The Glial cells Are support cells that protect and hold neurons together. There are more glial cells than neurons in our brain.
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. As we grow older, their number decreases. In fact, glial cells go through more changes than neurons.
Specifically, some glial cells transform their gene expression patterns with age. For example, which genes are activated or deactivated when they reach 80 years. Mainly change in brain area such as hippocampus (Memory) and the Black substance (movement). Even the amount of glial cells in each person can be used to deduce their age.
The main differences between neurons and glial cells are that the latter do not participate directly in the 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 can not 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 almost exclusively on neurons. However, glial cells have many important functions that were previously unknown. For example, it has recently been discovered that they participate in the communication between brain cells, blood flow and in the intelligence .
However, there is much to be discovered about glial cells, since they release many substances whose functions are not yet known and seem to be related to different neurological pathologies.
Brief History of Glial Cells
On April 3, 1858 Rudolf Virchow announced the concept of neuroglia in a lecture at the Institute of Pathology, University of Berlin. This conference was titled"Spinal Marrow and Brain". Virchow spoke of neuroglia as the connective tissue of the brain or"nervous cement."
This conference was published in a book called"Cell Pathology." It became one of the most influential medical publications of the nineteenth century. Thanks to this book, the concept of neuroglia spread all over the world.
In 1955, when Albert Einstein passed away, the brain was extirpated to study it in detail. For this they kept it in a container filled with formaldehyde. The scientists examined cuts of his brain trying to answer the reason for his exceptional abilities.
The popular belief is that the brain was larger than normal, but it was not. Neither did they find any more neurons in the account, nor were they larger.
After many studies, in the late 1980s they found that Einstein's brain had more glial cells. Above all, in a structure called associative cortex. This is responsible for interpreting the information. Participates in complex functions such as memory or the language .
This surprised the scientists since they had always thought that the glial cells only served to hold together the neurons.
The researchers had ignored the glial cells for a long time because of the lack of communication between them. In contrast, neurons communicate through the synapse using action potentials. That is, electrical impulses that are transmitted between neurons to send messages.
However, glial cells do not produce action potentials. Although recent discoveries show that these cells exchange information not by electrical means, but by chemical means.
In addition, not only communicate with each other but also with the neurons, enhancing the information they transmit.
Functions
The main functions of glial cells are as follows:
- Keep the central nervous system together. These cells are located around the neurons and hold them fixed in place.
- The glial cells attenuate the physical and chemical effects that the rest of the organism can have on the neurons.
- Control the flow of nutrients and other chemicals needed for neurons to exchange signals with each other.
- Isolate some neurons from others by preventing neuronal messages from mixing.
- Eliminate and neutralize the wastes of neurons that have died.
- Enhance the Neuronal synapses . Studies have shown that if there are no glial cells neurons and their connections fail. For example, in a rodent study, it was observed that neurons alone made very few synapses.
However, when they added a class of glial cells called astrocytes, the amount of synapses increased markedly and synaptic activity increased tenfold.
They have also discovered that astrocytes release a substance known as thrombospondin, which facilitates the formation of neuronal synapses.
- Contribute to neural pruning. When our nervous system Is developing, create neurons and connections (synapses) to spare.
At a later stage of development, neurons and leftover connections are trimmed, which is known as neuronal pruning. It appears that glial cells stimulate this task along with the immune system.
It is true that in some neurodegenerative diseases there is a pathological pruning, due to the anomalous functions of the glia. This occurs, for example, in the Alzheimer disease .
- They participate in learning, since some glial cells cover the axons, forming a substance called myelin. Myelin is an insulation that causes nerve impulses to 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 the learning .
Types of glial cells
There are three types of glial cells in the central nervous system of the adult. These are: astrocytes, oligodendrocytes and microglial cells. Each of them is described below.
Astrocytes
Astrocyte means"star-shaped cell". They are in brain and in the spinal cord . Its main function is to maintain, in various forms, a suitable chemical environment for the neurons to exchange information.
