Las asombrosas posibilidades de la neuroplasticidad y la neurogénesis

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 explicó cómo cambian las conexiones entre las neuronas (células cerebrales) como resultado de disparos repetitivos. Entonces, cuando repites un movimiento como balancear un palo de golf una y otra vez, las vías neuronales involucradas en el control de ese movimiento se vuelven más fuertes y más rápidas. No solo las sinapsis existentes (uniones entre neuronas) comienzan a dispararse de manera más eficiente, sino que se forman nuevas sinapsis y se reclutan otras neuronas para participar en la acción. Como resultado, su swing de golf se vuelve más automático, confiable y contundente cuanto más practica.

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 Neurogénesis, 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 investigación sobre neuroplasticidad y neurogénesis aún está en su infancia. Comprender cómo funcionan estos procesos presenta oportunidades increíbles para prevenir y recuperarse de enfermedades neurodegenerativas, mejorar la calidad de vida a medida que envejecemos y alcanzar nuestro potencial humano. En esta publicación, hablaré sobre cómo se producen nuevas neuronas en el cerebro, cómo el cerebro se adapta al uso y qué puede hacer usted para aumentar y disminuir la neurogénesis.

Cómo nacen nuevas neuronas y qué sucede después

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.

Stem-cells: Prospects for treating dementia. mayo 2008.

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.

A medida que continúe la investigación sobre la neurogénesis, los científicos descubrirán cosas más interesantes sobre cómo y dónde se producen en el cerebro nuevas neuronas. Pero producir nuevas neuronas es solo la primera pieza del rompecabezas. ¿Puede la migración neuronal (el movimiento de los neuroblastos) ocurrir en todo el cerebro, en función de dónde necesitemos nuevas neuronas? La investigación solo está comenzando a responder esta pregunta, pero hasta ahora parece que la respuesta es sí.

“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. marzo 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.

Llevando sus neuronas a donde deben ir: neuroplasticidad dependiente de la actividad y migración neuronal

La neuroplasticidad dependiente de la actividad es lo que sucede cuando las neuronas existentes forman nuevas sinapsis y crean nuevos circuitos basados en cómo elegimos usar nuestro cerebro; esto es aprendizaje. Tu cerebro se adapta constantemente a cómo se usa. Si practica el tango todos los días, las vías neuronales involucradas en el control de sus movimientos de baile se volverán cada vez más fuertes y eficientes. El secreto para usar la neuroplasticidad dependiente de la actividad a su favor es aprender su nueva habilidad y luego practicar, practicar y practicar.

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.

Ahora los científicos están descubriendo que si estimula una determinada área del cerebro, hace que las neuronas migren hacia esa área; esto es migración neuronal dependiente de la actividad.

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.

¿Estás listo para aprovechar al máximo tu neuroplasticidad y neurogénesis? Primero, debe dejar de hacer cosas que inhiben la neurogénesis. Entonces, debe comenzar a hacer cosas —¡o seguir haciéndolas! - que promuevan la neurogénesis. Finalmente, debe estimular la neuroplasticidad y la neurogénesis en las áreas del cerebro donde desea que ocurran, realizando las actividades en las que desea mejorar.

¿Qué se interpone en nuestro camino para producir nuevas neuronas?

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.

Siete formas de estimular la neurogénesis

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.

Por qué puede ser difícil comprender la neuroplasticidad y la neurogénesis

Todo el mundo comprende el "entrenamiento del cerebro" en un nivel práctico; es por eso que practicamos nuestras habilidades una y otra vez para mejorarlas. Sabemos que el aprendizaje y la repetición funcionan. Otra cosa es adoptar la neuroplasticidad y la neurogénesis como procesos fisiológicos que puede controlar y tomar medidas para mejorar. Medir los cambios en la función de su cerebro es difícil porque no puede verlos de la misma manera que puede ver los cambios en su cuerpo o realizar un seguimiento de las mejoras en su rendimiento físico.

También puede ser fácil pensar que estás imaginando cosas. Es normal que su estado de ánimo, energía, concentración, habilidades cognitivas y nivel de estrés fluctúen de un día a otro. ¿Cómo saber si algo diferente representa un cambio estructural y funcional en su cerebro?

Cuando realice un cambio en su estilo de vida, como dormir más, reducir la ingesta de alcohol, comenzar a meditar o hacer más ejercicio, observe los efectos. ¿Te sientes más feliz o menos estresado? ¿Puede concentrarse mejor en el trabajo? Observe cómo se siente todos los días a medida que continúa con su nuevo hábito y comenzará a ver tendencias. Cuando una tendencia persiste a lo largo del tiempo, puede estar seguro de que está haciendo un cambio duradero en su cerebro.

Y trate de imaginarse los cambios que tienen lugar en su cerebro. Cuando sea más rápido en su juego de computadora, visualice los circuitos neuronales que controlan su juego de manera eficiente iluminando y enviando sus mensajes. Cuando se dé cuenta de que reacciona menos a eventos potencialmente estresantes, imagine la calma en su cerebro cuando decide no activar su respuesta al estrés.

Tenemos tanto control sobre la salud y el funcionamiento de nuestro cerebro como nuestro cuerpo. Pero debido a que la ciencia es bastante nueva y los cambios en el cerebro son más difíciles de medir para nosotros, no hemos llegado a dominar la mejor manera de entrenar nuestro cerebro y mantener su salud a lo largo de nuestra vida. Por ahora, puede ser necesario un poco de fe y mucha conciencia de sí mismo para mantener su cerebro en la mejor forma posible.

Lectura recomendada:

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