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Can exercise make the brain more fit? That absorbing question inspired a new study at the University of South Carolina during which scientists assembled mice and assigned half to run for an hour a day on little treadmills, while the rest lounged in their cages without exercising. Earlier studies have shown that exercise sparks neurogenesis, or the creation of entirely new brain cells. But the South Carolina scientists were not looking for new cells. They were looking inside existing ones to see if exercise was whipping those cells into shape, similar to the way that exercise strengthens muscle.
For centuries, people have known that exercise remodels muscles, rendering them more durable and fatigue-resistant. In part, that process involves an increase in the number of muscle mitochondria, the tiny organelles that float around a cell's nucleus and act as biological powerhouses, helping to create the energy that fuels almost all cellular activity. The greater the mitochondrial density in a cell, the greater its vitality. Brain cells are also fueled by mitochondria. But until now, no one has known if a similar response to exercise occurs in the brain.
Like muscles, many parts of the brain get a robust physiological workout during exercise. “The brain has to work hard to keep the muscles moving,” says J. Mark Davis, a professor of exercise science and senior author of the new mouse study. Scans have shown that metabolic activity in many parts of the brain surges during workouts, but it was unknown whether those active brain cells were actually adapting and changing. To see, the South Carolina scientists exercised their mice for eight weeks.
At the end of the two months, the researchers had both groups complete a run to exhaustion on the treadmill. Not surprisingly, the running mice displayed much greater endurance than the loungers. They lasted on the treadmills for an average of 126 minutes, versus 74 minutes for the unexercised animals. More interesting, though, was what was happening inside their brain cells. When the scientists examined tissue samples from different portions of the exercised animals' brains, they found markers of upwelling mitochondrial development in all of the tissues. Some parts of their brains showed more activity than others, but in each of the samples, the brain cells held newborn mitochondria. There was no comparable activity in brain cells from the sedentary mice.
Dr. Davis speculates re-energized brain cells could behave like mitochondrial-drenched muscle cells, becoming more resistant to fatigue and, since bodily fatigue is partly mediated by signals from the brain, allowing you to withstand more exercise. In effect, exercising the body may train the brain to allow you to exercise more, amplifying the benefits. Revitalized brain cells also, at least potentially, could reduce mental fatigue and sharpen your thinking “even when you're not exercising,” Dr. Davis says.
11. We learn from Paragraphs 1 and 2 that South Carolina's experiment is ______.
[A] the first study on how exercise strengthens the brain
[B] to discover new brain cells created by exercise
[C] to find the way exercise strengthens muscle
[D] similar to the studies on the link between exercise and health
12. According to the text, which of the following is true of mitochondria?
[A] They exist in muscles but not in brain.
[B] They can only provide energy to muscle cells.
[C] Their function is to make people fatigue-resistant.
[D] Their number can affect cells' vitality.
13. The word “sedentary” (Line 7, Para. 4) most probably means ______.
[A] remaining
[B] inactive
[C] abnormal
[D] sick
14. Which of the following is true of the result of the experiment?
[A] It is totally beyond the researchers' expectation.
[B] It proves that exercise can affect brain cells' vigor.
[C] There are no mitochondria in the loungers' brains.
[D] The activities of the mice's brains are similar to each other's.
15. Which of the following would Dr. Davis most probably agree on?
[A] Re-energized brain cells enable people to bear more exercise.
[B] Body exercise cannot bring instant effect to brain.
[C] Re-energized brain cells can only reduce mental tiredness.
[D] The effect of exercise on brain works only when people are exercising.