White matter is a component of the central nervous system, in the brain and superficial spinal cord, and consists mostly of glial cells and myelinatedaxons 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 in prepared specimens is due to its usual preservation in formaldehyde.
White matter, long thought to be passive tissue, actively affects how the brain learns and functions. While 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.
White matter is composed of bundles of myelinatednerve 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 (across cortical areas) and is roughly the same number as those that communicate between the two hemispheres in the brain's largest white tissue structure, the 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
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. 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.
White matter is the tissue through which messages pass between different areas of gray matter within the central nervous system. Using a computer network as an analogy, the grey 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:
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.
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.
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.
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 (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. Or, gross changes in the diameter or packing density of the axon might cause the change. A more recent DTI study by Sampaio-Baptista and colleagues reported changes in white matter with motor learning along with increases in myelination.
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.
^ abSchuz, 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
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^ abcMarner, L; Nyengaard, JR; Tang, Y; Pakkenberg, B (2003). "Marked loss of myelinated nerve fibers in the human brain with age". The Journal of comparative neurology462 (2): 144–52. doi:10.1002/cne.10714. PMID12794739.
^Saladin, Kenneth (2012). Anatomy & Physiology: The Unity of Form and Function. New York: McGraw Hill. p. 531. ISBN978-0-07-337825-1.