The neurologists, for a better part of the 20th Century, assumed that the central nervous system in the mammals become structurally stable and fixed. Due to the complicated structure and function of the mammalian brain, it was contemplated that it was not possible to reshape the neural network during the adulthood. This assumption was put forward because of the lack of any evidence, pointing out any structural changes in the brain in the latter stages. In the advancing years however, dramatic changes occurred in this type of thinking as the result of the advances in experimental methods. Today, it is clearly known that the brain is a n organ that undergo constant changes throughout the life instead of a fixed structure.
Could your brain repair itself? – Ralitsa Petrova
The evidence of neurogenesis (the multiplication of the neurons) that was seen in adults in 60’s was ignored. Around 20 years later, the evidence regarding the neurogenesis was first revealed. In 1984, Paton and Nottebohm observed, for the first time, newly generated neurons in singing birds. In addition, it was learned that the newly formed neurons could be functionally integrated into the neural networks and it was shown that they could react to auditory stimulations. As the result of the discovery of the retrovirus through further studies, imaging the cell activity in the brain became even easier. After adding a gene that provides bioluminescence to the retroviruses, the multiplication of the cells and their integration to the networks was observed more clearly. After this phase, evidence of neurogenesis was found for various mammalian species as well. Today, it is acknowledged that some types of neuron are constantly generated and are added to the neural networks in adults. Exactly in which part of brain the neurogenesis occurs is still a subject of debate. Despite this, the hippocampus has been considered as an area of the brain, where, new neurons are generated throughout the life span.
Hippocampus hosts a heterogeneous neuron population that differs in terms of age, morphologic characteristics and connections. The mature neurons constantly come together with their synapses and dendrites to form neural networks and form new neurons throughout the life cycle.
In adult rodents, several thousand new neurons are produced everyday in the dentate gyrus (DG) section of the hippocampus. In this way, %6 of the cell population in that area is renewed. Although the most of these newly generated DG cells (%60-80) die within a months due to apoptosis (pre-programmed cell death) , a large quantity survives and is integrated into new neural networks in a functional manner.
The neurogenesis in hippocampus in mammals decreases by the age. It is debatable whether the rate of growth of the cells in DG outmatches the apoptosis. Some studies indicate that the total number of neurons in DG in the rats does not increase. However, other studies showed an increase in the total number of neurons in this area in mice. Nevertheless, it is generally accepted that the apoptosis in DG and the number of the newly formed neurons vary depending on the genetic factors and behaviors.
How the food you eat affects your brain – Mia Nacamulli
In a fully grown living being, do the neurons, freshly generated in DG, make a different contribution apart from the other neurons when they join into a neural network in hippocampus? According to the findings of the research, conducted so far, the new neurons, after they join into a new network, may be tasked for conditioning, data processing and the memory configuration. In a fully grown DG, join the neural networks in hippocampus to decode new information. As suggested by the results of the new researches, the presence of the freshly minted neurons is necessary for a long term episodic memory.
As the result, the findings of the studies show that the new neurons in adults are crucial for the hippocampus related functions, especially the ones associated with memory and learning.
The neurogenesis in the adults is not only a source for the neurons, it also shows variance according to the environmental signals during the development. Thanks to this feature, the living beings are able to adapt to their environments.
Despite the findings, the purpose of the new neurons in DG area has not been exactly revealed yet. What the various factors affecting the brain plasticity is also yet to be known. Deciphering the mechanism of neurogenesis in the brain and revealing all the factors that affect this mechanism by the future studies will be encouraging for learning difficulties and attention deficit disorders.