Anatomy & Organs

White Matter – Structure, Function & Diseases

White substance

The white matter can be understood as the counterpart of the gray matter in the brain . It consists of pathways (nerve fibers) whose white color comes from their medullary structure.

The white matter is part of the central nervous system and is also known as the substantia alba or marrow or marrow substance . In the spinal cord , it lies next to the gray matter . There it is divided into anterior, lateral and posterior strands. In the brain, the white nerve fibers are located in the inner regions and are surrounded by the gray matter. The myelinated pathways, i.e. myelinated extensions of nerve cells , also show accumulations of gray nerve cell bodies. These are the so-called core areas in the spinal cord and brain.

What is the white matter?

The myelin sheaths responsible for the white color of the substance are formed by so-called glial cells in the central nervous system. These also belong to the white matter. On the other hand, the bodies of the nerve cells are hardly found in this area, apart from the development before birth.

White matter is mostly superficial in the spinal cord and brainstem areas . Nerve fibers from a common origin and with the same destination are grouped into bundles, strands, or pathways. In the cerebrum, the white matter is in the central area and is also arranged in strands. The course of the nerve cords continues via the brainstem area and the so-called cerebellar stalks to the marrow of the cerebellum .

Anatomy & Structure

In terms of volume, the white matter fills almost half of the human brain. Overall, it can be imagined as a complicated system of several million connecting cables. Each of these strands has an extension of nerve cells that recognizes, transmits and transmits signals. Science speaks of an axon .

It is usually wrapped in the fatty myelin that gives the substance its white coloration. The bundles, strands and pathways of the nerves divide again and possibly reconnect, so that brain areas that are far away from each other can also be connected. Thus, the white matter is very important for all processes in the brain that are related to learning. If the nerve cords show disturbances, this can have an extremely negative effect on the mental performance of the person. Imaging techniques available today can clearly visualize the white matter and point to its causal effect on possible mental and psychiatric disorders.

They also show the influence of the white matter on intelligence and thinking ability. This proves that the nerve fibers determine the flow of information between the individual brain areas to a much greater extent than assumed. An active brain that is challenged to be active may increase its white matter. When a person learns something new or, for example, acquires many new skills on a musical instrument, the white matter in the brain increases quantitatively. So she’s trainable, which wasn’t originally thought possible. Conversely, it also becomes clear to what extent the white matter contributes to the decline in the versatile ability to think in old age.

Function & Tasks

New insights have also been gained over the past few years about the myelin, that fatty whitish coat around the pathways. At first it was assumed that this so-called myelin sheath was used solely to insulate the nerve fibers. Later, however, the question arose as to why some fibers do not have a cladding while others have a thin or thick one.

For a long time it was not entirely clear why the myelin sheath has microscopically small gaps ( Nodes of Ranvier ) every millimeter. It has now become clear that nerve impulses travel around a hundred times faster on a wrapped (myelinated) pathway than on an exposed one. Thanks to the “insulating tape”, the electrical signals jump over the node rings, so to speak. This is noticeable both in the central nervous system and in the various extremities.

Diseases

The lifelong development of the human white matter is characterized by ups and downs. During childhood and adolescence , their volume increases relatively evenly. It increases until the age of 40 to 50. Then, however, the white matter reduces again more or less slowly.

Accordingly, mental performance gradually decreases. The flow of information between the individual brain regions comes to a standstill because the number of nerve fibers covered with myelin decreases. There is research that suggests the total length of myelinated fibers in a person by the age of 20 is about 149,000 kilometers, but by the time they are 80 it has dropped to about 82,000 kilometers. However, this does not necessarily mean that older people lose the knowledge they have acquired. It usually remains well preserved into old age. The brain has the ability to compensate for certain deficits on its own.

A meaningful experiment with younger and older subjects found that motor responses slow down with age. However, the researchers suspected that behind this increased reaction threshold was a strategy of the brain to avoid hasty and possibly incorrect reactions. In fact, the older test subjects reacted more slowly than the young ones, but also achieved a lower error rate. It has also been found that older people, despite their white matter deficits, are better able to activate certain areas of the brain compared to younger people.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.