One of the most remarkable discoveries of modern neuroscience is that the adult brain is capable of producing new neurons in various mammalian species, including humans. For more than 30 years, science denied this possibility, which has now been confirmed by several important studies .
In addition to increasing in number, it seems that they play relevant roles. Several lines of evidence suggest that these new neurons have an essential role in cognitive flexibility, learning, and especially in “pattern separation memory,” which allows us to recall similar situations without mixing them up, such as different encounters with the same person.
A recent study by Swedish neuroscientist Jonas Frisén, published in the journal Science , confirmed that this neurogenesis is continuous in the human brain. However, experiments with animals have shown that chronic stress can impede neurogenesis and even impair normal brain physiology in the hippocampus region .
This is a warning sign for those suffering from affective disorders, including depression, anxiety, burnout , and post-traumatic stress disorder, which affect more than 1 billion people worldwide, according to the World Health Organization (WHO).
At the same time, these findings raise the interesting possibility that therapies inspired by neurogenesis – including aerobic exercise and meditation – could become an important tool in psychiatry and psychology.
An academic and personal interest
This topic is especially important to me because, in addition to being a neuroscientist at the Institute of Collective Health of the Federal University of Western Pará (ISCO-UFOPA), I also fought for two years against this invisible enemy that plagues minds. I experienced symptoms of depression and burnout . The phenomenological sensation, however, was of being caged, disconnected, without motivation.
In my case, definitive recovery didn’t come from pharmacological interventions, but from a radical change in environment and behavior that I came to call “Neural Symbiosis.” It’s a concept that deals with a reconciliation with nature , through the movement and sensory stimuli it provides, to induce neuroplasticity, the brain’s ability to adapt to new situations.
I published a review of the scientific literature on this topic in the journal Frontiers in Neuroscience. In it, I compile recent scientific evidence on the potential of neurogenesis-inspired therapies to reduce the symptoms of these affective disorders. But before going into detail about that, I would like to highlight some theories that help to understand the relevance of this type of approach.
Contrast of ancestors
If we analyze the issue from an evolutionary perspective, the stark discrepancy between the environment in which our brains evolved, from the African savannas, to the “concrete jungles” where we live today is evident. Our ancestors spent 99% of their evolutionary history as nomads and hunter-gatherers. Our genes and neural circuits were sculpted not only for movement, but for processing a sensorially rich environment.
Daniel Lieberman , an evolutionary biologist at Harvard, calls our current condition an “evolutionary mismatch disease.” He shows in his studies that the origin of many modern diseases can be interpreted as a lack of synchronicity between our bodies and the current environment, rich in high-calorie foods and scarce in physical exercise.
In other words, it’s a mismatch between the evolution of our biology and the demands of daily life, resulting in problems such as weight gain and behavioral dependencies. And sedentary lifestyles are only part of the problem. We produce microplastics that accumulate in the brain and ultra-processed foods that cause cancer.
Furthermore, there is a decline in sensory and motor skills. Smooth walls, artificial light, screens, and air conditioning are also part of our disordered evolution.
It’s as if we’re trying to run Pleistocene software on modern hardware. We are primates, whose cerebral cortex has expanded beyond our evolutionary adaptation. The result of our sensory and motor deprivation in natural environments is a systemic collapse of the nervous system induced by the chronic stress of modern societies. And this contributes to affective disorders.
Friendly fire of immunity
Another important factor contributing to affective disorders is excessive inflammation in the brain. I also discussed this issue in another scientific review published in the journal Neural Regeneration Research.
Part of the problem begins in the microglia, which are one of the main defense cells of the Central Nervous System against infections. They actively patrol for invading microorganisms and, if any are detected, unleash a series of chemical responses and recruit other cells to help eliminate them. Although essential, sometimes this inflammation ends up killing some neurons along with the microglia.
Furthermore, these cells also play a dual role , killing neurons after acute and chronic neurological disorders. It seems that the stress caused by some neural disorders confuses this system, which is triggered erroneously. This generates what I have termed the “friendly fire hypothesis”: neuroinflammation caused by the body itself, which can lead to various affective disorders, including depression and anxiety.
How new neurons can help
I believe the missing link between the hostile environment in which many of us live and mental health is neuroplasticity. Well-established studies show that neurons generated in the hippocampus during adulthood are necessary for the normal expression of stress-related hormones . Mice that lack this neurogenesis respond worse to various stressful situations.
Some studies are already beginning to test the effect of mental and physical training aimed at increasing neurogenesis in the hippocampus of mammals. For example, a conditioning program ( MAP-training ), proposed by the American neuropsychologist Tracey Shors, suggests 30 minutes of meditation with 30 minutes of aerobic exercise, done twice a week for eight weeks. Scientific studies show that MAP-Training reduces depressive symptoms in people with major depressive disorder and anxiety.
Of particular interest were the pioneering studies, conducted in the late 1990s, by the Dutch neuroscientist Henriette van Praag, then at the Salk Institute for Biological Studies in San Diego, United States. She showed that rodents that run on wheels attached to their laboratory cages have 40% more neurons in the hippocampus than those that do not run.
Furthermore, cages with enhanced motor and sensory stimulation also induce more adult hippocampal neurogenesis than animals raised in simple cages.
A review published in Nature Neuroscience argues that it is difficult to replicate environmental enrichment studies in humans. In the laboratory, they compare artificial and controlled situations. In humans, this is almost never possible. Each person has a unique life history, difficult to measure, which makes direct comparisons inaccurate.
Nevertheless, the increase in relative stimuli has proven relevant in contexts such as rehabilitation after a stroke, for example. In these cases, the focus is not on comparing final results, but on individual trajectories over time, integrating different areas of knowledge.
Furthermore, in some countries, such as Japan, forest therapies that use contact with nature to increase neuroplasticity are already a reality. (The Conversation)
[By Walace Gomes Leal, Neuroscientist and professor at the Institute of Collective Health, Federal University of Western Pará]
