Les incroyables possibilités de la neuroplasticité et de la neurogenèse
You may have heard the phrase “neurons that fire together wire together.” This short phrase summarizes the synaptic plasticity theory of learning described by Canadian psychologist Donald Hebb in his 1949 book The Organization of Behavior.
Hebb a expliqué comment les connexions entre les neurones (cellules du cerveau) changent à la suite de décharges répétitives. Ainsi, lorsque vous répétez un mouvement comme balancer un club de golf encore et encore, les voies neuronales impliquées dans le contrôle de ce mouvement deviennent plus fortes et plus rapides. Non seulement les synapses existantes (jonctions entre neurones) commencent à se déclencher plus efficacement, mais de nouvelles synapses se forment et d'autres neurones sont recrutés pour participer à l'action. En conséquence, votre swing de golf devient plus automatique, fiable et puissant au fur et à mesure que vous vous entraînez.
That is neuroplasticity: your brain’s ability to change and adapt based on input and use. The concept of neuroplasticity had been previously proposed by others, most notably American psychologists William James and Karl Lashley, and Polish neuroscientist Jerzy Konorski, but it was largely ignored by the scientific community until Hebb brought the concept to the forefront in his groundbreaking book.
In 1965, scientists took another huge step forward in their understanding of the brain when a study demonstrated that new neurons are produced in the brains of adult rats. This was the first proof that Neurogenèse, the process by which new neurons are formed in the brain, occurred after birth. Up until that point, scientists believed that higher vertebrates (reptiles, birds, and mammals) were born with all the neurons they were going to have for their entire lives, and that neurons gradually died over time and never got replaced.
But was the same true for human brains? In 1998, researchers showed that adult human brains produced new neurons throughout life as well—and the brain-fitness industry boomed.
La recherche sur la neuroplasticité et la neurogenèse en est encore à ses balbutiements. Comprendre le fonctionnement de ces processus offre des opportunités incroyables pour prévenir et guérir des maladies neurodégénératives, améliorer la qualité de vie à mesure que nous vieillissons et atteindre notre potentiel humain. Dans cet article, je parlerai de la façon dont les nouveaux neurones sont produits dans le cerveau, de la façon dont le cerveau s'adapte à l'utilisation et de ce que vous pouvez faire pour augmenter et diminuer la neurogenèse.
Comment naissent les nouveaux neurones et que se passe-t-il ensuite
Neural stem cells and progenitor cells give rise to immature neurons called neuroblasts. These newborn brain cells can then stay where they are or migrate, and then mature and integrate into our neural circuitry.
The hippocampus, which plays an important role in memory and spatial navigation, is one of the brain areas where neurogenesis has been observed in humans. In Alzheimer’s disease, the rate of neurogenesis in the hippocampus progressively declines as the disease advances. Scientists have found that enhancing neurogenesis in rodents improves hippocampal size and function, and decreases symptoms of Alzheimer’s disease. Their findings suggest that stimulating neurogenesis in humans, through environmental enrichment, running, and other proven methods, may be a key therapeutic approach in preventing and reversing Alzheimer’s in humans. The size of the hippocampus in people with major depressive disorder is significantly reduced, suggesting that a similar approach could be used for treating depression.
Another part of the brain where neurogenesis occurs is the subventricular zone (SVZ) of the lateral ventricals. Neuroblasts produced in the SVZ migrate through the rostral migratory stream (RMS) to the olfactory bulb, which is involved in our sense of smell.
Research shows shows neurogenesis occurring in the cerebral cortex of adult mice, and one study of stroke sufferers shows evidence of new neurons in the human cerebral cortex (the outer layer of the brain that plays an important role in attention, perception, awareness, thought, memory, language, and consciousness). There are also cells that express stem cell markers in the adult cerebellum, which is responsible for coordinating movement.
And in 2003, scientists were surprised to find that neurogenesis occurs in the substantia nigra, the brain area affected in Parkinson’s disease. In 2016, researchers found that neural progenitor cells in the brains of adult mice replenish the dopaminergic neurons that are lost in Parkinson’s disease. The researchers suggest that neuron loss in Parkinson’s disease may result from inhibition of neurogenesis. They note that neurogenesis has been difficult to prove due to limitations of current cell lineage tracing methods, and they were able to demonstrate neurogenesis of nigral neurons using a new tracing model that they developed. Neurogenesis of nigral neurons is not limited to mice—viable neural stem cells have been found in the brains of people with Parkinson’s as well.
