Gray and White Matter: Structure and Functions

The human brain is a very complex structure. Besides, there are parts of the brain we still do not know much about. More importantly, medicine and science still do not know the cure for numerous nervous system diseases and how to repair the damage.

Despite this, scientists have managed to discover a lot about this mysterious organ that is the node of our personality and life.

Dividing its anatomical structure
into lobes, the left, and right hemisphere, and brain parts help us understand
its structure and, more importantly, its functions and mechanisms in a better
way.

Very common and meaningful classification of the brain structures refers to the gray and white matter diversification. In this article, we will discuss the nature of these two types of structures, their differences, significance, and functions.

Finally, we will address different conditions, diseases, and damages of the white and the gray matter, as well as its consequences.

Gray vs. White Matter

Gray matter is peculiar for containing the neuron cell bodies. The scientific name for the neuron bodies is soma (1).

White matter is specific for containing myelinated axons (1). These structures are long relays. They extend out from the neuron bodies. As their color is whiteish, these structures are called the white matter.

The color comes from a high lipid fat content in myelin. This structure connects the brain cells and is distributed in tracts or bundles.

Even though both types of matter are arranged throughout our CNS, i.e. they exist in both brain and spinal cord, we must emphasize one detail. At a system or classification level, the division on the white and gray matter is accurate.

However, there are mixed cell types. Such types exist both in white and gray matter. Therefore, things are not very black-and-white (or gray-and-white), nor simple.

Content of gray and white matter

Except for these basic differences, it is important to emphasize the contents of both of these types of matter. Namely, the gray matter contains glial cells, axon tracts, neuropil (glia, dendrites, and unmyelinated axons), as well as capillary blood vessels (1).

The white matter contains the glial cells responsible for the production of myelin (the oligodendrocytes) and the astrocytes (1).

Where is the Gray Matter located?

It is easy to detect where the gray matter concentration is the highest. Simply put, we look for the neuronal cell bodies. We know that they prevail in the cerebellum, brain stem or truncus encephali, and the cerebrum.

The majority of the neurons are located in the cerebellum (1). More precisely, it contains more than all other brain parts together. Gray matter is also present in the spinal cord. 

Moreover, there are regions in the
CNS that have an external layer of the gray matter. Those are, primarily, the
cerebrum and cerebellum. When it comes to the brain stem and its gray matter,
this part of the brain contains groups of neurons or nuclei embedded within
white matter tracts.

Where is the white matter located?

Axon tracts that represent the white matter are mostly found under the gray matter regions. This means they are located in deeper areas of the cerebellum and cerebrum.

In other cases, for example, the basal ganglia, the grey matter is embedded in the white matter. When it comes to the brain ventricles filled with fluid, they are also surrounded by white matter.

It is exceptionally interesting to
take a look at the spinal cord and the organization of the white and gray
matter in this region. Namely, the gray matter forms a butterfly-like structure
and it is covered with the white matter.

The Function of Gray and White Matter

First of all, the gray matter has a key role in controlling sensory and muscular activity (1). More precisely, the brain regions which predominantly contain the grey matter are involved in these processes.

For example, the cerebral cortex, which is the outer layer or surface structure of the brain, consists of gray matter columns that have the white matter positioned under the gray matter. The cerebral cortex has several important functions.

Those include learning, memory, cognitive processes, and attention. Furthermore, the function of gray matter in the cerebellum is related to motor control, balance, precision, and coordination.

When it comes to the function of
white matter, we can say that the white fatty myelin is important for the
insulation of the axons. This allows faster signal transport and directly
affects the normal sensory and motor functions.

Gray matter diseases

Gray matter damages and diseases cause significant dysfunctions of different systems, primarily the difficulties in interpreting the sensory information, as well as the motor information receipt system.

As a result, patients suffer from paralysis, muscle weakness, as well as tingling sensations.

When it comes to the origin of these diseases, they are classified as neurodegenerative diseases. Those include frontotemporal dementia and Alzheimer's disease. Unfortunately, millions of people worldwide suffer from these neurodegenerative diseases.

