GLIAL CELLS: FUNCTIONS, TYPES AND DISEASES - GEOGRAFÍA - 2025

The glial cells are supporting cells that protect neurons and hold them together. The set of glial cells is called glia or neuroglia. The term "glia" comes from the Greek and means "glue", that is why they are sometimes referred to as "nervous glue".

Glial cells continue to grow after birth and as we age, their number decreases. In fact, glial cells go through more changes than neurons. There are more glial cells than neurons in our brain.

Specifically, some glial cells transform their gene expression patterns with age. For example, which genes are turned on or off when you reach 80 years of age. They mainly change in brain areas such as the hippocampus (memory) and the substantia nigra (movement). Even the number of glial cells in each person can be used to deduce their age.

The main differences between neurons and glial cells is that the latter do not participate directly in synapses and electrical signals. They are also smaller than neurons and do not have axons or dendrites.

Neurons have a very high metabolism, but they cannot store nutrients. That is why they need a constant supply of oxygen and nutrients. This is one of the functions performed by glial cells; without them, our neurons would die.

Studies throughout history have focused practically exclusively on neurons. However, glial cells have many important functions that were previously unknown. For example, they have recently been found to be involved in communication between brain cells, blood flow, and intelligence.

However, there is much to discover about glial cells, since they release many substances whose functions are not yet known and seem to be related to different neurological pathologies.

Features

The main functions of glial cells are as follows:

They enhance neuronal synapses (connections)

Certain studies have shown that if there are no glial cells, neurons and their connections fail. For example, in a rodent study, neurons alone were found to make very few synapses.

However, when they added a class of glial cells called astrocytes, the number of synapses increased dramatically and synaptic activity increased 10 times.

They have also discovered that astrocytes release a substance known as thrombospondin, which facilitates the formation of neuronal synapses.

They contribute to neural pruning

When our nervous system is developing, excess neurons and connections (synapses) are created. At a later stage of development, leftover neurons and connections are cut back, known as neural pruning.

Glial cells appear to stimulate this task in conjunction with the immune system. It is true that in some neurodegenerative diseases there is pathological pruning, due to the abnormal functions of the glia. This occurs, for example, in Alzheimer's disease.

They participate in learning

Some glial cells coat the axons, forming a substance called myelin. Myelin is an insulator that makes nerve impulses travel faster.

In an environment in which learning is stimulated, the level of myelination of neurons increases. Therefore, it can be said that glial cells promote learning.

Other functions

- Keep the central nervous system attached. These cells are found around neurons and hold them in place.

- Glial cells attenuate the physical and chemical effects that the rest of the body can have on neurons.

- They control the flow of nutrients and other chemicals necessary for neurons to exchange signals with each other.

- They isolate some neurons from others, preventing neural messages from mixing.

- They eliminate and neutralize the waste of neurons that have died.

Glial cell types

The four different types of glial cells found in the central nervous system: ependymal cells (light pink), astrocytes (green), microglial cells (red), and oligodendrocytes (light blue). Source: Artwork by Holly Fischer / CC BY (https://creativecommons.org/licenses/by/3.0)

There are three types of glial cells in the adult central nervous system. These are: astrocytes, oligodendrocytes, and microglial cells. Each of them is described below.

Astrocytes

Fibrous astrocytes

Astrocyte means "star-shaped cell." They are found in the brain and spinal cord. Its main function is to maintain, in various ways, a suitable chemical environment for neurons to exchange information.

In addition, astrocytes (also called astrogliacytes) support neurons and remove waste from the brain. They also serve to regulate the chemical composition of the fluid that surrounds neurons (extracellular fluid), absorbing or releasing substances.

Another function of astrocytes is to feed neurons. Some processes of astrocytes (which we can refer to as the arms of the star) wrap around blood vessels, while others wrap around certain areas of neurons.

These cells can move throughout the central nervous system, extending and retracting its processes, known as pseudopods ("false feet"). They travel in much the same way as amoebae. When they find some debris from a neuron they gobble it up and digest it. This process is called phagocytosis.

