By GRETCHEN REYNOLDS
For some time, scientists studying exercise have been puzzled by physical activity’s two seemingly incompatible effects on the brain. On the one hand, exercise is known to prompt the creation of new and very excitable brain cells. At the same time, exercise can induce an overall pattern of calm in certain parts of the brain.
Most of us probably don’t realize that neurons are born with certain predispositions. Some, often the younger ones, are by nature easily excited. They fire with almost any provocation, which is laudable if you wish to speed thinking and memory formation.
But that feature is less desirable during times of everyday stress. If a stressor does not involve a life-or-death decision and require immediate physical action, then having lots of excitable neurons firing all at once can be counterproductive, inducing anxiety.
Studies in animals have shown that physical exercise creates excitable neurons in abundance, especially in the hippocampus, a portion of the brain known to be involved in thinking and emotional responses.
But exercise also has been found to reduce anxiety in both people and animals.
How can an activity simultaneously create ideal neurological conditions for anxiety and leave practitioners with a deep-rooted calm, the Princeton researchers wondered?
So they gathered adult mice, injected them with a substance that marks newborn cells in the brain, and for six weeks, allowed half of them to run at will on little wheels, while the others sat quietly in their cages.
Afterward, the scientists determined each group’s baseline nervousness. Given access to cages with open, well-lighted areas, as well as shadowy corners, the running mice were more willing to cautiously explore and spend time in open areas, an indication that they were more confident and less anxious than the sedentary animals.
The researchers also checked the brains of some of the runners and the sedentary mice to determine how many and what varieties of new neurons they contained.
As expected, the runners’ brains teemed with many new, excitable neurons. The sedentary mice’s brains also contained similar, volatile newborn cells, but not in such profusion.
The runners’ brains, however, also had a notable number of new neurons specifically designed to release the neurotransmitter GABA, which inhibits brain activity, keeping other neurons from firing easily. In effect, these are nanny neurons, designed to shush and quiet activity in the brain.
In the runners’ brains, there were large new populations of these cells in a portion of the hippocampus, the ventral region, associated with the processing of emotions. (The rest of the hippocampus, the dorsal region, is more involved with thinking and memory.)
What role these nanny neurons were playing in the animals’ brains and subsequent behavior was not altogether clear.