biology

White matter

White matter is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand White matter rather than just read about it. In short: White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, 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.

Key takeaways

White matter belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect White matter to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of White matter from memory before moving on to harder problems.

Reference excerpt

White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, 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. Its white color in prepared specimens is due to its usual preservation in formaldehyde. It appears pinkish-white to the naked eye otherwise, because myelin is composed largely of lipid tissue veined with capillaries.

Structure

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

Grey matter

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

Location White matter forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. Aggregates of grey matter such as the basal ganglia (caudate nucleus, putamen, 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 fluid-filled cerebral ventricles (lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle) are also located deep within the cerebral white matter.

Myelinated axon length One small study found that men have more white matter than women both in volume and in length of myelinated axons, and that volume and length reduced with age. 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 woman 82,000 km. Most of this reduction is due to the loss of thinner fibers. However, this reduction may correlate with men having larger brains than women and with brain size reducing with age.

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. 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, 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 microstructure, though prolonged abstinence may in certain cases reverse such white matter changes. White matter lesions on magnetic resonance imaging are linked to several adverse outcomes, such as cognitive impairment and depression. White matter hyperintensities are often found in patients with vascular dementia, particularly with small vessel/subcortical subtypes of vascular dementia. Environmental exposures can negatively impact on white matter. For example, exposure to air pollution in early childhood has been associated with microstructural changes in white matter: higher levels of ambient nitrogen dioxide in the first four years of life have been found to be linked to decreased white matter integrity.

Volume Smaller volumes (in terms of group averages) of white matter might be associated with larger deficits in attention, declarative memory, executive functions, intelligence, and academic achievement. However, volume change is continuous throughout one's lifetime due to neuroplasticity, 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 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.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with White matter

Start with the simplest possible case. Write down what White matter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to White matter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about White matter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of White matter

In research: White matter appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses White matter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: White matter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Central nervous system, Neuroanatomy, so understanding it makes those chapters shorter.
In everyday life: Look for White matter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study White matter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what White matter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain White matter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is White matter in simple terms?

Why does White matter matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study White matter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on White matter.