The unique connections of the adolescent brain

Those who live in a house with teenagers are surely familiar with the rolling of the eyes, the moods, the endless conversations with friends behind the closed bedroom door, and in general a feeling of “you don’t understand us”. The physical changes they go through are easy to see – they gain height, their bodies and voices change. But the changes that take place in the brain cannot be seen from the outside.

dictation Originally published on the Davidson Institute for Science Education website

For years, the accepted approach to understanding the changes that occur in the brain during adolescence was that they stem mainly from a process of self-organization called synaptic pruning. According to this approach, during childhood, many connections (synapses) between nerve cells are formed in the brain. During adolescence, the brain cleans out the weak or useless connections in order to be more efficient and act faster. New researchwhich was done on mice, showed that this process does exist, but along with it, areas are also formed along the nerve cell extensions that have points that are characterized by a particularly high density of synapses.

Previous studies that examined the number of connections between nerve cells during adolescence in animals And at mankindfound that during this period there was a decrease in the amount of synapses, to the side Additional structural changes. The innovation in the current study is that this time the researchers did not settle for a general average of the number of synapses, but examined where they are scattered along the nerve cell extensions.

In order to prepare the mice’s brains for microscopic examination, the researchers used methods that make it possible to distinguish details in the tissue at an extremely high level of detail of only hundreds of nanometers. Brains of mice of various ages were examined: one-week-old, two-week-old and three-week-old puppies, as well as adult mice aged 8-12 weeks. The division allowed the researchers to follow the changes that occur in the mice’s brains from the age of two weeks until adulthood, a period that more or less corresponds to puberty in humans.

The researchers focused on the area of ​​the brain that processes sensory input, and within it a layer of cells that receives information from the mouse’s whisker hairs. Advanced microscopic methods allowed them to examine all parts of the nerve cell: the cell body and the many branches that branch off from it called “dendritic tree”. They followed the dendrite – a kind of branch that comes out of the body of the nerve cell and splits later. The high resolution allowed them to identify dendritic spines along its length – a kind of protrusions on which there are synapses used to pass signals and information between nerve cells.

The researchers also identified that the dendritic spines are not uniformly distributed along the branches. In certain areas, a particularly high density of dendritic spines with synapses on them was seen, and the researchers called these areas “hotspots”. No such sites were found in the brains of very young mice, but only in the brains of mice from puberty onwards. When the mice had their whisker hairs cut, from which the input to this area of ​​the brain comes, there were fewer connection sites along the dendrite. In other words, the findings show that sensory input is essential for the normal development of the neural connections in this area.

But the most intriguing discovery in the study is related to psychiatric diseases: post-mortem autopsies previously performed on the brains of people with schizophrenia found that their density of dendritic spines was lower than normal. Therefore, the researchers wanted to test it in genetically engineered mice that simulate disease schizophrenia.

It was found that in young puppies there was no difference in the amount of synapses between the transgenic mice and healthy mice. On the other hand, during adolescence, a significant increase in the number of dendritic spines and the amount of synapses at the link sites was recorded in the healthy mice, while in the transgenic mice no such sites developed at all. This finding shows that it is possible that the reduction in the density of dendritic spines in the brain of schizophrenia patients is not related to excessive pruning of the synapses, but rather to a failure in the formation of synapses at the link sites during the critical time window in adolescence.

The research findings raise fascinating research directions, but it should be remembered that the research was only performed on mice. It is true that mice are mammals, like humans, and there is a certain similarity in the structure of their cerebral cortex and ours, but further studies are required in order to examine whether a similar process actually occurs in humans as well. In addition, the researchers focused on a certain type of nerve cells in the cerebral cortex and it is possible that in other types of cells and in other areas of the brain, which were not examined in the study, other developmental processes occur.