It is very common that, when talking about a person’s intelligence, we refer specifically to a very specific type of cells: neurons. Thus, it is normal for those who we attribute low intelligence in a derogatory way to be called mononeuronal. However, The idea that the brain is essentially equivalent to a set of neurons is increasingly outdated
The human brain contains more than 80 billion neurons, but this only represents 15% of the total cells in this set of organs.
The remaining 85% is occupied by another type of microscopic bodies: the so-called glial cells Taken together, these cells They form a substance called glia or neuroglia which extends through all the recesses of the nervous system.
Currently, glia is one of the fields of study with the greatest progress in neuroscience, in search of revealing all his tasks and interactions they make for the nervous system to function as it does. And the brain currently cannot be understood without understanding the involvement of glia.
The term neuroglia was coined in 1856 by the German pathologist Rudolf Virchow. This is a word that in Greek means “neural glue (glia)”, since at the time of its discovery Neurons were thought to be linked together to form nerves and, what’s more, that the axon was a group of cells rather than a part of the neuron. For this reason, it was assumed that these cells that were found near the neurons were there to help structure the nerve and facilitate the union between them, and nothing more. A rather passive and auxiliary role, in short.
In 1887, the famous researcher Santiago Ramón y Cajal came to the conclusion that neurons were independent units and that they were separated from the others by a small space that was later known as synaptic space. This served to disprove the idea that axons were anything more than parts of independent nerve cells. However, the idea of glial passivity remained Today, however, Its importance is being discovered to be much greater than previously assumed
In a way, it is ironic that the name that has been given to neuroglia is that. It is true that it does help in the structure, but it not only performs this function, but is also there for protection, repair of damage, improves the nervous impulse, offers energy, and even controls the flow of information, among many more functions discovered. They are a powerful tool for the nervous system.
Types of glial cells
The neuroglia It is a set of different types of cells that have in common that they are found in the nervous system and are not neurons
There are quite a few different types of glial cells, but I will focus on talking about the four classes that are considered most important, as well as explaining the most notable functions discovered to date. As I have said, this field of neuroscience advances more every day and surely in the future there will be new details that are unknown today.
1. Schwann cells
The name of this glia cell is in honor of its discoverer, Theodore Schwann, best known as one of the fathers of the Cell Theory This type of glial cell is the only one found in the Peripheral Nervous System (PNS), that is, in the nerves that run throughout the body.
While studying the anatomy of nerve fibers in animals, Schwann observed cells that were united along the axon and that gave the sensation of being something like small “pearls”; Beyond this, he gave them no more importance. In future studies, it was discovered that these microscopic bead-like elements were actually myelin sheaths, an important product generated by this type of cell.
Myelin is a lipoprotein that provides insulation against the electrical impulse to the axon , that is, it allows the action potential to be maintained for longer and at a greater distance, making the electrical shots go faster and not disperse across the membrane of the neuron. That is, they act like the rubber that covers a cable.
Schwann cells They have the ability to secrete several neurotrophic components, including “Nerve Growth Factor” (NGF) , the first growth factor found in the nervous system. This molecule serves to stimulate the growth of neurons during development. Furthermore, as this type of neuroglia wraps around the axon as if it were a tube, it also has an influence on marking the direction in which it should grow.
Beyond this, it has been seen that when a PNS nerve has been damaged, FCN is secreted so that the neuron can grow again and regain its functionality This explains the process by which the temporary paralysis suffered by muscles after suffering a tear disappears.
The three different Schwann cells
For the first anatomists there were no differences in Schwann cells, but with advances in microscopy it has been possible to differentiate up to three different types, with well-differentiated structures and functions. The ones I have been describing are the “myelinated” ones, since they produce myelin and are the most common.
However, In neurons with short axons, another type of Schwann cell called “unmyelinated” is found , since it does not produce myelin sheaths. These are larger than the previous ones, and inside they house more than one axon at a time. Apparently they do not produce myelin sheaths, since their own membrane already serves as insulation for these smaller axons.
The last type of this form of neuroglia is found in the synapses between neurons and muscles. They are known as terminal or perisynaptic Schwann cells (between the synapse). The function currently given to it was revealed thanks to the experiment carried out by Richard Robitaille, a neurobiologist at the University of Montreal. The test consisted of adding false messengers to these cells to see what would happen. The result was that the response expressed by the muscle was altered. In some cases the contraction increased, in other times it decreased. The conclusion was that This type of glia regulates the flow of information between the neuron and the muscle
2. Oligodendrocytes
Within the Central Nervous System (CNS) there are no Schwann cells, but neurons have another form of myelin coating thanks to an alternative type of glial cells. This function is carried out the last of the large types of neuroglia discovered: the one made up of oligodendrocytes
Their name refers to how the first anatomists who found them described them; a cell with a multitude of small extensions. But the truth is that the name does not accompany them much, since some time later, a pupil of Ramón y Cajal, Pío del Río-Hortega, designed improvements in the staining used at the time, revealing the true morphology: a cell with a pair of long extensions, like arms
Myelin in the CNS
One difference between oligodendrocytes and myelinated Schwann cells is that the former do not wrap their body around the axon, but rather They do it with their long extensions, as if they were the tentacles of an octopus , and it is through them that myelin is secreted. Furthermore, myelin in the CNS is not only there to insulate the neuron.
