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White matter

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White matter
Micrograph showing white matter with its characteristic fine meshwork-like appearance (left of image - lighter shade of pink) and grey matter, with the characteristic neuronal cell bodies (right of image - dark shade of pink). HPS stain.
Latin substantia alba
Gray's subject #
MeSH [1]
Human brain right dissected lateral view, showing grey matter (the darker outer parts), and white matter (the inner and prominently whiter parts).

White matter is one of the two components of the central nervous system and consists mostly of glial cells and myelinated axons that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centers. 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.

The other main component of the brain is grey matter (actually pinkish tan due to blood capillaries), which is composed of neurons. 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.

White matter, long thought to be passive tissue, actively affects how the brain learns and functions. Whilst grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.[1]


Large scaleEdit

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.[2]

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.[3] Schüz and Braitenberg note "As a rough rule, the number of fibres of a certain range of lengths is inversely proportional to their length."[3]377


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.[citation needed] White matter in nonelderly adults is 1.7-3.6% blood.[4]

Myelinated axon lengthEdit

Men have 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.[5] 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.[5] Most of this reduction is due to the loss of thinner fibers.[5]


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, or bundles of axons which connect one part of the brain to another and to the spinal cord, within the white matter:

  1. Projection tracts extend vertically between higher and lower brain and spinal cord centers, and carry information between the cerebrum and the rest of the body. The cortico spinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord. Other projection tracts carry signals upward to the cerebral cortex. Superior to the brainstem, such tracts form a broad, dense sheet called the internal capsule between the thalamus and basal nuclei, then radiate in a diverging, fanlike array to specific areas of the cortex.
  2. Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. The great majority of commissural tracts pass through the large corpus callosum. A few tracts pass through the much smaller anterior and posterior commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.
  3. Association tracts connect different regions within the same hemisphere of the brain. Long association fibers connect different lobes of a hemisphere to each other whereas short association fibers connect different gyri within a single lobe. Among their roles, association tracts link perceptual and memory centers of the brain.[6]
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.[citation needed]For example, in the historic case of Phineas Gage, a railroad foreman who survived a large iron rod being shot through his brain in 1848, the white matter damage was far more extensive than the damage to gray matter.[7]

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[8] used diffusion tensor imaging (DTI) to demonstrate changes in white matter volume as a result of learning a new motor task (i.e. 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.[9]


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.

Clinical relevanceEdit

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.[10]

See alsoEdit


  1. Fields, Douglas (March 2008). White Matter. Scientific American 298 (3): 54–61.
  2. Klein, S.B., & Thorne, B.M. Biological Psychology. Worth Publishers: New York. 2007.
  3. 3.0 3.1 Schuz, A. Braitenberg, V. (2002). "The human cortical white matter: Quantitative aspects of cortico-cortical long-range connectivity". Cortical Areas: Unity and Diversity, Conceptual Advances in Brain Research. pp 377–386 Taylor and Francis London. ISBN 978-0-415-27723-5
  4. (1990). Cerebral blood flow, blood volume and oxygen utilization. Normal values and effect of age. Brain : a journal of neurology 113 ( Pt 1): 27–47.
  5. 5.0 5.1 5.2 (2003). Marked loss of myelinated nerve fibers in the human brain with age. The Journal of comparative neurology 462 (2): 144–52.
  6. Saladin, Kenneth (2012). Anatomy & Physiology: The Unity of Form and Function, 531, New York: McGraw Hill.
  7. Van Horn, John D., Irimia, A., Torgerson, C.M., Chambers, M.C., Kikinis, R., Toga, A.W. (2012). Mapping connectivity damage in the case of Phineas Gage. PLoS One 7 (7): e37454.
  8. Training induces changes in white-matter architecture. Nature Neuroscience. URL accessed on 2009-10-11.
  9. White Matter Matters. Dolan DNA Learning Center. URL accessed on 2009-10-19.
  10. Assaf Y, Pasternak O (2008). Diffusion tensor imaging (DTI)-based white matter mapping in brain research: a review. J. Mol. Neurosci. 34 (1): 51–61.

External links Edit

Wikimedia Commons has media related to:
[[Commons: Category:White matter

| White matter


Spinal cord

epidural space, dura mater, subdural space, arachnoid mater, subarachnoid space, pia mater, denticulate ligaments, conus medullaris, cauda equina, filum terminale, cervical enlargement, lumbar enlargement, anterior median fissure, dorsal root, dorsal root ganglion, dorsal ramus, ventral root, ventral ramus, sympathetic trunk, gray ramus communicans, white ramus communicans

grey matter: central canal, substantia gelatinosa of Rolando, reticular formation, substantia gelatinosa centralis, interneuron, anterior horn, lateral horn, posterior horn (column of Clarke, dorsal spinocerebellar tract)

white matter: anterior funiculus: descending (anterior corticospinal tract, vestibulospinal fasciculus, tectospinal tract), ascending (anterior spinothalamic tract, anterior proper fasciculus)

lateral funiculus: descending (lateral corticospinal tract, rubrospinal tract, olivospinal tract), ascending dorsal spinocerebellar tract, ventral spinocerebellar tract, spinothalamic tract, lateral spinothalamic tract, anterior spinothalamic tract, spinotectal tract, posterolateral tract, lateral proper fasciculus, medial longitudinal fasciculus

posterior funiculus: fasciculus gracilis, fasciculus cuneatus, posterior proper fasciculus


Mesencephalon (midbrain)

cerebral peduncle: midbrain tegmentum (periaqueductal gray, ventral tegmentum, nucleus raphe dorsalis), pretectum, substantia nigra, red nucleus, pedunculopontine nucleus, medial longitudinal fasciculus, medial lemniscus, rubrospinal tract, lateral lemniscus

tectum: corpora quadrigemina, inferior colliculi, superior colliculi

cerebral aqueduct: oculomotor nucleus, trochlear nucleus, Edinger-Westphal nucleus

Template:Commissural fibers


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