Au fur et à mesure que la recherche sur la neurogenèse se poursuit, les scientifiques découvriront des choses plus intéressantes sur comment et où dans le cerveau de nouveaux neurones sont produits. Mais produire de nouveaux neurones n'est que la première pièce du puzzle. La migration neuronale - le mouvement des neuroblastes - peut-elle se produire dans tout le cerveau, en fonction de l'endroit où nous avons besoin de nouveaux neurones ? La recherche commence seulement à répondre à cette question, mais jusqu'à présent, il semble que la réponse soit oui.
“Neuroblasts derived from the ventricular‐subventricular zone (V‐SVZ) are a special population that can migrate over long distances in the adult brain; this is an important characteristic for providing new neurons for neuronal regeneration to areas that are distant from the germinal zone, especially in large primate brains. The long‐distance directional migration of these neurons is controlled by various endogenous and exogenous factors.”
–Mechanisms of neuronal migration in the adult brain. mars 2017.
For example, after a brain injury, neuroblasts produced in the V-SVZ migrate toward the injury to repopulate the injured brain tissue. The neuroblasts are guided toward the injury by chemoattractants and structures including blood vessels and neuroblast chains. The immature neurons tend to change direction and explore their environment more than those that stay within the well-traveled rostral migratory stream (RMS). They’re forging new pathways through the brain, so their migration is inefficient. When they do arrive at the site of the injury, they mature into functional neurons.
A 2017 study by researchers at University of Alabama at Birmingham showed something else that happens after new neurons are born. Neuroblasts produced in the hippocampal dentate gyrus synapse with existing neurons in the cerebral cortex, taking the place of older neurons in the neural circuitry. So instead of creating additional synapses, neuroblasts replace and weed out less-fit neurons, which then die. This finding shows the role that new neurons play in “neural pruning,” the process by which neurons that aren’t used on a regular basis are eliminated in order to increase the efficiency of regularly-used circuits.
Amener vos neurones là où ils doivent aller : neuroplasticité dépendante de l'activité et migration neuronale
La neuroplasticité dépendante de l'activité est ce qui se passe lorsque les neurones existants forment de nouvelles synapses et créent de nouveaux circuits en fonction de la façon dont nous choisissons d'utiliser notre cerveau ; c'est l'apprentissage. Votre cerveau s'adapte constamment à la façon dont il est utilisé. Si vous pratiquez le tango tous les jours, les voies neuronales impliquées dans le contrôle de vos mouvements de danse deviendront continuellement plus fortes et plus efficaces. Le secret pour utiliser la neuroplasticité dépendante de l'activité en votre faveur est d'apprendre votre nouvelle compétence, puis de pratiquer, pratiquer, pratiquer.
Neurologists in Germany did brain scans of people as they learned to juggle and compared them with controls. The scans clearly showed how brain areas involved in processing and storing complex visual motion increased in size over three months as the volunteers learned to juggle. Then three months later, after discontinuing the juggling practice, these brain areas had decreased in size, losing about half of the gray matter they had gained by juggling.
Similar cortical reorganization occurs in people and animals who have lost limbs due to amputation. After amputation, people and animals adapt by increasing use of their intact limbs; for example, someone who has lost their right hand will start using their left hand much more than they used to. Parts of the motor and somatosensory cortices that control and sense the intact body parts adapt to increased use by growing in size, while the parts that control and sense the lost limb decrease in size or sometimes move into nearby brain areas due to lack of use.
Maintenant, les scientifiques découvrent que si vous stimulez une certaine zone du cerveau, cela provoque la migration des neurones vers cette zone ; c'est la migration neuronale dépendante de l'activité.
In 2009, researchers in South Korea showed how hippocampal neurons migrate toward a site of electrical stimulation. Neurons communicate with each other via electrical signals called action potentials and chemical signals called neurotransmitters. When we’re using a part of our brain, electrical activity in that area increases, and this experiment clearly demonstrated how increasing electrical activity attracts new neurons. Similar results have been found using cortical neurons.
Êtes-vous prêt à tirer le meilleur parti de votre neuroplasticité et de votre neurogenèse ? Tout d'abord, vous devez arrêter de faire des choses qui inhibent la neurogenèse. Ensuite, vous devez commencer à faire des choses – ou continuer à les faire ! – qui favorisent la neurogenèse. Enfin, vous devez stimuler la neuroplasticité et la neurogenèse dans les zones cérébrales où vous souhaitez qu'elles se produisent, en faisant les activités dans lesquelles vous souhaitez vous améliorer.