When a large amount of damaged tissue has to be destroyed, these cells will multiply, producing enough new cells to reach the goal. Once this tissue is cleaned, the astrocytes will occupy the empty space formed a lattice. Also, a specific class of astrocytes will form scar tissue that seals the area.

Oligodendrocytes

Neuronal cell diagram showing oligodendrocytes and myelin sheath. Source: Andrew c

This type of glial cell supports the processes of neurons (axons) and produces myelin. Myelin is a substance that covers the axons, isolating them. Thus, it prevents the information from spreading to nearby neurons.

Myelin helps nerve impulses travel faster through the axon. Not all axons are covered in myelin.

A myelinated axon resembles a necklace of elongated beads, since myelin is not continuously distributed. Rather, it is distributed into a series of segments with uncovered parts between them.

A single oligodendrocyte can produce up to 50 myelin segments. When our central nervous system develops, the oligodendrocytes produce extensions that are subsequently wound repeatedly around a piece of axon, thus producing the layers of myelin.

The unmyelinated parts of an axon are called Ranvier's nodules, after their discoverer.

Microglial cells or microgliocytes

Microglial cells. Source: No machine-readable author provided. GrzegorzWicher ~ commonswiki assumed (based on copyright claims). / Public domain

They are the smallest glial cells. They can also act as phagocytes, that is, ingesting and destroying neuronal waste. Another function they develop is the protection of the brain, defending it from external microorganisms.

Thus, it plays an important role as a component of the immune system. These are responsible for the inflammation reactions that occur in response to brain injury.

Ependymal cells

They are cells that line the ventricles of the brain that are filled with cerebrospinal fluid, and the central canal of the spinal cord. They have a cylindrical shape, similar to that of mucosal epithelial cells.

Diseases that affect glial cells

There are multiple neurological diseases that show damage to these cells. Glia has been linked to disorders such as dyslexia, stuttering, autism, epilepsy, sleep problems, or chronic pain. In addition to neurodegenerative diseases such as Alzheimer's disease or multiple sclerosis.

Some of them are described below:

Multiple sclerosis

It is a neurodegenerative disease in which the patient's immune system mistakenly attacks the myelin sheaths in a certain area.

Amyotrophic lateral sclerosis (ALS)

In this disease there is a progressive destruction of motor neurons, causing muscle weakness, problems with speaking, swallowing and breathing that progress.

It seems that one of the factors involved in the origin of this disease is the destruction of glial cells that surround motor neurons. This may explain why the degeneration starts in one area and spreads to adjacent areas.

Alzheimer disease

It is a neurodegenerative disorder characterized by general cognitive impairment, mainly memory deficits. Multiple investigations suggest that glial cells may play an important role in the origin of this disease.

It appears that changes occur in the morphology and functions of glial cells. Astrocytes and microglia cease to fulfill their neuroprotective functions. Thus the neurons remain subjected to oxidative stress and excitotoxicity.

Parkinson's disease

This disease is characterized by motor problems due to a degeneration of neurons that transmit dopamine to areas of motor control such as the substantia nigra.

It seems that this loss is associated with a glial response, especially in the microglia of astrocytes.

Autism spectrum disorders

It appears that the brains of children with autism are larger than those of healthy children. These children have been found to have more neurons in some areas of the brain. They also have more glial cells, which can be reflected in the typical symptoms of these disorders.

Also, there appears to be a malfunction of the microglia. As a consequence, these patients suffer from neuroinflammation in different parts of the brain. This causes the loss of synaptic connections and neuronal death. Perhaps for this reason, there is less connectivity than normal in these patients.

Affective disorders

In other studies, decreases in the number of glial cells have been associated with different disorders. For example, Öngur, Drevets and Price (1998) showed that there was a 24% reduction in glial cells in the brain of patients who had suffered from affective disorders.

Specifically, in the prefrontal cortex, in patients with major depression, this loss being more pronounced in those with bipolar disorder. These authors suggest that the loss of glial cells may be the reason for the reduced activity seen in that area.

There are many more conditions in which glial cells are involved. More research is currently underway to determine its exact role in multiple diseases, primarily neurodegenerative disorders.

References

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