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White matter

Recommended video: White matter [33:06]
Structure of the white matter of the spinal cord in cross section.

The term 'white matter' refers to regions within the central nervous system that contain tracts of myelinated axons with supporting cells including astrocytes and microglia, as well as blood vessels and very few neuronal cell bodies. The presence of lipid-rich myelin sheaths encompassing the myelinated axons give these areas their unique white appearance.

White matter is one of two distinct areas of tissue within the brain and spinal cord, the other being gray matter, which contains neuronal cell bodies, dendrites, telodendria, and unmyelinated axons. 

In the brain, white matter is situated beneath the gray matter of the cortex and connects various gray matter areas within the brain to each other. It contains three major groups of axons: association fibers, commissural fibers, and projection fibers. Association fibers are confined to each hemisphere. Commissural fibers on the other hand connect corresponding areas of both hemispheres, while projection fibers link the cerebral cortex of each hemisphere with nuclei of subcortical regions, brainstem and spinal cord.

The white matter within the cerebellum is termed the arbor vitae (Latin for "tree of life"), due to its tree-like distribution pattern. It comprises afferent and efferent fibers that run to and from the cerebellar cortex.

The white matter of the spinal cord is located externally and surrounds the centrally placed gray matter. It is composed of abundant nerve cell processes which form large ascending and descending bundles (tracts). The spinal white matter is typically organized into three large, paired masses: the dorsal, lateral, and ventral funiculi. Each funiculus contains a number of specific tracts, which carry afferent (sensory) and efferent (motor) signals to different levels of the spinal cord and relays information to and from the brain

The main function of white matter is communication between different brain regions and between the brain and spinal cord.

Terminology English: White matter
 Substantia alba
Definition Tracts of myelinated axons within the central nervous system tissue
Structure Brain: Association fibers, commissural fibers and projection fibers
Cerebellum: Afferent and efferent pathways
Spinal cord: Dorsal, lateral, and ventral funiculi
Function Communication between different areas of the central nervous system.

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