Surely, you have often heard the statement: "Nerve cells do not recover." However, this one of the popular misconceptions was recently refuted by scientists.
On October 15, 1999, the journal Science published a study by Elizabeth Gould and Charles Gross, employees of the Department of Psychology at Princeton University. It showed that the great primate brain produces several thousand new neurons per day throughout life. This process has been called neurogenesis. In the same year, it was discovered that neurogenesis is also carried out in the human brain. However, the process itself was discovered even earlier.
In 1965, scientist Dzhokhev Altman discovered it in the hippocampus (part of the brain) of a rat, and 15 years later, an employee of Rockefeller University Fernando Notteb discovered it in canaries. According to Notteba's discovery, songbirds form nerve cells in the "vocal center" of their brain.
Nevertheless, despite the fact that nerve cells are restored, it is not out of place to take care of neurons, because, as you know, they often die under conditions of severe stress, with injuries, poisoning, etc. However, the function of dead nerve cells is taken over by living cells … So, one healthy nerve cell can replace up to nine dead.
The popular expression "Nerve cells do not recover" is perceived by everyone from childhood as an immutable truth. However, this axiom is nothing more than a myth, and new scientific data refutes it.
Nature puts a very high margin of safety in the developing brain: during embryogenesis, a large excess of neurons is formed. Almost 70% of them die before the child is born. The human brain continues to lose neurons after birth, throughout life. This cell death is genetically programmed. Of course, not only neurons die, but also other cells of the body. Only all other tissues have a high regenerative capacity, that is, their cells divide, replacing the dead. The regeneration process is most active in the epithelial cells and hematopoietic organs (red bone marrow). But there are cells in which the genes responsible for reproduction by division are blocked. In addition to neurons, these cells include heart muscle cells. How do people manage to maintain intelligence until very old age,if nerve cells die and are not renewed?
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One of the possible explanations: not all neurons “work” simultaneously in the nervous system, but only 10% of neurons. This fact is often cited in popular and even scientific literature. I have repeatedly had to discuss this statement with my domestic and foreign colleagues. And none of them understands where this figure came from. Any cell lives and "works" at the same time. In each neuron, metabolic processes take place all the time, proteins are synthesized, nerve impulses are generated and transmitted. Therefore, leaving the hypothesis of "resting" neurons, let us turn to one of the properties of the nervous system, namely, to its exceptional plasticity.
The meaning of plasticity is that the functions of the dead nerve cells are taken over by their surviving "colleagues", who increase in size and form new connections, compensating for the lost functions. The high, but not unlimited, efficiency of such compensation can be illustrated by the example of Parkinson's disease, in which there is a gradual death of neurons. It turns out that until about 90% of neurons in the brain die, the clinical symptoms of the disease (trembling of the limbs, limitation of mobility, unsteady gait, dementia) do not appear, that is, the person looks practically healthy. This means that one living nerve cell can replace nine dead.
But the plasticity of the nervous system is not the only mechanism that allows the preservation of intelligence to a ripe old age. Nature also has a fallback - the emergence of new nerve cells in the brain of adult mammals, or neurogenesis.
The first report on neurogenesis appeared in 1962 in the prestigious scientific journal Science. The article was titled "Are New Neurons Forming in the Brain of Adult Mammals?" Its author, Professor Joseph Altman from Purdue University (USA), with the help of an electric current, destroyed one of the structures of the rat's brain (the lateral geniculate body) and injected there a radioactive substance that penetrates the newly emerging cells. A few months later, the scientist discovered new radioactive neurons in the thalamus (part of the forebrain) and cerebral cortex. Over the next seven years, Altman published several more studies proving the existence of neurogenesis in the brain of adult mammals. However, then, in the 1960s, his work caused only skepticism among neuroscientists, their development did not follow.
And only twenty years later neurogenesis was "rediscovered", but already in the brain of birds. Many songbird researchers have noticed that during each mating season, the male canary Serinus canaria sings a song with new "knees". Moreover, he does not adopt new trills from his brothers, since the songs were updated in isolation. Scientists began to study in detail the main vocal center of birds, located in a special section of the brain, and found that at the end of the mating season (in canaries, it occurs in August and January), a significant part of the neurons of the vocal center died, probably due to excessive functional load … In the mid-1980s, Professor Fernando Notteboom from Rockefeller University (USA) managed to showthat in adult male canaries, the process of neurogenesis occurs in the vocal center constantly, but the number of formed neurons is subject to seasonal fluctuations. The peak of neurogenesis in canaries occurs in October and March, that is, two months after mating seasons. That is why the "music library" of the male canary's songs is regularly updated.