As Martin Schwab demonstrated in 1988, the deposition of myelin on the axon in cultured neurons hinders their growth. Searching for an explanation, Schwab and his team managed to purify several myelin proteins that cause this inhibition: Nogo, MAG and OMgp. The curious thing is that it has been seen that in the early stages of brain development, the myelin protein MAG stimulates the growth of the neuron, performing an inverse function to the neuron in adults. The reason for this inhibition is a mystery, but scientists hope that its role will soon be known
Another protein found in the 90s, this time by Stanley B. Prusiner, is also found in myelin: the Prion Protein (PrP). Its function in a normal state is unknown, but in a mutated state it becomes a Prion and generates a variant of Creutzfeldt-Jakob disease, commonly known as mad cow disease. The prion is a protein that gains autonomy, infecting all glial cells, which generates neurodegeneration
3. Astrocytes
This type of glial cell was described by Ramón y Cajal. During his observations of neurons, he noticed that near the neurons there were other cells, of a stellate shape; hence its name. It is located in the CNS and the optic nerve, and is possibly one of the glia that carries out the greatest number of functions Its size is two to ten times larger than that of a neuron, and it has very diverse functions.
Blood-brain barrier
Blood does not flow directly into the CNS. This system is protected by the Blood-Brain Barrier (BBB), a highly selective permeable membrane. Astrocytes actively participate in it, being in charge of filtering what can happen to the other side and what cannot Mainly, they allow the entry of oxygen and glucose, to feed the neurons.
But what happens if this barrier is damaged? In addition to the problems caused by the immune system, groups of astrocytes move to the damaged area and join together to form a temporary barrier and stop the bleeding.
Astrocytes have the ability to synthesize a fibrous protein known as GFAP, with which they gain robustness, in addition to secreting another protein that allows them to gain impermeability. In parallel, astrocytes secrete neurotrophs to stimulate regeneration in the area
Recharging the potassium battery
Another of the described functions of astrocytes is their activity to maintain the action potential. When a neuron generates an electrical impulse, it collects sodium ions (Na+) to become more positive towards the outside. This process by which the electrical charges on the outside and inside of neurons are manipulated produces a state known as depolarization, which causes electrical impulses to be born that travel through the neuron until they end up in the synaptic space. During your trip, The cellular environment always seeks balance in the electrical charge, which is why it loses potassium ions (K+) on this occasion to equalize with the extracellular medium.
If this always happened, in the end a saturation of potassium ions would be generated on the outside, which would mean that these ions would stop leaving the neuron, and this would translate into the inability to generate the electrical impulse. This is where astrocytes come into the picture, who They absorb these ions inside to clean the extracellular space and allow more potassium ions to continue to be secreted Astrocytes have no problem with charging, since they do not communicate by electrical impulses.
The last of the four most important forms of neuroglia is microglia This was discovered before oligodendrocytes, but was thought to come from blood vessels. It occupies between 5 to 20 percent of the CNS glia population , and its importance is based on the fact that it is the basis of the brain’s immune system. By having the protection of the Blood-Brain Barrier, the free passage of cells is not allowed, and this includes those of the immune system. Thus, The brain needs its own defense system, and this is made up of this type of glia
The CNS immune system
This glial cell has great mobility, which allows it to react quickly to any problem it encounters in the CNS. Microglia have the ability to devour damaged cells, bacteria and viruses, as well as release a series of chemical agents with which to fight invaders. But The use of these elements can cause collateral damage, since it is also toxic to neurons Therefore, after the confrontation they have to produce, just as astrocytes do, neurotrophic drugs to facilitate the regeneration of the affected area.
Previously I talked about damage to the BBB, a problem that is generated in part by the side effects of microglia when leukocytes cross the BBB and enter the brain. The interior of the CNS is a new world for these cells, and they react to everything unknown as if it were a threat, generating an immune response against it. Microglia initiate defense, causing what we could say a “civil war” which generates a lot of damage to neurons.
Communication between glia and neurons
As you have seen, glia cells carry out a wide variety of tasks. But one section that has not been clear is whether neurons and glia communicate with each other. The first researchers already realized that glia, unlike neurons, do not generate electrical impulses. But this changed when Stephen J. Smith verified how they communicate, both among themselves and with neurons
Smith had the intuition that neuroglia use the calcium ion (Ca2+) to transmit information, since this element is the most used by cells in general. Somehow, he and his colleagues jumped into the pool with this belief (after all, the “popularity” of an ion doesn’t tell us much about its specific functions either), but they were right.
These researchers designed an experiment that consisted of a culture of astrocytes to which fluorescent calcium was added, which allows their position to be seen through fluorescence microscopy. In addition, he added a very common neurotransmitter, glutamate, to the medium. The result was immediate. for ten minutes They were able to see how the fluorescence entered the astrocytes and traveled between the cells as if it were a wave With this experiment they demonstrated that the glia communicate between themselves and with the neuron, since without the neurotransmitter the wave does not start.
The latest we know about glial cells
Through more recent research, it has been discovered that glia detect all types of neurotransmitters. What’s more, both astrocytes and microglia have the ability to manufacture and release neurotransmitters (although these elements are called gliotransmitters because they originate from glia), thus influencing the synapses of neurons.
A current field of study is to see up where glia cells influence the general functioning of the brain and complex mental processes such as learning, memory or sleep.
