Dynamism of the Brain. An Organ in Continuous Remodeling

A MOVING ORGAN
The brain is an organ in constant change. Unlike the rest of our organs, cells and some specialized areas of our brain are remodeled along our entire life in response to everyday situations. For example, they are modified when we learn something new and they also change when that new learning is consolidated in what we know as memory, but it is also modified in other situations.Think, for example, of someone we know who has suffered some kind of substantial brain damage as a result of a severe blow or head trauma, ischemia, embolism, or hemorrhage.

We know that, as a result of the damage, some neurons die and the person has an alteration or loss of some of the functions controlled by the damaged area. However, over time, we can see that the person can recover the lost functions at least partially.

Laboratory studies suggest that this occurs because some of the surviving neurons surrounding the damaged area can remodel themselves by changing their structure and way of communicating with other neurons, allowing for the restoration of lost or altered functions. Also, regions far from the damage zone, but connected to it, can modify their function and help functional recovery. This ability to reorganize the brain, in conjunction with routine processes such as the ability to learn and remember, occurs due to a fundamental property of brain cells that we call nerve plasticity or nerve cell plasticity.

The term nerve plasticity refers to the ability of the cells of the nervous system (neurons) to modify the way they communicate with each other. In general terms, neurons are made up of a cell body, an axon that usually sends information through the axon terminal, and a dendritic tree with dendritic spines, which receive information. The contacts established by axons of some neurons with dendritic spines of other neurons are called synapses and are the basis of communication in the brain.

According to the Allen Institute, up to 84,000 neurons and half a billion synapses can be found in a piece of mouse brain the size of a grain of sand (Neergaard, 2025). Another equally interesting and even difficult to imagine fact is that it is estimated that in the human brain there are 86 billion neurons! Just to give us an idea of what this means, the human brain has as many neurons as there are stars in the Milky Way, and that neuron can establish up to 15,000 synapses with another neuron.

The complexity of brain communication is staggering, and the proper functioning of the brain as a single organ can be understood from the specialization of functions in specific brain regions and structures and the formation of synaptic circuits that communicate some structures with each other. Nowadays we know that there are structures that are responsible for processing movement; others, for the ability to locate oneself in space, or to learn a path and remember it; still others are involved in the generation and understanding of language, and each of them can change along our life.

We also know nerve connections can change continuously and in different ways throughout life. For example, new synapses can be generated from the birth and morphological modification of the dendritic tree or its spines (the small structures in a dendrite where synapses are established from contact with an axon); but the content of various molecules that allow synaptic communication and that inhibit, strengthen, or modulate it can also be modified. Thus, the nerve cells in the brain and the communication they establish are in continuous remodeling.

INTERNATIONAL FRONTIER RESEARCH
There are several research groups and laboratories at UNAM dedicated to the study of nerve plasticity, for which animal models are used and which concentrate their efforts on understanding which neurons participate in various functions or how each synaptic circuit behaves during classic learning processes, but also how regions are reorganized or how synaptic communication that has been affected by different types of brain damage is remodeled. 

In some efforts to better understand the cellular plasticity events that occur after a process of damage to the brain and that could explain the restoration of functions, our working group at the Institute of Biomedical Research has collaborated with groups in Germany using cutting-edge techniques that have allowed us, for example, to see the response of groups of cells to the presentation of visual stimuli before and after a brain injury. As a result of our observations, we identified that certain sets of cells in the brain that did not respond to a given visual stimulus before the damage occurred, do so after the damage occurred at the expense of other sets of cells, so that the latter could contribute to reestablishing a visual function altered by brain injury (Zepeda et al. 2003, 2004). In another, more recent collaboration, we found that some neurons that are located very close to an area of brain damage show growth of their dendritic tree and that some of their dendrites are directed to the area of damage, which could be a response to replace the connections that were lost due to the death of damaged neurons (Jungenitz et al., 2024). 

It is important to note that, even though neurosciences have advanced significantly, expanding our understanding of some responses that accompany the processes of learning, memory, and recovery of functions after brain damage in animal models, there are still many questions about the application in humans of the results obtained in research laboratories. For this reason, finding mechanisms for collaboration between basic neurosciences and medicine is becoming increasingly relevant. 

On the other hand, research on frontier issues such as the one we are dealing with here requires not only collaboration between scientific disciplines, but also between scientists across borders. The examples mentioned here show the importance of international collaborations and the role of our university in the generation of knowledge.

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