Neural pruning: what does it consist of and why is it important?

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Neuronal pruning is a mechanism that discards unnecessary synapses, allowing the brain to function more optimally. Understanding it, and being able to intervene on it, could provide us with the key to facing different diseases.

The nervous system is made up of millions of neurons that connect to form complex networks. Since pregnancy, The body produces a huge number of neurons that fulfill various functions. However, not all the cells that are generated end up being useful and are eliminated with neuronal pruning.

This is a natural process of our body that occurs at different stages of life.. It is said to be essential for optimizing brain activity and learning. However, some evidence indicates that it could be related to certain well-studied mental pathologies.

What is neural pruning?

Neurons are the anatomical and functional unit, as small as they are valuable, of the nervous system. These cells form organs, tissues and are responsible for receiving and transmitting information through neuronal networks. For example, when sound waves reach the ear, they stimulate nerve cells in that region. From there, a nervous impulse is sent to the brain, which processes that information and interprets it as sound.

During pregnancy, When we are embryos, nerve cells begin to be produced in quantity. This process remains constant until approximately two years of age. In this way, a child has many more neurons than any adult. Thus, it is said that the function of this overproduction is to facilitate children’s learning.

Let us remember that everything we learn is fixed in the brain in the form of synaptic patterns. These are nothing more than connections that create various groups of neurons with each other. Therefore, the more neurons there are available, the easier it will be to establish links and build patterns.

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However, as expected, not all neurons that are born become part of some network. These “excess” nerve cells are discarded with neuronal pruning. As its name says, it consists of removing non-functional neurons in order to optimize brain activity.

How does synaptic pruning work?

We know that From the age of two, a regulatory pruning process begins that eliminates non-functional connections. For example, the cerebral visual cortex releases a series of synapses with the spinal cord that promote the development of vision.

Then, In adolescence, another axon pruning occurs that remains active until adulthood. To date, the mechanisms that activate and regulate synaptic pruning in the nervous system are not entirely clear.

Fractalakin, complementary proteins and microglial cells

Sakai (2020) published an article in which he points out different hypotheses about neuronal pruning. One of them mentions fractalaquine, a molecule that signals to the brain the need to activate microglial cells. The latter are part of our defenses and are responsible for engulfing pathogens to avoid diseases.

In an experiment with mice, fractalaquine disrupted communication between neurons and microglial cells. The researchers noticed that these mice had immature synaptic connections that would not otherwise exist.

On the other hand, complementary proteins fulfill the function of signaling possible pathogenic cells to eliminate them. Thus, It is suggested that these molecules, together with fractalaquine, would mark circuits with low activity to destroy them..

Why is neuronal pruning important in development?

The evidence accumulated to date indicates that the overproduction of neurons could be an evolutionary mechanism. More synaptic connections improve learning in several ways. Because of this, it is often said that children are like “sponges”, capable of learning very quickly.

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Nevertheless, too many connections can also slow down the travel of the nerve impulse. As a result, response times could be slower and this would be fatal if we are in a dangerous situation. Furthermore, they would demand more energy from the body and this is not useful for adaptation.

Neuronal pruning is then performed to remove synaptic pathways that are not frequently used. Consequently, those that do are activated, reinforced and optimized.

Thus, Pruning favors learning that allows us to better adjust to the environment that surrounds us. Therefore, it is possible to affirm that synaptic pruning is an essential mechanism for the development of the nervous system.

Synaptic pruning and its role in mental illness

There are reasons to believe that axon pruning could play a key role in the development of certain pathologies. Sellgren et al. (2019) published a work on synaptic pruning in patients with schizophrenia. Their conclusions were that excess synaptic pruning can precipitate or produce the clinical picture.

Similarly, other works point out the possible role of synaptic pruning in the symptoms of Alzheimer’s disease. However, since we do not understand how this mechanism works, it is not possible to say with certainty.

To close, It is suggested that neuronal pruning could be altered by various factors, such as age and genetic defects. If we were able to identify these variables, perhaps we could regulate the symptoms of certain pathologies that are associated with this mechanism.