Qu'est-ce qui fait obstacle à us produire de nouveaux neurones ?
Four of the lifestyle factors that inhibit neurogenesis the most are are stress, alcohol consumption, sleep deprivation, and diet.
Chronic stress decreases neurogenesis in the hippocampus, and also lowers the chances that the new neurons that do get produced will survive. Neuroinflammation (the inflammatory response of the immune system when it occurs in the brain) that results from stress also likely prevents the production of new neurons. And sadly, stress experienced in childhood can inhibit neurogenesis in adulthood. Constant stimulation and worrying about the future is not good for brain health—we need to slow down and relax our minds in the same way we rest our bodies.
Regular alcohol consumption reduces the size of the hippocampus to a degree proportional to the amount that people drink. The same is true for overall brain atrophy; researchers found that brain volume decreases in proportion to alcohol consumed, and the effect is measurable even in light and moderate drinkers in comparison to non-drinkers. Luckily, it seems that the effects can be reversed. While heavy alcohol consumption inhibits neurogenesis, subsequent abstinence allows neurogenesis to return to relatively normal levels in a short period of time.
Both sleep deprivation and sleep fragmentation (disrupted sleep) inhibit neurogenesis in the hippocampus. Even a single day of sleep deprivation reduces the rate at which new neural cells are produced. Not to worry—normal rates of neurogenesis can be recovered within about two weeks after adequate sleep is resumed.
Researchers have found a number of dietary factors that prevent the production of new neurons. It probably won’t come as a surprise that eating a high-fat diet is one, and consuming refined sugars is another. Being deficient in vitamin A and vitamin B can also inhibit neurogenesis. And pay attention to the texture of your food: eating a diet of soft foods decreases neurogenesis, while eating solid foods that require more chewing increases production of new neurons.
Sept façons de stimuler la neurogenèse
Exercise is one of the best ways to increase neurogenesis, largely because it boosts production of brain-derived neurotrophic factor (BDNF). BDNF is a protein that acts like Miracle-Gro for brain cells: it stimulates the growth of neuroblasts, helps them survive, and encourages the formation of new synapses. The positive effects of exercise are enhanced by environmental enrichment (EE), which can include being in new, stimulating surroundings or going outdoors.
Sustained aerobic exercise like running increases neurogenesis, while resistance exercise has not been shown to have the same effect. Dr. John Ratey, the author of Spark: The Revolutionary New Science of Exercise and the Brain, recommends doing both aerobic exercise and activities that demand focus and coordination, like martial arts, dance, rock climbing, and yoga, in order to fully stimulate your brain.
Learning improves the chances that neuroblasts will survive, mature, and integrate into neural circuitry. This is why continuing to stimulate your brain by learning new things throughout life is so important. For best results, scientists recommend exercising first in order to increase the production of new neurons, then spending time learning your new skill to help the new neurons survive and integrate. And be sure to let your learning be fun; when learning becomes stressful, it can decrease neurogenesis.
In rat experiments, sex increases the number of new neurons in the hippocampus. But regular sexual activity is best; a single sexual encounter also increases levels of the stress hormone corticosterone. Daily sexual encounters for 14 consecutive days do not raise corticosterone levels, and actually decrease anxiety behavior while still promoting neurogenesis. It’s also interesting to note that female rats only experience an increase in neuronal survival when they’re in control of the sexual encounter. Researchers speculate that when they’re not in control, female rats experience stress which prevents their new neurons from surviving. Neither of these experiments have been replicated in humans, but it’s easy to see how the same principles apply to short-term versus long-term relationships and male-female dynamics.
Living in groups helps us survive, so it’s not surprising that socializing promotes production of new brain cells. Neurogenesis is higher in socialized rats compared to rats kept in isolation. Isolated rats show a significant reduction in BDNF in the hippocampus, and they also perform worse on memory tasks. Social isolation also increases stress, and even cancels out the positive effects of running on neurogenesis. So if you live alone, be sure to make socializing part of your regular routine.
There is a long list of foods that promote neurogenesis. You may have heard that blueberries are brain food, and it turns out that strawberries are too (it’s the polyphenols). Grapeseed extract, turmeric, green tea, and resveratrol (found in peanuts, tree nuts, grapes, cocoa, wine, and berry fruits) also promote the growth of new neurons. And fatty fish, seaweed, algae, walnuts, and flax, chia, and hemp seeds contain omega-3 polyunsaturated fatty acids (PUFAs), which have been shown to promote hippocampal neurogenesis and improve spatial memory.
