BRAIN WHITE MATTER: FUNCTION AND STRUCTURE (WITH PICTURES) - NEUROPSYCHOLOGY - 2025

The white matter is a part of the nervous system that is mainly made up of neuron axons. That is, by the part of the neurons that is responsible for transmitting the information processed by the cell nucleus to other neurons.

The white substance or matter is part of the spinal cord and is made up of a large number of nerve fibers. For this reason, this type of substance is observed mainly in the innermost regions of the brain.

Representation of the white matter and gray matter.

Gray matter is usually observed in the regions adjacent to the white matter. The name of white matter is due to the fact that this part of the nervous system has a large proportion of myelinated fibers and provides a whitish color.

The main function of the white matter is to unite the spinal segments with each other, and the spinal cord with the brain.

Characteristics of the white matter

The white matter is a whitish tissue that is part of the central nervous system. It is located mainly in the spinal cord and is formed by the extensions of neurons that carry electrical signals to synaptic regions and by glial cells.

Thus, the white matter is mainly characterized by being a brain region that does not contain nuclei of neurons.

The role of the white matter is to ensure a good flow of information in the nervous system and to connect different regions of the brain.

For this reason, the white matter is characterized by containing high amounts of myelin. Myelin is a substance that covers the axons of most neurons and provides a white appearance.

Likewise, myelin has the main function of accelerating the transmission of information. The transmission acceleration is carried out, since the myelin allows that the information does not have to pass in a straight and continuous way through the axon, but can travel through small jumps between the myelin sheaths.

This type of brain communication is called saltatory transmission, and as the white matter is the region of the brain with the highest amounts of myelin, the transmission of information from this structure is characterized by being very fast.

One of the pathologies most associated with the functioning and structure of the white matter is multiple sclerosis. Although this brain region is related to many other conditions and pathologies.

Features

The main function of the white matter is to carry out a correct transmission of brain information. In fact, recently researchers at the University of South Carolina have coined the term "scaffold" for white matter.

This term refers to the importance of the transmission of information carried out by the white matter. According to the researchers, the network of connections in the white matter defines the information architecture that supports brain function.

According to the scientist Van Horn, although all the connections in the brain are highly important, there are particular links that are especially relevant.

- Brain communication

In this sense, the white matter seems to be of primary importance in brain communication. Damages or conditions in this region can affect the functioning of multiple brain structures and involve a wide variety of physical and neurological alterations.

The white matter has a high level of involvement in allowing humans to transfer the electrochemical pulses emitted by the brain to the rest of the body.

Thus, it can be determined that the white matter is responsible for coordinating communication between the different systems of the human body. This fact involves both the functioning of regions inside and outside the brain.

It is for this reason that the axons of neurons predominate in the white matter, since this is the part that is capable of transmitting information to another neuron.

The white matter acts as a communication bridge between different regions of the brain that contain the cells of the neurons. These areas of the brain are, essentially, neural highways, areas of communication and transmission of information between regions of the brain.

- Other functions

Historically it was understood that the main and only function of the white matter was to transmit information from some regions of the brain to others. In this way, the white matter was interpreted as a passive structure that was limited to transferring neuronal orders.

However, more recent research has shown that this is not entirely the case. Although the main function of the white matter still lies in the transmission of information, it has been shown that it could participate in the performance of other activities.

Cognitive and emotional processes

The white matter is closely related to cognitive and emotional processes, and it is currently agreed that it is an important element in the performance of such actions.

The participation of white matter in the development of emotional and cognitive processes lies in the connection speed that it provides to the brain.

Memorization and learning

The rapid transmission of information carried out by the white matter allows the construction of neural networks, which can govern a great variety of cognitive processes.

Specifically, the neural networks generated by the white matter seem to be closely linked in memorization and learning activities. Likewise, they participate in the management of cognitive resources and executive functions.

In this way, today it is interpreted that white matter is a very important element of the brain that greatly affects the development and use of people's intellectual capacities.

Structure

Right lateral view of the dissected human brain, showing gray matter (the outer parts darker) and white matter (the inner parts and prominently whiter). Source: John A Beal, PhD Dep't. of Cell Biology and Anatomy, Louisiana State University Shreveport Center for Health Sciences / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)

The white matter is located mainly in the spinal cord and is made up of a large number of nerve fibers known as glia.

If the white matter is transected, it is seen to be covered by a layer of gray matter.

Although it is characterized by being a substance with a high content of myelinated fibers that run longitudinally, the white matter also contains a certain amount of unmyelinated fibers, that is, fibers without myelin.