In addition, astrocytes (also called astrogliocytes) support neurons and eliminate brain waste. 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 prolongations of astrocytes (which we can refer to as the arms of the star) wind around the blood vessels, while others do around certain areas of neurons.
This structure caught the attention of the famous Italian histologist Camillo Golgi. He thought it was because the astrocytes administered nutrients to the neurons and they were released from the waste from the blood capillaries.
Golgi proposed in 1903 that the nutrients traveled from the blood vessels to the cytoplasm of the astrocytes, before passing to the neurons. The Golgi hypothesis has now been confirmed. This has been integrated with new knowledge.
For example, it has been found that astrocytes receive glucose from the capillaries, and convert it into lactate. This is the chemical that is produced in the first phase of glucose metabolism.
Lactate is released into the extracellular fluid that surrounds the neurons for them to absorb. This substance supplies the neurons with a fuel that can metabolize faster than glucose.
These cells can move throughout the central nervous system, extending and retracting their extensions, known as pseudopodia ("false feet"). They travel in much the same way amoebas do. When they find some scrap of a neuron they swallow it and digest it. This process is called phagocytosis.
When a large number of damaged tissues have to be destroyed, these cells will multiply, producing enough new cells to reach the target. Once the tissue is cleared, the astrocytes will occupy the empty space formed by a lattice. In addition, a particular class of astrocytes will form a healing tissue that will seal the area.
Oligodendrocytes
This type of glial cell provides support to neuron (axon) extensions and produces myelin. Myelin is a substance that overlays axons by isolating them. Thus, it prevents information from spreading to nearby neurons.
Myelin serves for nerve impulses to travel more quickly through the axon. Not all axons are covered with myelin.
A myelinated axon resembles a collar of elongated beads, since myelin is not distributed continuously. Rather, it is distributed in a series of segments existing between them uncovered parts.
A single oligodendrocyte can produce up to 50 myelin segments. When our central nervous system develops, the oligodendrocytes produce extensions that are subsequently rolled repeatedly around a piece of axon, thus producing the myelin layers.
The parts that are not myelinated from an axon are called Ranvier nodules, by their discoverer.
Microglial cells or microgliocytes
They are the smaller glial cells. They can also act as phagocytes, ie, ingesting and destroying neural 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 a brain injury.
Diseases affecting glial cells
There are multiple neurological diseases that manifest damage in these cells. Glia has been linked with disorders such as dyslexia , the stammering , he autism , the epilepsy , Sleep problems, or chronic pain . In addition to neurodegenerative diseases such as Alzheimer's disease or multiple sclerosis.
Here are some of them:
- Multiple sclerosis It is a neurodegenerative disease in which the patient's immune system mistakenly attacks the myelin sheaths of a given area.
- Amyotrophic Lateral Sclerosis (THE A): In this disease there is a progressive destruction of the Motor neurons , Causing muscle weakness problems to talk, swallow and breathe that are advancing.
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 the reason why degeneration begins in a particular area and extends to adjacent areas.
- Alzheimer disease: Is a neurodegenerative disorder characterized by general cognitive impairment, mainly due to 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 fail to fulfill their neuroprotection functions. Thus neurons remain subject to oxidative stress and excitotoxicity.
- Parkinson's disease: This disease is characterized by motor problems due to a degeneration of the transmitting neurons Dopamine To motor control zones such as Black substance .
It seems that this loss is associated with a glial response, especially of the astroglia microglia.
- Autistic Spectrum Disorders: It appears that the brains of children with autism have more volume than healthy children. It has been found that these children 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.
In addition, there appears to be a malfunction of the microglia. As a consequence these patients suffer neuroinflammation in different parts of the brain. This causes loss of synaptic connections and neuronal death. Perhaps for this reason there is less connectivity than normal in these patients.
- Affective Disorders: Other studies have found decreases in the number of glial cells associated with various disorders. For example, Öngur, Drevets and Price (1998) showed that there was a 24% reduction of glial cells in the brain of patients who had suffered affective disorders.
Specifically, in the prefrontal cortex, in patients with major depression, being more pronounced this loss in those who suffered Bipolar disorder . These authors suggest that glial cell loss 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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