Synaptogenesis: how are connections created between neurons?
Synaptogenesis is the process by which synapses are created , that is, connections between one neuron and another cell of the nervous system, or between two neurons. Although synaptogenesis is especially intense during early development, environmental influences influence the consolidation and disappearance of synapses throughout life.
To understand how the connections between neurons are created It is important to understand first what characteristics define this type of cells and what exactly are the synapses. It is equally important to clarify the meaning of various concepts related to synaptogenesis, such as brain plasticity and neurogenesis.
- Related article: "Types of neurons: characteristics and functions"
Neurons and synapses
Neurons are cells of the nervous system specialized in receiving and transmitting electrochemical impulses. These signals allow very varied functions to take place, such as the secretion of neurotransmitters and hormones, the contraction and distension of muscles, perception, thought or learning.
The morphology of neurons differentiates them from other types of cells. Specifically, they consist of three main parts: soma, dendrites and axon .
The soma, or cellular body, is where the processes and functions of the neuron are organized; the dendrites are short extensions that receive the electrical impulses; and the axon is a longer appendage that sends signals from the soma to other cells.
When the neural impulses reach the end of the axon (terminal button) the neuron secretes neurotransmitters, chemical compounds that favor or inhibit the action of the receptor cell. In the case of neurons, as we have said, it is the dendrites that capture these signals.
We call "synapses" the connection between one neuron and another cell , especially if this is also a neuron. When two synaptan cells, the membrane of the presynaptic neuron releases neurotransmitter into the synaptic space, the area where it binds with the receptor or postsynaptic cell; it receives the impulse, which regulates its activity.
The creation of numerous synapses between neurons and other cells, which occurs naturally during neurodevelopment, causes networks or circuits to form neuronal very complex These networks are fundamental for the proper functioning of the nervous system and therefore of the processes that depend on it.
- Related article: "What is the synaptic space and how does it work?"
How are connections created between neurons?
The ends of axons and dendrites (before birth, in the latter case) contain extensions with a conical shape that develop in the direction of another cell and cause the neuron to grow and approach it, allowing the synaptic connection. The name that these extensions receive is "cones of neural growth".
The growth cones are guided by neurotrophic factors secreted by target neurons. These chemical compounds attract or repel the axon of the presynaptic neuron; in this way they indicate where it should grow. Once the axon connects to the postsynaptic cell, the neurotrophic factors signal it and stop growing.
This process, which takes place in different degrees throughout life, is called synaptogenesis and allows brain plasticity, that is, the faculty by which our nervous system grows, changes and restructures. Plasticity is based on learning and practice , which make the connections between neurons consolidate or weaken.
The frequency of synaptogenesis is greater in some stages of life, especially during early development. However, the Environmental stimulation favors synaptogenesis at any stage of life.
Synaptogenesis throughout development
At the beginning of the embryonic development there is a massive proliferation of neuroblasts (precursors of the neurons) in the inner zone of the neural tube; this moment is known as "neurogenesis" . Subsequently, the neuroblasts migrate to the outside of the neural tube through the radial glia, thus extending throughout the nervous system.
Neuroblasts become neurons during the process of cell differentiation, which depends on the information contained in the genes and occurs once the migration is complete. The differentiation also depends in part on the induction, that is, on the influence of the neurons of a region on the development of the neuroblasts that reach it.
Synaptogenesis begins around the fifth month of fetal development, but the critical period takes place after birth.During this first synaptogenesis an excess of neurons and connections between them is created; later the less powerful synapses will disappear (apoptosis) and the definitive structure will settle with the learning.
During adolescence, synaptogenesis and apoptosis intensify again, although the changes are not as significant as those that occur in early development. Errors in any of these stages can favor the appearance of neurodevelopmental disorders , like schizophrenia.
Although the genes in part determine the characteristics of synaptogenesis in each individual, studies with twins and cloned animals have shown that the distribution of synapses varies in genetically equal or very similar individuals. This confirms the importance of learning and the specific environment in synaptogenesis.