The idea of fasting is not a popular one—most people don’t like being hungry. But the practice of intermittent fasting is a new trend due to research showing its benefits for a wide range of health conditions. We evolved to survive during periods of fasting because food wasn’t always available. As a result, intermittent fasting is built into our physiology, and research shows that it benefits our health in many ways. Intermittent fasting lowers our insulin level and blood pressure, reduces inflammation, helps clear out toxins and damaged cells, improves our stress resistance, lengthens our lifespan, and reduces incidence of diabetes, obesity, and cancer. And of course, it increases neurogenesis and levels of BDNF, and prevents neuron death in the hippocampus.
The simplest form of intermittent fasting limits all food intake to a certain window of time, like 6 hours a day. Researchers recommend the circadian rhythm fasting approach, in which you start eating in the morning and finish consuming all your food by mid-afternoon or early evening. A 2018 study of men with prediabetes clearly showed the positive effects of early time-restricted feeding (eTRF): lower insulin levels, improved insulin sensitivity, and lower blood pressure and oxidative stress. Surprisingly, the eTRF group also had less desire to eat in the evening. Eating a big meal in the evening is probably not good for our health, and researchers suggest that the ways our circadian rhythms affect our body temperature, biochemical reactions, hormone levels, physical activity, and digestion of food may be why.
Finally, you can stimulate neurogenesis with meditation. The cerebral cortex of people who practice meditation is thicker in the areas associated with attention, interoception (internal sense of your body), and sensory processing. Differences in cortical thickness correlates with meditation experience, and meditation may offset age-related cortical thinning. Experienced meditators also have larger hippocampi than non-meditators, and yoga and meditation increase levels of BDNF.
Pourquoi il peut être difficile de comprendre la neuroplasticité et la neurogenèse
Tout le monde comprend «l'entraînement cérébral» à un niveau pratique - c'est pourquoi nous pratiquons les compétences encore et encore pour nous améliorer. Nous savons que l'apprentissage et la répétition fonctionnent. C'en est une autre d'embrasser la neuroplasticité et la neurogenèse en tant que processus physiologiques que vous pouvez contrôler et prendre des mesures pour les améliorer. Il est difficile de mesurer les changements dans votre fonction cérébrale, car vous ne pouvez pas les voir de la même manière que vous pouvez voir les changements dans votre corps ou suivre les améliorations de vos performances physiques.
Il peut aussi être facile de penser que vous imaginez des choses. Il est normal que votre humeur, votre énergie, votre concentration, vos capacités cognitives et votre niveau de stress fluctuent d'un jour à l'autre. Comment savoir si quelque chose de différent représente un changement structurel et fonctionnel dans votre cerveau ?
Lorsque vous modifiez votre mode de vie, comme dormir plus, réduire votre consommation d'alcool, commencer à méditer ou faire plus d'exercice, remarquez les effets. Vous sentez-vous plus heureux ou moins stressé ? Êtes-vous capable de mieux vous concentrer au travail ? Remarquez comment vous vous sentez chaque jour pendant que vous continuez votre nouvelle habitude, et vous commencerez à voir des tendances. Lorsqu'une tendance persiste dans le temps, vous pouvez être sûr que vous apportez un changement durable dans votre cerveau.
Et essayez d'imaginer les changements qui se produisent dans votre cerveau. Lorsque vous devenez plus rapide dans votre jeu sur ordinateur, visualisez les circuits neuronaux qui contrôlent votre jeu s'allumant efficacement et envoyant leurs messages. Lorsque vous vous retrouvez à réagir moins à des événements potentiellement stressants, imaginez le calme dans votre cerveau alors qu'il décide de ne pas activer votre réponse au stress.
Nous avons autant de contrôle sur la santé et le fonctionnement de notre cerveau que sur notre corps. Mais parce que la science est relativement nouvelle et que les changements dans le cerveau sont plus difficiles pour us pour mesurer, nous n'avons pas tout à fait maîtrisé la meilleure façon d'entraîner nos cerveaux et de maintenir leur santé tout au long de notre vie. Pour l'instant, il faudra peut-être un peu de foi et beaucoup de conscience de soi pour garder votre cerveau dans la meilleure forme possible.
Lecture recommandée:
The Pain Relief Secret: How to Retrain Your Nervous System, Heal Your Body, and Overcome Chronic Pain by Sarah Warren, CSE
Somatics: Reawakening the Mind’s Control of Movement, Flexibility and Health by Thomas Hanna
