The human brain is a complex structure. If we observe it from the outside, we see a gelatinous mass of an approximately greyish colour , with numerous protuberances, grooves and convolutions that cover its surface. Inside it, however, we can see a series of structures of a more whitish colour .

This change of colour is not accidental: the neurons that make up the brain have different parts with different functions, having delimited the existence of two types of matter or substances throughout the entire nervous system: the grey substance, in which we find mainly somas or nuclei of the neurons, and the white substance, also called white matter .

The white substance

The white substance is that part of the nervous system configured mainly by axons of neurons, that is, the part of the neurons in charge of transmitting the information processed by the soma through the rest of the system. Although the grey matter (also called grey matter) is especially visible in the cerebral cortex and the interior of the spinal cord, the white matter can be found more easily in the internal structures of the brain and in the most external part of the cord .

The whitish colouring of this substance is due to the presence of myelin, a substance that covers the axons of a large part of the neurons. The main function of this myelin is to accelerate the transmission of information . This acceleration is due to the fact that, thanks to the myelin, the information does not have to pass in a straight and continuous way through the axon, but rather it is carried out through small jumps between the myelin sheaths (this type of communication is called jumping transmission).

Basic functions

The main function of white matter is the correct transmission of brain information . This substance has a great implication in allowing the human being to transfer the electrochemical pulses emitted by the brain to the rest of the body. In this way we can consider that it coordinates the communication between the different systems of the human body, both inside and outside the brain. Thanks to it, distant parts of the nervous system can maintain the necessary contact to work together.

That is why where there is white substance, the axons of the neurons are particularly predominant, which means that these areas of the brain that are white are, in essence, neuronal highways , areas of communication between parts of the brain.

Other recently discovered functions

Traditionally, it has been assumed that what we have seen is the main function of the white substance, the latter being believed to be a passive element that merely transferred orders from the nucleus of the neuron to other cells. However, more recent research indicates that the white matter, apart from the mere transmission of information, is related to different cognitive and emotional elements .

This is because the connection and speed offered by the substance allows the construction of neural networks that can govern different processes . Specifically, it greatly affects memory and learning, as well as the management of cognitive resources and executive functions. Thus, it has been indicated that the white substance greatly affects the development and use of intelligence .

Structure and internal configuration

As we have indicated, the white substance is predominantly formed by myelinated axons, which are the part of the neuron in charge of projecting the nerve impulse towards relatively remote areas, with maximum speed and efficiency. This does not mean that somas, or even axons without myelin, cannot be found, but their proportion is much lower than that of the grey substance, which produces the visual effect that white predominates in those regions.

Apart from these components, also contains a large number of glial cells, structures that support and maintain the neurons . Myelin is not the only substance associated with these glial cells, there is a great variety of these that serve to keep the neurons functioning correctly.

The tracts of the brain

Both inside and outside the central nervous system, the white substance is organised in the form of bundles of nerve fibres . So-called tracts or projection nerve fibres send the information processed by the grey matter to the various body regions outside the brain. A second type of white matter fibres are association fibres that connect different brain regions in the same hemisphere . The third and last type corresponds to the interhemispheric commissures , which connect structures of different hemispheres.

Within the brain there are a large number of structures configured mainly by white matter. One of the most visible and remarkable is the corpus callosum, one of the interhemispheric commissures, of great relevance that joins the two cerebral hemispheres and transmits the information between them.

When the white substance fails

As we already know, there are numerous disorders produced by damage to the structures of the brain, of a neurological nature. Taking into account that the speed of processing is largely due to the presence of myelin and the need for information to travel effectively and efficiently in order to coordinate our actions, the presence of white matter damage can cause disorders such as the following: fatigue, psychomotor slowness, lack of coordination and muscular weakness, blurred vision, difficulty in remembering, deficit in executive functions and in intellectual capacities are some of the frequent symptoms of the malfunctioning of the white substance.

Some of the disorders that affect or are affected by the white matter are multiple sclerosis (in which an inflammation of the white matter takes place that produces a demyelination of the neurons), Alzheimer’s and other dementias, ADHD (in subjects with this disorder a lower amount of white matter has been observed) or dyslexia (difficulties being linked to the speed of processing).

Bibliographic references:

  • Fields, D. (2008). White Matter Matters. Scientific American, p. 54.
  • Tirapau-Ustarroz, J., Luna-Lario, P., Hernáez-Goñi, P., & García-Suescun, I. (2011). Relationship between white matter and cognitive functions. Journal of Neurology, 52 (12), 725-742.