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NYT: How Exercise Makes Us Happy

July 7, 2011

Image via NYT.com

It’s long been a research-supported fact that exercise makes us feel good and can help significantly reduce stress. But how and why this works at a cellular level has, so far, been hard to figure out.

In a post this week on the New York Times’ Well blog, writer Gretchen Reynolds digs a little deeper into this mystery, sharing the findings of a recent National Institute of Mental Health study that suggests not only does exercise relate to emotion, but also that it may not require a lot of physical activity to provide lasting emotional resilience.

Via NYT.com:

For the experiment, researchers at the institute gathered two types of male mice. Some were strong and aggressive; the others were less so. The alpha mice got private cages. Male mice in the wild are territorial loners. So when then the punier mice were later slipped into the same cages as the aggressive rodents, separated only by a clear partition, the big mice acted like thugs. They employed every animal intimidation technique and, during daily, five-minute periods when the partition was removed, had to be restrained from harming the smaller mice, which, in the face of such treatment, became predictably twitchy and submissive.

After two weeks of cohabitation, many of these weaker mice were nervous wrecks. When the researchers tested them in a series of stressful situations away from the cages, the mice responded with, as the scientists call it, “anxiety-like behavior.” They froze or ran for dark corners. Everything upset them. “We don’t use words like ‘depressed’ to describe the animals’ condition,” said Michael L. Lehmann, a postdoctoral fellow at the institute and lead author of the study. But in effect, those mice had responded to the repeated stress by becoming depressed.

But that was not true for a subgroup of mice that had been allowed access to running wheels and nifty, explorable tubes in their cages for several weeks before they were housed with the aggressive mice. These mice, although wisely submissive when confronted by the bullies, rallied nicely when away from them. They didn’t freeze or cling to dark spaces in unfamiliar situations. They explored. They appeared to be, Dr. Lehmann said, “stress-resistant.”

“In people, we know that repeated applications of stress can lead to anxiety disorders and depression,” Dr. Lehmann said. “But one of the mysteries” of mental illness “is why some people respond pathologically to stress and some seem to be stress-resistant.”

To discern what was different, physiologically, about the stress-resistant mice, the scientists looked at brain cells using stains and other techniques. They determined that neurons in part of the rodents’ medial prefrontal cortex, an area of the brain involved in emotional processing in animals and people, had been firing often and rapidly in recent weeks, as had neurons in other, linked parts of the brain, including the amygdala, which is known to handle feelings of fear and anxiety.

The animals that had not run before moving in with the mean mice showed much less neuronal activity in these portions of the brain.

Dr. Lehmann said that he believed that the running was key to the exercised animals’ ability to bounce back from their unpleasant housing conditions.

“Of course,” Reynolds continues, “mice are not people.” But, she points out, hierarchies, marked by bullying and resulting stress,  are found in the human world all the time—from school yards to offices.

Perhaps the most reassuring finding of the study is that a large amount of exercise was not needed for the mice to become emotionally resilient—they only ran as often and for as long as they wanted. Dr. Lehmann concluded that moderate amounts of exercise seem to be the best for relieving stress.

 

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