Overview of the Nervous System - Brain, Spinal Cord, and Nerve Disorders - MSD Manual Consumer Version (2024)

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  Overview of the Nervous System

  
  

The nervous system has 2 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).

• 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

• 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 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.

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 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 causes random, flowing movements that are involuntary and cannot be suppressed (called chorea). Sometimes a muscle or a group of muscles jerk... 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 (involved in the conduction of nerve impulses) may become inflamed and lost (disrupting communication between nerve cells), 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). Symptoms occur suddenly... 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.