White matter refers to areas of the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

(CNS) that are mainly made up of myelinated

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

s, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution of action potentials, acting as a relay and coordinating communication between different brain regions. White matter is named for its relatively light appearance resulting from the lipid content of myelin. However, the tissue of the freshly cut brain appears pinkish-white to the naked eye because myelin is composed largely of lipid tissue veined with

capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

. Its white color in prepared specimens is due to its usual preservation in formaldehyde.

Structure

White matter

White matter is composed of bundles, which connect various grey matter areas (the locations of nerve cell bodies) of the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a ve ...

to each other, and carry nerve impulses between neurons. Myelin acts as an insulator, which allows electrical signals to jump, rather than coursing through the axon, 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 The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental m ...

. Schüz and Braitenberg note "As a rough rule, the number of fibres of a certain range of lengths is inversely proportional to their length." White matter in nonelderly adults is 1.7–3.6% blood.

Grey matter

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

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...

s. 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 A microscope slide is a thin flat piece of glass, typically 75 by 26 mm (3 by 1 inches) and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is mounted (secured) on the slide, and then b ...

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.

Location

White matter forms the bulk of the deep parts of the brain and the superficial parts of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...

. Aggregates of grey matter such as the basal ganglia ( caudate nucleus,

putamen The putamen (; from Latin, meaning "nutshell") is a round structure located at the base of the forebrain (telencephalon). The putamen and caudate nucleus together form the dorsal striatum. It is also one of the structures that compose the basal ...

, globus pallidus, substantia nigra, subthalamic nucleus, nucleus accumbens) and brainstem nuclei ( red nucleus, 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 fastigial nucleus is located in the cerebellum. It is one of the four deep cerebellar nuclei (the others being the nucleus dentatus, nucleus emboliformis and nucleus globosus), and is grey matter embedded in the white matter of the cereb ...

). The fluid-filled cerebral ventricles (lateral ventricles,

third ventricle The third ventricle is one of the four connected ventricles of the ventricular system within the mammalian brain. It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ...

cerebral aqueduct The cerebral aqueduct (aqueductus mesencephali, mesencephalic duct, sylvian aqueduct or aqueduct of Sylvius) is a conduit for cerebrospinal fluid (CSF) that connects the third ventricle to the fourth ventricle of the ventricular system of the br ...

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 ve ...

) are also located deep within the cerebral white matter.

Myelinated axon length

Men have more white matter than women both in volume and in length of myelinated axons. At the age of 20, the total length of myelinated fibers in men is 176,000 km while that of a woman 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. However, this study only included 36 participants.

Function

White matter is the tissue through which messages pass between different areas of grey matter within the central nervous system. 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. Unlike grey matter, which peaks in development in a person's twenties, the white matter continues to develop, and peaks in middle age.

Research

Multiple sclerosis (MS) is the most common of the inflammatory demyelinating diseases of the central nervous system which affect white matter. In MS lesions, the myelin sheath around the axons is deteriorated by

inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...

. Alcohol use disorders are associated with a decrease in white matter volume. Amyloid plaques in white matter may be associated with Alzheimer's disease and other neurodegenerative diseases. Other changes that commonly occur with age include the development of

leukoaraiosis Leukoaraiosis is a particular abnormal change in appearance of white matter near the lateral ventricles. It is often seen in aged individuals, but sometimes in young adults. On MRI, leukoaraiosis changes appear as white matter hyperintensitie ...

, which is a rarefaction of the white matter that can be correlated with a variety of conditions, including loss of myelin pallor, axonal loss, and diminished restrictive function of the blood–brain barrier. There is also evidence that substance abuse may damage white matter microstucture, though prolonged abstinence may in certain cases may reverse such white matter changes. White matter lesions on magnetic resonance imaging are linked to several adverse outcomes, such as

cognitive impairment Cognitive deficit is an inclusive term to describe any characteristic that acts as a barrier to the cognition process. The term may describe * deficits in overall intelligence (as with intellectual disabilities), * specific and restricted defici ...

and depression. White matter hyperintensity are more than often present with

vascular dementia Vascular dementia (VaD) is dementia caused by problems in the supply of blood to the brain, typically a series of minor strokes, leading to worsening cognitive abilities, the decline occurring piecemeal. The term refers to a syndrome consisti ...

, particularly among small vessel/subcortical subtypes of vascular dementia.

Volume

Smaller volumes (in terms of group averages) of white matter might be associated with larger deficits in attention,

declarative memory Explicit memory (or declarative memory) is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and c ...

, executive functions,

intelligence Intelligence has been defined in many ways: the capacity for abstraction, logic, understanding, self-awareness, learning, emotional knowledge, reasoning, planning, creativity, critical thinking, and problem-solving. More generally, it can be des ...

, and academic achievement. However, volume change is continuous throughout one's lifetime due to

neuroplasticity Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that differs from how it p ...

, and is a contributing factor rather than determinant factor of certain functional deficits due to compensating effects in other brain regions. The integrity of white matter declines due to aging. Nonetheless, regular

aerobic exercise Aerobic exercise (also known as endurance activities, cardio or cardio-respiratory exercise) is physical exercise of low to high intensity that depends primarily on the aerobic energy-generating process. "Aerobic" is defined as "relating to, inv ...

appears to either postpone the aging effect or in turn enhance the white matter integrity in the long run. Changes in white matter volume due to inflammation or injury may be a factor in the severity of obstructive sleep apnea.

Imaging

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. 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 (e.g. 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.

References

External links

* {{DEFAULTSORT:White Matter Central nervous system Neuroanatomy