In the late 1980s, neurogenesis was also discovered in adult amphibians in the laboratory of the Leningrad scientist Professor A. L. Polenov.
Where do new neurons come from if nerve cells are not dividing? The source of new neurons in both birds and amphibians turned out to be neuronal stem cells from the wall of the brain ventricles. During the development of the embryo, it is from these cells that the cells of the nervous system are formed: neurons and glial cells. But not all stem cells turn into cells of the nervous system - some of them “hide” and wait in the wings.
It has been shown that new neurons arise from stem cells of the adult organism and in lower vertebrates. However, it took almost fifteen years to prove that a similar process occurs in the mammalian nervous system.
The development of neuroscience in the early 1990s led to the discovery of "newborn" neurons in the brains of adult rats and mice. They were found mostly in the evolutionarily ancient parts of the brain: the olfactory bulbs and the hippocampal cortex, which are mainly responsible for emotional behavior, stress response and regulation of mammalian sexual functions.
Just like in birds and lower vertebrates, in mammals, neuronal stem cells are located near the lateral ventricles of the brain. Their transformation into neurons is very intensive. In adult rats, about 250,000 neurons are formed from stem cells per month, replacing 3% of all neurons in the hippocampus. The lifespan of such neurons is very high - up to 112 days. Neuronal stem cells travel a long way (about 2 cm). They are also able to migrate to the olfactory bulb, turning into neurons there.
The olfactory bulbs of the mammalian brain are responsible for the perception and primary processing of various odors, including the recognition of pheromones - substances that in their chemical composition are close to sex hormones. Sexual behavior in rodents is regulated primarily by the production of pheromones. The hippocampus is located under the cerebral hemispheres. The functions of this complex structure are associated with the formation of short-term memory, the realization of certain emotions and participation in the formation of sexual behavior. The presence of constant neurogenesis in the olfactory bulb and hippocampus in rats is explained by the fact that in rodents these structures bear the main functional load. Therefore, the nerve cells in them often die, which means that they need to be renewed.
In order to understand what conditions affect neurogenesis in the hippocampus and olfactory bulb, Professor Gage from Salk University (USA) built a miniature city. The mice played there, did physical education, looked for exits from the labyrinths. It turned out that in "urban" mice new neurons arose in a much larger number than in their passive relatives, mired in a routine life in a vivarium.
Stem cells can be removed from the brain and transplanted into another part of the nervous system, where they become neurons. Professor Gage and his colleagues conducted several similar experiments, the most impressive of which was the following. A piece of brain tissue containing stem cells was transplanted into the destroyed retina of a rat eye. (The light-sensitive inner wall of the eye has a "nervous" origin: it consists of modified neurons - rods and cones. When the light-sensitive layer is destroyed, blindness sets in.) The transplanted brain stem cells turned into retinal neurons, their processes reached the optic nerve, and the rat regained its sight! Moreover, during transplantation of brain stem cells into an intact eye, no transformations occurred with them. Probably, when the retina is damaged, some substances are produced (for example,the so-called growth factors), which stimulate neurogenesis. However, the exact mechanism of this phenomenon is still not clear.
Scientists were faced with the task of showing that neurogenesis occurs not only in rodents, but also in humans. To this end, researchers under the guidance of Professor Gage recently performed sensational work. In one of the American oncology clinics, a group of patients with incurable malignant neoplasms took the chemotherapeutic drug bromodioxyuridine. This substance has an important property - the ability to accumulate in the dividing cells of various organs and tissues. Bromodioxyuridine is incorporated into the DNA of the mother cell and is stored in daughter cells after the mother's cells divide. Pathological studies have shown that neurons containing bromodioxyuridine are found in almost all parts of the brain, including the cerebral cortex. So these neurons were new cells that emerged from stem cell division. The find unconditionally confirmed that the process of neurogenesis also occurs in adults. But if in rodents neurogenesis occurs only in the hippocampus, then in humans, it is likely that it can capture more extensive areas of the brain, including the cerebral cortex.
Recent studies have shown that new neurons in the adult brain can be formed not only from neuronal stem cells, but from blood stem cells. The discovery of this phenomenon has caused euphoria in the scientific world. However, the publication in the journal "Nature" in October 2003 cooled enthusiastic minds in many ways. It turned out that blood stem cells do indeed penetrate the brain, but they do not turn into neurons, but merge with them, forming binuclear cells. Then the "old" nucleus of the neuron is destroyed, and it is replaced by the "new" nucleus of the blood stem cell. In the rat's body, blood stem cells mainly merge with the giant cells of the cerebellum - the Purkinje cells, although this happens quite rarely: only a few merged cells can be found in the entire cerebellum. More intense fusion of neurons occurs in the liver and heart muscle. It is not yet clear what the physiological meaning is in this. One of the hypotheses is that blood stem cells carry with them new genetic material, which, entering the "old" cerebellar cell, prolongs its life.
