How the Brain Learns and Creates Memories
As fundamental as the neurotransmitters are, there’s another class of master molecules that, over the past 15 years, has dramatically changed our understanding of connections in the brain. I’m talking about a family of proteins referred to as "factors," the most prominent of which is the brain-derived neurotrophic factor (BDNF). Whereas neurotransmitters carry out signaling, neurotrophins, such as BDNF, build and maintain the infrastructure itself.
Once it became clear to researchers that BDNF was present in the hippocampus, the area of the brain related to memory and learning, they set out to test whether it was a necessary ingredient in the process. Learning requires strengthening the affinity between neurons through a dynamic mechanism called long-term potentiation (LTP). When the brain is called on to take in information, the demand naturally causes activity between neurons. The more activity, the stronger the attraction becomes, and the easier it is for the signal to fire and make the connection.
Say you’re learning a French word. The first time you hear it, nerve cells recruited for a new circuit fire a glutamate signal between each other. If you never practice the word again, the attraction between the synapses involved diminishes, weakening the signal. You forget.
The discovery that astonished memory researchers -- and earned Columbia University neuroscientist Eric Kandel a share of the 2000 Nobel Prize -- is that repeated activation, or practice, causes the synapses themselves to swell and make stronger connections. A neuron is like a tree that, instead of leaves, has synapses along its dendritic branches. Eventually new branches sprout, providing more synapses to further solidify the connections. These changes are called synaptic plasticity, which is where BDNF takes center stage.
Early on, researchers found that if they sprinkled BDNF onto neurons in a petri dish, the cells automatically sprouted new branches, producing the same structural growth required for learning. I call BDNF Miracle-Gro for the brain. BDNF also binds to receptors at the synapse, unleashing the flow of ions to increase the voltage and immediately improve the signal strength. Inside the cell, BDNF activates genes that call for the production of more BDNF, as well as serotonin and proteins that build up the synapses. BDNF directs traffic and engineers the roads, as well. Overall, it improves the function of neurons, encourages their growth, and strengthens and protects them against the natural process of cell death.
The More Your Body Exercises, the Better Your Brain Functions
So how does the brain amp up its supply of BDNF? Exercise. In 1995, I was doing research for my book, A User’s Guide to the Brain, when I came across a one-page article in the journal Nature about exercise and BDNF in mice. There was scarcely more than a column of text, yet it said everything. According to the study’s author, Carl Cotman, director of the Institute for Brain Aging and Dementia at the University of California-Irvine, exercise seemed to elevate Miracle-Gro, or BDNF, throughout the brain.
By showing that exercise sparks the master molecule of the learning process, BDNF, Cotman nailed down a biological connection between movement and cognitive function. He set up an experiment to measure the levels of BDNF in the brains of mice that work out.
Unlike humans, rodents seem to enjoy physical activity, and Cotman’s mice ran several kilometers a night. When their brains were injected with a molecule that binds to BDNF and scanned, not only did the scans of the running rodents show an increase in BDNF over controls, but the farther each mouse ran, the higher the levels were.
As the stories of BDNF and exercise developed together, it became clear that the molecule was important not merely for the survival of neurons but also for their growth (sprouting new branches) and, thus, for learning. Cotman showed that exercise helps the brain learn.
“One of the prominent features of exercise, which is sometimes not appreciated in studies, is an improvement in the rate of learning, and I think that’s a cool take-home message,” Cotman says. “Because it suggests that, if you’re in good shape, you may be able to learn and function more efficiently.”
Indeed, in a 2007 study, German researchers found that people learn vocabulary words 20 percent faster following exercise than they did before exercise, and that the rate of learning correlated directly with levels of BDNF. Along with that, people with a gene variation that robs them of sufficient BDNF levels are more likely to have learning deficiencies. Without the so-called Miracle-Gro, the brain closes itself to the world.
Which isn’t to say that going for a run will turn you into a genius. “You can’t just inject BDNF and be smarter,” Cotman points out. “With learning, you have to respond to something in a different way. But the something has to be there.” And without question, what that something is matters.