White matter

White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to its usual preservation in formaldehyde. A 20 year-old male has around 176,000 km of myelinated axons in his brain.

The other main component of the brain is grey matter (actually pinkish tan due to blood capillaries). A third colored component found in the brain that appears darker due to higher levels of melanin in dopaminergic neurons than its nearby areas is the substantia nigra.

Note that white matter can sometimes appear darker than grey matter on a microscope slide because of the type of stain used.

Large scale
White matter is composed of bundles of myelinated nerve cell processes (or axons), which connect various grey matter areas (the locations of nerve cell bodies) of the brain to each other, and carry nerve impulses between neurons. Myelin acts as an insulator, increasing the speed of transmission of all nerve signals.

The total number of long range fibers within a cerebral hemisphere is 2% of the total number of cortico-cortical fibers and is roughly the same number as those that communicate between the two hemispheres in Corpus callosum. Schüz and Braitenberg note "As a rough rule, the number of fibres of a certain range of lengths is inversely proportional to their length" 377

Microscopic
Cerebral- and spinal white matter do not contain dendrites, which can only be found in grey matter along with neural cell bodies, and shorter axons. White matter in nonelderly adults is 1.7-3.6% blood.

Myelinated axon length
Men have slightly more white matter than females both in volume and in length of myelinated axons. At the age of 20, the total length of myelinated fibers in males is 176,000 km while that of a female is 149,000 km. There is a decline in total length with age of about 10% each decade such that a man at 80 years of age has 97,200 km and a female 82,000 km. Most of this reduction is due to the loss of thinner fibers.

Function
White matter is the tissue through which messages pass between different areas of gray matter within the nervous system. Using a computer network as an analogy, the gray matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together. The white matter is white because of the fatty substance (myelin) that surrounds the nerve fibers (axons). This myelin is found in almost all long nerve fibers, and acts as an electrical insulation. This is important because it allows the messages to pass quickly from place to place.

There are three different kinds of tracts within the white matter: 1.	Projection tracts, which send action potentials from the cortex to other brain regions, out of the brain to muscles, or into the brain from sense receptors. 2.	Commissural tracts, which carry information between the left and right hemispheres of the brain over bridges known as commissures. These tracts allow the two hemispheres of the brain to communicate with one another. 3.	Association tracts, which carry information between lobes within the same hemisphere. Long association fibers connect different lobes of a hemisphere with one another and short association fibers connect different gyri within a single lobe.

The brain in general (and especially a child's brain) can adapt to white-matter damage by finding alternative routes that bypass the damaged white-matter areas, and can therefore maintain good connections between the various areas of gray matter.

Unlike gray matter, which peaks in development in a person's twenties, the white matter continues to develop, and peaks in middle age (Sowell et al., 2003). This claim has been disputed in recent years, however.

A 2009 paper by Jan Scholz and colleagues used diffusion tensor imaging (DTI) to demonstrate changes in white matter volume as a result of learning a new motor task (juggling). The study is important as the first paper to correlate motor learning with white matter changes. Previously, many researchers had considered this type of learning to be exclusively mediated by dendrites, which are not present in white matter. The authors suggest that electrical activity in axons may regulate myelination in axons. Similarly, the cause may be gross changes in the diameter or packing density of the axon.

Location
White matter forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. Aggregates of gray matter such as the basal ganglia (caudate nucleus, putamen, globus pallidus, subthalamic nucleus, nucleus accumbens) and brain stem nuclei (red nucleus, substantia nigra, cranial nerve nuclei) are spread within the cerebral white matter.

The cerebellum is structured in a similar manner as the cerebrum, with a superficial mantle of cerebellar cortex, deep cerebellar white matter (called the "arbor vitae") and aggregates of grey matter surrounded by deep cerebellar white matter (dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus). The fluid-filled cerebral ventricles (lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle) are also located deep within the cerebral white matter.

Types of astrocytes
In 1983, M. C. Raff et al. discovered that tissue samples originating from the optic nerves of rats contained two morphologically distinct types of astrocytes.
 * So-called "Type 1 astrocytes" had a fibroblasts appearance and resided in both gray matter and white matter.
 * "Type 2 astrocytes" had a neuron-like appearance and resided in white matter alone (Sherman, Chris).

Clinical relevance
Multiple Sclerosis (MS) is one of the most common diseases which affect white matter. In MS lesions, the myelin shield around the axons has been destroyed by inflammation.

Changes in white matter known as amyloid plaques are associated with Alzheimer's disease and other neurodegenerative diseases. White matter injuries ("axonal shearing") may be reversible, while gray matter regeneration is less likely. Other changes that commonly occur with age include the development of leukoaraiosis, which is a rarefaction of the white matter that can be caused by a variety of conditions, including loss of myelin, axonal loss, and a breakdown of the blood-brain barrier.

The study of white matter has been advanced with the neuroimaging technique called diffusion tensor imaging where magnetic resonance imaging (MRI) brain scanners are used. As of 2007, more than 700 publications have been published on the subject.