Synaptogenesis: How are connections between neurons created?

Synaptogenesis is the process by which synapses are created, that is, connections between a 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 first understand what characteristics define these types of cells and what exactly synapses are. 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 a wide variety of functions to take place, such as the secretion of neurotransmitters and hormones, contraction and distention of muscles, perception, thinking 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 cell body, is where the processes and functions of the neuron are organized; the dendrites They are short extensions that receive electrical impulses; and the axon it is a longer appendage that sends signals from the soma to other cells.

When 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 "synapse" the connection between a neuron and another cell, especially if this is also a neuron. When two cells synapt, 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 essential for the proper functioning of the nervous system and therefore of the processes that depend on it.

• Related article: "What is synaptic space and how does it work?"

How are connections between neurons created?

The ends of the axons and dendrites (before birth, in the latter case) contain extensions with conical shape that develop in the direction of another cell and make the neuron grow and approach it, allowing connection synaptic. The name given to these extensions is "neural growth cones."

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 to grow. Once the axon connects with the postsynaptic cell, neurotrophic factors signal it to it and it stops growing.

This process, which takes place to varying degrees throughout life, is called synaptogenesis and allows brain plasticity, that is, the faculty by which our nervous system grows, changes and is restructure. Plasticity is based on learning and practice, which cause the connections between neurons to strengthen or weaken.

The frequency of synaptogenesis is higher in some stages of life, especially during early development. Nevertheless, the environmental stimulation favors synaptogenesis at any stage of life.

Synaptogenesis throughout development

At the beginning of embryonic development, there is a massive proliferation of neuroblasts (precursors of neurons) in the inner zone of the neural tube; this moment is known as "neurogenesis". Subsequently, the neuroblasts migrate out of the neural tube through the radial glia, thus spreading throughout the nervous system.

Neuroblasts become neurons during the process of cell differentiation, which depends on the information contained in genes and occurs after migration is complete. Differentiation also depends in part on induction, that is, on the influence of neurons in a region on the development of the neuroblasts that reach it.

Synaptogenesis begins around the fifth month of fetal development, but the critical period occurs after birth. During this first synaptogenesis, an excess of neurons and connections between them are 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, as the schizophrenia.

Although genes determine in part the characteristics of synaptogenesis in each individual, twin studies and with cloned animals have shown that the distribution of synapses varies in genetically equal or very Similar. This confirms the importance of learning and the specific environment in synaptogenesis.