White matter is a component of the central nervous system, in the brain and superficial
spinal cord, 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 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.
Structure White matter is composed of bundles of myelinated
nerve cell processes, which connect various grey matter areas 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 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 The other main component of the brain is grey
matter, which is composed of neurons. The substantia nigra is a third colored component
found in the brain that appears darker due to higher levels of melanin in dopaminergic
neurons than its nearby areas. Note that white matter can sometimes appear darker than grey
matter on a microscope slide because of the type of stain used. Cerebral- and spinal white
matter do not contain dendrites, neural cell bodies, or shorter axons, which can only be
found in grey matter. White matter in nonelderly adults is 1.7-3.6%
blood. 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 and brain stem 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 and aggregates of grey matter surrounded by deep cerebellar white matter.
The fluid-filled cerebral ventricles are also located deep within the cerebral white matter.
Myelinated axon length It was historically thought that 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.
Function 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 that surrounds the
nerve fibers. 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 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.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. Unlike gray matter, which peaks in development
in a person's twenties, the white matter continues to develop, and peaks in middle age. This
claim has been disputed in recent years, however. A 2009 paper by Jan Scholz and colleagues
used diffusion tensor imaging to demonstrate changes in white matter volume as a result
of learning a new motor task. 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. Clinical significance
Multiple Sclerosis 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 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
brain scanners are used. As of 2007, more than 700 publications have been published
on the subject. References
External links White Matter Atlas
White+matter at eMedicine Dictionary