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 particularly intense during early development, environmental influences influence the consolidation and disappearance of synapses throughout life.

To understand how connections between neurons are created it is important to first understand what characteristics define this type of cell and what exactly are synapses. It is equally important to clarify the meaning of various concepts related to synaptogenesis, such as brain plasticity and neurogenesis.

Neurons and synapses

Neurons are cells of the nervous system specialized in receiving and transmitting electrochemical impulses. These signals allow a variety of 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 the neurons differentiates them from other types of cells. Specifically, they consist of three main parts: soma, dendrites and axon .

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The soma, or cell body, is where the processes and functions of the neuron are organized; dendrites are short extensions that receive 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 favour 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 the connection between a neuron and another cell “synapse”, especially if it is also a neuron. When two cells synapse, the membrane of the presynaptic neuron releases neurotransmitter into the synaptic space, the area where it binds to the receptor or postsynaptic cell; the latter receives the impulse, which regulates its activity.

The creation of numerous synapses between neurons and other cells, which occurs naturally during neurodevelopment, causes very complex neuronal networks or circuits to form . These networks are fundamental for the correct functioning of the nervous system and therefore of the processes that depend on it.

How do you create connections between neurons?

The ends of axons and dendrites (before birth, in the latter case) contain cone-shaped extensions that develop in the direction of another cell and cause the neuron to grow and approach it, allowing synaptic connection. The name given to these extensions is “neural growth cones”.

The growth cones are guided by neurotrophic factors secreted by the target neurons. These chemicals attract or repel the axon of the presynaptic neuron, thus telling it where to grow. Once the axon connects to the postsynaptic cell, the neurotrophic factors signal it and it stops growing.

This process, which takes place to varying degrees throughout life, is called synaptogenesis and allows for brain plasticity, that is, the ability for our nervous system to grow, change and restructure. Plasticity is based on learning and practice , which causes connections between neurons to become stronger or weaker.

The frequency of synaptogenesis is higher in some stages of life, especially during early development. However, the environmental stimulation favours 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 neurons) in the internal area of the neural tube; this moment is known as “neurogenesis” . The neuroblasts then migrate out of the neural tube through the radial glia, thus spreading throughout the entire nervous system.

Neuroblasts are converted into neurons during the process of cell differentiation, which depends on the information contained in the genes and occurs after the migration is completed. Differentiation also depends in part on induction, i.e. the influence of neurons in a region on the development of the neuroblasts that reach that region.

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 learning.

During adolescence synaptogenesis and apoptosis intensify again, although the changes are not as significant as those in early development. Errors at any of these stages may favour the appearance of neurodevelopmental disorders , such as schizophrenia.

Although genes partly determine the characteristics of synaptogenesis in each individual, studies with twins and cloned animals have shown that the distribution of synapses varies in individuals who are genetically the same or very similar. This confirms the importance of learning and specific environment in synaptogenesis.