Discovering the Power to Change Your Brain
Scientists all the way back to Ramón y Cajal -- who won the Nobel Prize in 1906 for proposing that the central nervous system was made up of individual neurons that communicate at what he termed “polarized junctions” -- have theorized that learning involves changes at the synapses. Despite the accolades, most scientists didn’t buy it. It took psychologist Donald Hebb to stumble onto the first hint of evidence.
The lab rules were loose in those days, and, apparently, Hebb thought it would be fine if he brought home some lab rats as temporary pets for his children. The arrangement turned out to be mutually beneficial: When he returned the rats to the lab, Hebb noticed that, compared to their cage-bound peers, they excelled in learning tests. The novel experience of being handled and toyed with somehow improved their learning ability, which Hebb interpreted to mean that it changed their brains. In his acclaimed 1949 textbook, The Organization of Behavior: A Neuropsychological Theory, he described the phenomenon as “use-dependent plasticity.” The theory was that the synapses rearrange themselves under the stimulation of learning.
Hebb’s work ties in with exercise because physical activity counts as novel experience, at least as far as the brain is concerned. In the 1960s, a group of psychologists at Berkeley formalized an experimental model called “environmental enrichment” as a way to test use-dependent plasticity. Rather than take rodents home, the researchers outfitted their cages with toys, obstacles, hidden food, and running wheels. They also grouped the animals together, so they could socialize and play.
It wasn’t all peace and love, though, and eventually the rodents’ brains were dissected. Living in an environment with more sensory and social stimuli, the lab tests showed, altered the structure and function of the brain. The rats fared better on learning tasks, and their brains weighed more compared to those housed alone in bare cages.
In a seminal study, in the early 1970s, neuroscientist William Greenough used an electron microscope to show that environmental enrichment made the neurons sprout new dendrites. The branching caused by the environmental stimulation of learning, exercise, and social contact caused the synapses to form more connections, and those connections had thicker myelin sheaths.
Now we know that such growth requires BDNF. This remodeling of the synapses has a huge impact on the circuits’ capacity to process information, which is profoundly good news. What it means is that you have the power to change your brain. All you have to do is lace up your running shoes.
How to Grow and Nurture New Neurons
For the better part of the twentieth century, scientific dogma held that the brain was hardwired once it was fully developed in adolescence -- meaning we’re born with all the neurons we’re going to get. We can only lose neurons as life goes on.
Guess what? Neurons do grow back -- by the thousands -- through a process called neurogenesis. They divide and propagate like cells in the rest of the body. Neurons are born as blank-slate stem cells, and they go through a developmental process in which they need to find something to do in order to survive. Most of them don’t. It takes about 28 days for a fledgling cell to plug into a network. If we don’t use the newborn neurons, we lose them. Exercise spawns neurons, and the environmental enrichment helps those cells survive.
The first solid link between neurogenesis and learning came from Fred Gage, a neuroscientist of the Salk Institute, and his colleague Henriette van Praag. They used a rodent-sized pool filled with opaque water to hide a platform just beneath the surface in one quadrant. Mice don’t like water, so the experiment was designed to test how well they remembered, from an earlier dip, the location of the platform -- their escape route. When comparing inactive mice with others that hit the running wheel for four kilometers a night, the results showed that the runners remembered where to find safety more quickly. The sedentary ones floundered before figuring it out.
When the mice were dissected, the active mice had twice as many new stem cells in the hippocampus as the inactive ones. Speaking generally about what they found, Gage says: “There is a significant correlation between the total number of cells and [a mouse’s] ability to perform a complex task. And if you block neurogenesis, mice can’t recall information.”
Although all this research has been done in rodents, you can see how it might relate to those progressive schools that exercise students before class begins: Gym class provides the brain with the right tools to learn, and the stimulation in the kids’ classes encourages those newly developing cells to plug into the network, where they become valuable members of the signaling community. The neurons are given a mission. And it seems that cells spawned during exercise are better equipped to spark this process.
Anyone for a run?