White matter
Encyclopedia
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 dopamine
rgic neuron
s 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.
ated nerve cell
processes (or axon
s), 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
s, 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.
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.
. 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.
s.
(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.
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
is composed largely of lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
tissue veined with capillaries. Its white color is due to its usual preservation in formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
. 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
Grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil , glial cells and capillaries. Grey matter contains neural cell bodies, in contrast to white matter, which does not and mostly contains myelinated axon tracts...
(actually pinkish tan due to blood capillaries). A third colored component found in the brain that appears darker due to higher levels of melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
in dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...
rgic neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s than its nearby areas is the substantia nigra
Substantia nigra
The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in...
.
Note that white matter can sometimes appear darker than grey matter
Grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil , glial cells and capillaries. Grey matter contains neural cell bodies, in contrast to white matter, which does not and mostly contains myelinated axon tracts...
on a microscope
Optical microscope
The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
slide
Microscope slide
A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...
because of the type of stain used.
Large scale
White matter is composed of bundles of myelinMyelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
ated nerve cell
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
processes (or axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s), which connect various grey matter areas (the locations of nerve cell bodies) of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
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
Corpus callosum
The corpus callosum , also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication...
. 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 dendriteDendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
s, which can only be found in grey matter
Grey matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil , glial cells and capillaries. Grey matter contains neural cell bodies, in contrast to white matter, which does not and mostly contains myelinated axon tracts...
along with neural cell bodies
Soma (biology)
The soma , or perikaryon , or cyton, is the bulbous end of a neuron, containing the cell nucleus. The word "soma" comes from the Greek σῶμα, meaning "body"; the soma of a neuron is often called the "cell body"...
, 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 cordSpinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
. Aggregates of gray matter
Gray Matter
"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...
such as the basal ganglia
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...
(caudate nucleus
Caudate nucleus
The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...
, putamen
Putamen
The putamen is a round structure located at the base of the forebrain . The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that comprises the basal ganglia. Through various pathways, the putamen is connected to the substantia nigra and globus pallidus...
, globus pallidus
Globus pallidus
The globus pallidus also known as paleostriatum, is a sub-cortical structure of the brain. Topographically, it is part of the telencephalon, but retains close functional ties with the subthalamus - both of which are part of the extrapyramidal motor system...
, subthalamic nucleus
Subthalamic nucleus
The subthalamic nucleus is a small lens-shaped nucleus in the brain where it is, from a functional point of view, part of the basal ganglia system. Anatomically, it is the major part of subthalamus. As suggested by its name, the subthalamic nucleus is located ventral to the thalamus. It is also...
, nucleus accumbens
Nucleus accumbens
The nucleus accumbens , also known as the accumbens nucleus or as the nucleus accumbens septi , is a collection of neurons and forms the main part of the ventral striatum...
) and brain stem nuclei (red nucleus
Red nucleus
The red nucleus is a structure in the rostral midbrain involved in motor coordination. It comprises a caudal magnocellular and a rostral parvocellular part.-Function:...
, substantia nigra
Substantia nigra
The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in...
, cranial nerve nuclei) are spread within the cerebral white matter.
The cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
is structured in a similar manner as the cerebrum, with a superficial mantle of cerebellar cortex, deep cerebellar white matter (called the "arbor vitae
Arbor vitae (anatomy)
The arbor vitae is the cerebellar white matter, so called for its branched, tree-like appearance. It brings sensory and motor information to and from the cerebellum....
") and aggregates of grey matter surrounded by deep cerebellar white matter (dentate nucleus
Dentate nucleus
The dentate nucleus is located within the deep white matter of each cerebellar hemisphere, and it is the largest single structure linking the cerebellum to the rest of the brain. It is the largest and most lateral, or farthest from the midline, of the four pairs of deep cerebellar nuclei, the...
, globose nucleus
Globose nucleus
The globose nucleus is one of the deep cerebellar nuclei. It is located medial to the emboliform nucleus and lateral to the fastigial nucleus. This nucleus contains primarily large and small multipolar neurons....
, emboliform nucleus
Emboliform nucleus
The emboliform nucleus lies immediately to the medial side of the nucleus dentatus, and partly covering its hilus. It is one among the four pairs of cerebellar nuclei, which are from lateral to medial: the dentate, interposed , and fastigial nuclei...
, and fastigial nucleus
Fastigial nucleus
The fastigial nucleus or nucleus fastigii refers specifically to the concentration of gray matter nearest to the middle line at the anterior end of the superior vermis, and immediately over the roof of the fourth ventricle, from which it is separated by a thin layer of white matter...
). The fluid-filled cerebral ventricles
Ventricular system
The ventricular system is a set of structures containing cerebrospinal fluid in the brain. It is continuous with the central canal of the spinal cord.-Components:The system comprises four ventricles:* right and left lateral ventricles* third ventricle...
(lateral ventricles, third ventricle
Third ventricle
The third ventricle is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid ....
, cerebral aqueduct, fourth ventricle
Fourth ventricle
The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral ventricles, the third ventricle, and the 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 astrocyteAstrocyte
Astrocytes , also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord...
s.
- 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 SclerosisMultiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
(MS) is one of the most common diseases which affect white matter. In MS lesions, the myelin
Myelin
Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
shield around the axons has been destroyed by inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
.
Changes in white matter known as amyloid
Amyloid
Amyloids are insoluble fibrous protein aggregates sharing specific structural traits. Abnormal accumulation of amyloid in organs may lead to amyloidosis, and may play a role in various neurodegenerative diseases.-Definition:...
plaques are associated with Alzheimer's disease
Alzheimer's disease
Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...
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
Leukoaraiosis
Leukoaraiosis or periventricular white matter disease is a term for changes in the cerebral white matter that can be detected with high frequency by CT and MRI in aged individuals. It is a descriptive term for rarefaction of the white matter. It is also commonly referred to as white matter...
, 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
Blood-brain barrier
The blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion...
.
The study of white matter has been advanced with the neuroimaging
Neuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
technique called diffusion tensor imaging where magnetic resonance imaging
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRI) brain scanners are used.
As of 2007, more than 700 publications have been published on the subject.