So, new neurons can arise from stem cells even in the adult brain. This phenomenon is already widely used for the treatment of various neurodegenerative diseases (diseases accompanied by the death of neurons in the brain). Stem cell preparations for transplantation are obtained in two ways. The first is the use of neuronal stem cells, which are located around the ventricles of the brain in both the embryo and the adult. The second approach is the use of embryonic stem cells. These cells are located in the inner cell mass at an early stage of embryo formation. They are able to transform into almost any cell in the body. The biggest challenge in working with embryonic cells is to make them transform into neurons. New technologies make it possible to do this.
Some hospitals in the United States have already formed "libraries" of neuronal stem cells derived from embryonic tissue, and are being transplanted into patients. The first attempts at transplantation are yielding positive results, although today doctors cannot solve the main problem of such transplants: the rampant multiplication of stem cells in 30-40% of cases leads to the formation of malignant tumors. So far, no approach has been found to prevent this side effect. But despite this, stem cell transplantation will undoubtedly be one of the main approaches in the treatment of such neurodegenerative diseases as Alzheimer's and Parkinson's, which have become the scourge of developed countries.
Nerve tissue is restored at any age, - assured the famous German neuroscientist Professor at the University of Göttingen Harold Huther. - At 20 years old, the process is intense, and at 70 - slowly. But it goes.
The scientist cited as an example the observation of the Canadian colleagues of the elderly nuns - 100 years or more. Magnetic resonance imaging showed: their brains are in order - no manifestations of senile dementia.
And the whole thing, according to the professor, is in the way of life and thinking of these women, who literally restore their brain structures and their conductivity. And a similar miracle happens due to the fact that the nuns are modest, have stable ideas about the structure of the world, an active life position and pray, hoping to change people for the better.
Huther explained that the main destroyer of nerve cells is stress, which also suppresses the brain's ability to regenerate. And harmony with oneself contributes to it. And this is what the professor advises in this regard: to measure dreams with reality, to be able to organize your life, and not to go, as they say, with the flow, to understand the meaning of life - at least your own, to have strong social connections - good relations with as many people - especially close ones.
And further. According to Huter, nothing helps nerve cell regeneration more than a problem for which a person has found a solution. And so that the problems are not too burdensome, the professor recommends learning something. Even in old age. To keep the taste for life.
The rate at which nerve cells regenerate was measured by Swedish scientists from the Karolinska Institute. It turned out that it can reach 700 new neurons per day.
The researchers were helped by … ground-based nuclear tests, which were carried out in the 50s of the last century. Then they strongly polluted the environment with a radioactive isotope - carbon-14. But its level dropped after it was banned in 1963 to detonate atomic bombs in the atmosphere.
The nerve cells of people who caught ground nuclear explosions "sucked" the isotope in an increased concentration. It is embedded in the DNA strands. Scientists used it for the so-called radiocarbon dating of living tissues. Carbon-14 made it possible to determine the age of the cells. And it turned out that they - nerve cells - appeared at different times. That is, along with the old, new ones were born.
Likewise, Canadians at the University of Toronto have shown that heart muscle cells are capable of regeneration. The living pump of a 25-year-old man is capable of producing newborn cells in an amount of up to 1 percent per year of the organ weight. By the age of 75, the productivity of the "factory" drops to 0.45 percent. But it does not disappear at all.
Why do we hardly remember our childhood?
It seems that Canadian researchers at the Neurobiology Laboratory at the Hospital for Sick Children in Toronto have figured out why most adults don't remember what happened to them in their first three years of life.
“It's not that children are bad at forming memories,” says Katherine Akers, one of the study's authors. They shape very well. When my daughter was 3 years old, I took her to the zoo. She told in detail about everything that she saw. Now she is 5 years old - she does not remember at all that she was in the zoo.
Experiments have shown that old events are erased from memory. Are erased during the birth of new brain cells.
Drinking and getting smarter?
The same Swedish scientists came to a startling conclusion. If the scandalous results of their recent research are to be believed, new nerve cells are also growing from regular drinking. They grow not just anywhere, but in the head - the most, it would seem, vulnerable part of the body of alcoholics.
However, scientists upset, not everything is so cloudless. Along with the cells, the craving for alcohol also grows. In Swedish experiments, mice, namely, they were watered, were indeed enriched with nerve cells. But at the same time they began to prefer vodka to water. According to Professor Stephen Brin, the head of the research, this explains the fact that people can go from moderate alcohol consumption rather quickly to unrestrained drinking.