Neurogenesis in the adult body

It was believed that neurons in the adult state in the body does not appear. Neurons only die, in particular because they are not used. But some experiments and observations did not coincide with this point of view. In particular, bird watching showed that the repertoire with which the male attracts the female is very diverse and changes with age - singing is a product of work and learning. This process is resumed and rerun every year — the bird forgets the skills and learns them anew. The male has areas of neurons in the brain that are responsible for singing more than those of the female during the period of singing, and during the quiet period of the same size. This dynamic change in the brain region led researchers to the idea that there is a mechanism that promotes the appearance of additional neurons. This assumption has been proven by experiments. The researchers concluded that the brain is capable of generating new neurons - neurogenesis. Neurogenesis is stimulated by the emergence of a new function, for which additional neurons are needed.

There are several discrete zones of neurogenesis in the adult body:

1) the subventricular zone (in the walls of the literal ventricles), in which the cells divide and from there the new cells migrate to the new cortex and olfactory bulb;

2) subgranular zone (in one layer of the dentate gyrus of the hippocampus) from where cells migrate for a short distance to the hippocampus;

3) local areas of the cortex - the temporal cortex and the prefrontal cortex (controversial statement, not everyone agrees with it).

Memory processes stimulate neurogenesis, every time a person tries to remember something, he stimulates the birth of new neurons.

Neurogenesis is most intensely represented in the hippocampus, it is he who responds to the novelty and compares it with the existing one. The cells that appeared in the hippocampus go exactly to the hippocampus.

The emergence of new neurons in an adult organism occurs as a compensatory injury, brain ischemia, the occurrence of convulsive electrical activity.

Neurogenesis is stimulated by :

1) body activity (enriched environment, physical activity, intensive social contacts);

2) training (the acquisition of knowledge, the development of skills, abilities);

3) the emergence of new needs .

Neurogenesis goes in several stages :

1) stem cell proliferation (cell division, sometimes secondary division of daughter cells occurs);

2) the migration of its daughter cells to their structural targets (the path to the olfactory bulb is well studied, since it is there that the neurons are most quickly replaced are replaced);

3) differentiation - the transformation of a stem cell into a specialized one (neuron or glia).

Not all stem cells are capable of division, division is vertical (one remains spare, and the second differentiates into the other) and horizontal (two identical cells). The cell after division has no processes, only catfish. Differentiation can take place already during the migration, the closer to the target zone, the more specialized the neuron. Migration can occur in all areas of the cortex, for example, in the case of injury and the need to replace dead neurons. New neurons are embedded in the damaged area, acquire the characteristics of the surrounding neurons. That is, the neuron acquires a full specialization on the spot. The very fact of embedding and specialization is repeatedly proven. For movement there are several landmarks in the brain. For movement, signals from the medium and receptors on the cell that will receive signals are needed. In some cases, the movement of the neuron goes along the glia, in others the axon grows (according to roughness or chemical signals), there is also a genetic program that makes you notice the markers and after them perform certain actions.

The chemical substance, known as BrdU getting into the brain, at the stage of proliferation - cell division, is connected to the DNA of cells. Connects to the newborn daughter cell, which thus receives the label.

The process of differentiation of new cells involves the expression of genes, successively capturing various regions of DNA.

The expression of the substance TiL is a sign of the belonging of a cell to a neuron, which is both at the initial stage of formation and at the stage of appearance of its mature form.

NeuN expression - formed neuron marker

GFAP - glial cell (astrocyte) marker

CNP - oligodendroglia marker.

The migration path is traced by counting the cells marked with one marker or another.

Neurogenesis in the olfactory bulb - daughter cells follow the path to the olfactory bulbs; a certain gene is responsible for the migration of stem cells. Knockout of the gene responsible for the migration of neuroblasts along the rostral path into both olfactory bulbs leads to the fact that the cells do not move, but collide with each other, and the animal loses the sense of smell.

Zones of neurogenesis differ in intensity and speed of the process.

Not all neurons that are built into the structure survive. Survival depends on how actively they begin to be used. There is a growth phase, a conservation phase and a death phase of newborn neurons.

Depression and stress reduce the volume of the hippocampus and the number of new neurons.

Antidepressants increase the number of new neurons in the hippocampus.

If you block neurogenesis, the antidepressants no longer relieve depression.

Stress disrupts neurogenesis in the hippocampus, which manifests itself at a behavioral level in the deterioration of learning outcomes:

-the effect of prenatal stress

-Stress associated with the long formation of difficult skills (finding a platform under water).

The results showed that, despite the training, neurogenesis is impaired.

Neurogenesis is also disturbed by drugs and alcohol (loss of cognitive abilities and memory). Alcohol does not affect the neurogenesis of the olfactory bulb, but it greatly reduces the hippocampal neurogenesis. The effect of alcohol consumption increases with time. Also under the influence of alcohol, the survival rate of newborn neurons is sharply reduced. The number of damaged cells grows, the nuclei are destroyed.

Apaptosis is a genetically programmed cell death. Newborn neurons die. For a cell to survive, it must get a lot of connections with the surrounding cells. We need strong synapses, the strength is determined by how strong the signal is sent by the cell to another neuron, and, accordingly, it receives a response in the form of a neurotropic factor that acts on the cell nucleus. If this is not the case, then the cell dies - the lack of a neurotropic factor causes the launch of the death genome - apaptosis. This is a system for utilizing unused neurons.

There is a similarity of neurogenesis in an adult organism with the maturation of neurons in ontogenesis.