Structurally, the nerve fibers that make up the white matter are characterized by joining the segments of the spinal cord to each other, as well as the spinal cord to the regions of the brain.

- Types of fibers

The fibers that contain the white matter in the posterior roots and that reach the posterior horn are characterized by presenting different morphologies. The shapes of these fibers depend mainly on the stimulus they transmit and can be divided into two large groups.

Exteroceptive fibers

The exteroceptive fibers are small in caliber and have reduced amounts of myelin. That is, they are unmyelinated fibers.

These fibers are mainly responsible for transmitting exteroceptive afferents through the regions of the spinal cord and the brain.

Proprioceptive fibers

The proprioceptive fibers are larger and thicker than the exteroceptive fibers. They are characterized by being myelinated fibers.

The fibers of the posterior root make contact with the neurons of the anterior horn directly or through interneurons whose bodies are located in the posterior horn.

The axons of the interneurons cross the anterior white commissure and the gray commissure to connect with the motor neurons on the opposite side.

- Organization levels

The white matter is characterized by presenting three fundamental levels of organization in the spinal cord.

At the base of the medulla is the segmental region of the white matter. This region is responsible for performing segmental reflex activities represented in the reflex arc.

Subsequently, there is the intersegmental region, which is responsible for linking the segmental mechanisms of the white matter.

Finally, in the suprasegmental area of ​​the spinal cord, spinal activities are coordinated through the higher brain centers.

- Cords of white matter

The white substance is made up of three different cords. The differentiation of each of these cords is made through its location in the spinal cord.

a) Anterior chord

The anterior cord is located between the ventral median fissure and the ventrolateral grooves of the spinal cord. It is characterized by containing motor tracts that control the movements associated with voluntary movements.

b) Side cord

The lateral cord lies between the ventrolateral sulci and the dorsolateral sulci. It contains fascicles related to voluntary movements, the lateral corticospinal tract and fascicles related to sensation.

c) Posterior chord

Finally, the third and last cord of the white matter is located between the dorsal median sulcus and the dorsolateral sulci.

The posterior cord is characterized by being subdivided into two fascicles in the cervical and upper thoracic region, thanks to the presence of the dorsal intermediate sulcus.

The two fascicles that contain the posterior cord are the gracilis fascicle (in the medial region) and the cueniform fascicle (in the lateral region). Both fascicles contain ascending fibers belonging to the epicritic tract, conscious proprioception, and vibratory sensitivity.

Tracts of the brain

The white matter is organized in the form of bundles of nerve fibers. This organization is observed both inside and outside the central nervous system.

The tracts of the brain are thus a set of projection nerve fibers that send information processed by gray matter to the different regions of the body located outside the brain.

Another type of white matter fibers are association fibers, which connect different brain regions of the same hemisphere.

Finally, the third and last type corresponds to the interhemispheric commissures, which contain structures from different cerebral hemispheres.

In addition, apart from the spinal cord, the brain is characterized by presenting a large number of structures that are mainly made up of white matter. The most important is the corpus callosum, an interhemispheric commissure that connects the two hemispheres of the brain.

Alterations in the white matter

Alterations in the structure and functioning of the white matter are related to a wide variety of pathologies.

The factors that have been most associated with lesions in the white matter are age and high blood pressure. That is, both the frequency and severity of lesions in the white matter increase with age and in the hypertensive population.

However, the spectrum of possibilities that can present in subjects with lesions in the white matter is very varied. From people with serious lesions in the white matter without any vascular risk factor to individuals who have multiple risk factors such as severe arterial hypertension.

In this sense, it is argued that lesions in the white matter can present a wide spectrum of risk factors, which today are not sufficiently studied and defined.

Despite this, at present there are more or less reliable data about the relationship of lesions in the white matter and different pathologies. The most important ones are:

- Alterations of the white matter and cognitive deterioration

Based on the existing data, it is not possible to establish a clear relationship between the presence of lesions in the white matter and cognitive impairment, since the studies are limited.

However, some research has shown that damage to the white matter positively correlates with frontal lobe alteration, affecting the speed of information processing, verbal fluency and executive functions.

- Alterations of the white matter and intracranial hemorrhages

Multiple studies have found that white matter abnormalities are much more common in subjects with intracerebral hemorrhages

These findings show that the hemorrhages are not only located in the basal ganglia, but also affect the lobar.

- Alterations of the white matter as producers of ischemic stroke or vascular death

Several clinical trials have shown that alterations in the white matter predispose to ischemic stroke.

Specifically, subjects who suffer an ischemic attack and alterations in the white matter have twice the risk of presenting a new stroke.

References

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