells and the internal communication between them. All thoughts, beliefs, memories, behaviors, and moods... read more 
and spinal cord Spinal Cord The spinal cord is a long, fragile tubelike structure that begins at the end of the brain stem and continues down almost to the bottom of the spine. The spinal cord consists of bundles of nerve... read more ) and the peripheral nervous system Overview of the Peripheral Nervous System The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord. Thus, the peripheral nervous... read more (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: 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 (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. 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, which bathes cells of the nervous system.
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. These cells 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 Overview of the Peripheral Nervous System The peripheral nervous system refers to the parts of the nervous system that are outside the central nervous system, that is, those outside the brain and spinal cord. Thus, the peripheral nervous... read more 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 Nerves The peripheral nervous system consists of more than 100 billion nerve cells (neurons) that run throughout the body like strings, making connections with the brain, other parts of the body, and... read more .)
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 Alzheimer Disease Alzheimer disease is a progressive loss of mental function, characterized by degeneration of brain tissue, including loss of nerve cells, the accumulation of an abnormal protein called beta-amyloid... read more , Huntington disease Huntington Disease Huntington disease is a hereditary disease that begins with occasional involuntary jerking or spasms, then progresses to more pronounced involuntary movements (chorea and athetosis), mental... read more , or Parkinson disease Parkinson Disease (PD) Parkinson disease is a slowly progressive degenerative disorder of specific areas of the brain. It is characterized by tremor when muscles are at rest (resting tremor), increased muscle tone... read more .
Oligodendrocytes may become inflamed and lost, causing multiple sclerosis Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more .
Bacteria or viruses can infect the brain or spinal cord, causing encephalitis Encephalitis Encephalitis is inflammation of the brain that occurs when a virus directly infects the brain or when a virus, vaccine, or something else triggers inflammation. The spinal cord may also be involved... read more or meningitis Introduction to Meningitis Meningitis is inflammation of the layers of tissue that cover the brain and spinal cord (meninges) and of the fluid-filled space between the meninges (subarachnoid space). Meningitis can be... read more .
A blockage in the blood supply to the brain can cause a stroke Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more .
Injuries Overview of Head Injuries Head injuries that involve the brain are particularly concerning. Common causes of head injuries include falls, motor vehicle crashes, assaults, and mishaps during sports and recreational activities... read more or tumors Tumors of the Nervous System can cause structural damage to the brain or spinal cord.