Even though changes in the white matter are also found in these diseases, scientists point out that tau neurofibrillary tangles and amyloid plaques are located in the gray matter.

That's why these diseases are most commonly linked to the changes in the gray matter found in different brain structures. Moreover, as expected, the brain part which loses the largest portion of neurons dictates the progression of the disease. As a result, Parkinson's disease motor disability symptoms are related to the changed in the substantia nigra, etc.

Finally, we must emphasize another group of gray matter damage. Those are the spinal cord injuries. If the axon bundles found in this part of the nervous system, i.e. the spinal cord are damaged, the link between the gray matter in the spinal cord and brain is either jeopardized or completely lost.

As a result, the patient can suffer from sensory issues, paralysis, and other dysfunctions which can be permanent if there is damage to the neuronal body.

White matter diseases

Even though many diseases affect both the white and the gray matter, it is important to note which diseases occur as a result of damage or trauma or initial changes in white or gray matter.

For example, neural diseases that result in disruption of the transit of nerve signals originate from changes in the white matter. The reason is simple: the white matter is responsible for the smooth and proper operation of the nervous system.

It is interesting to focus on MS or Multiple sclerosis. This is a disease that we still don't know the cure for. However, scientists are working on it and hopefully, we will be able to successfully treat it in the near future. Currently, we know that the protective axon coating made from the white fatty myelin is destroyed in the patients who suffer from MS (2).

As a result, they experience serious sensory or motor disruption. There are different kinds of MS, as well as different stages of the disease. Some patients state that their condition keeps going back and forth.

The reason is the fact that the relapsing-remitting MS is a condition in which the myelin coating is lost and can be repaired, but unfortunately lost again, several times (2). On the other hand, the progressive MS leads to a neuron death caused by the axonal damage, which is an irreversible state (2).

Moreover, we today know that the white matter disease leads to around one-fifth of strokes. The reason is the fact that diseases that affect white matter also affect the blood vessels in it. As with the usual strokes, they harden and prevent nutrients and oxygen from reaching the brain regions.

Conclusion

The gray matter which got its name for the dark, grayish color, contains neural cell bodies. Also, it houses the axon terminals, nerve synapses, and the dendrites. The brain zones in which it prevails include the cerebrum, the cerebellum, and truncus encephali or the brain stem.

It is also found in the spinal cord. Namely, it forms a butterfly-shaped structure in its central part. The posterior part of this structure is known as the dorsal gray horn. It is especially important for passing the sensory information to the brain through the ascending nerve signals.

The anterior part of this butterfly-like shape is known as the ventral gray horn. It is responsible for sending the descending nerve signals to the autonomic nerves to govern the motor activities. 

Damage of the gray matter, more precisely the dorsal gray horn may result in difficulties in interpreting the sensory information. Besides, damage to the ventral gray horn prevents the normal functioning of the motor information receipt system. These conditions result in paralysis, muscle weakness, as well as tingling sensations.

The
white matter is built of bundles of axons. The key feature is the myelin
coating. Myelin is a structure built from a mixture of lipids and proteins. It
is responsible for axon protection and conducting nerve signals to the spinal
cord.

If
the white matter is damaged, this can affect sensory functions, moving, and
appropriate reactions to the external stimuli.

References

1. Jiang G, Yin X, Li C, Li L, Zhao L, Evans AC, Jiang T, Wu J, Wang J. The Plasticity of Brain Gray Matter and White Matter following Lower Limb Amputation. Neural Plast. 2015;2015:823185. doi: 10.1155/2015/823185. Epub 2015 Oct 25. PMID: 26587289; PMCID: PMC4637496. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4637496/
2. Han XM, Tian HJ, Han Z, Zhang C, Liu Y, Gu JB, Bakshi R, Cao X. Correlation between white matter damage and gray matter lesions in multiple sclerosis patients. Neural Regen Res. 2017 May;12(5):787-794. doi: 10.4103/1673-5374.206650. PMID: 28616036; PMCID: PMC5461617. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5461617/