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  3. Of 100 billion nerve cells in the human brain, how many form after birth? For years, the official answer was "zero". Scientists

Of 100 billion nerve cells in the human brain, how many form after birth? For years, the official answer was "zero". Scientists

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问题     Of 100 billion nerve cells in the human brain, how many form after birth? For years, the official answer was "zero". Scientists thought people were born with all the neurons they’d ever have. But from 1980s, biologists overturned that doctrine, finding a reservoir of stem cells that became fresh neurons in two parts of the brains of adult birds, monkeys and humans. Those discoveries were stunning, but the next seemed to top them all. In 1999, psychologist Elizabeth Gould reported large numbers of new nerve cells in a third of the monkey brain, hinting that the same part in humans — the neocortex, which lets us reason and remember — was regenerating, too. If she was right, scientists would have to revise almost all their ideas about human memory, and doctors might someday find a way to treat Alzheimer’s patients by simply turning on the neural-construction equipment.
    The birth of new nerve cells, or "neurogenesis", is now confirmed in the original two parts of human brain, the hippocampus and olfactory bulb. But for the neocortex, the no-neurons theory lives — and it’s just gotten major boost.
    Until December, Gould’s study stood alone and unverified. Two neuroscientists have repeated her work in Science, but not her results. Where Gould saw new nerve cells in the neocortex, Rakic and Kormack see only glial cells, the "glue" that supports neurons. But they do spot new nerve cells in the other two areas. In a January review in Nature Neuroscience, Rakic charges Gould’s work with technical problems. Focusing on what appeared to be 100 new neurons, Rakic and Kornack found that every one was merely a new glial cell hiding behind an old neuron. Gould has a cross-sectioned image from her own study that she says shows one cell marked as new — and it’s clearly a neuron. But Rakic has an answer for that, too. The method that identified the cells as "new" finds DNA synthesis, which can happen in cells that aren’t actually dividing. Rakic says Gould’s tests were too sensitive, tagging "new" neurons that weren’t. Gould responses that Rakic’s methods just weren’t sensitive enough. But even she can’t explain why that might be.
    Rakic’s study squares with the idea that memory comes not from new nerve cells but from chemicals in the spaces between old ones.
    Gould’s team are circulating response to Rakic and Kornack and recreating two studies side by side to see if small differences in methods are to blame. Others are also redoing the tests; a Japanese team’s unpublished results echoes Rakic’s, while another team’s support Gould’s.
    Meanwhile work on less controversial new neurons marches forward. Neuroscientist Fred Gage, who’s just wrapped up a study of the function of new hippocampus nerve cells, says that’s as it should be. Still, until more studies confirm Rakic and Kornack, he’ll keep a close eye on the neocortex debate.
What can we know about Rakic’s study?
选项 A、It is the opposite with what scientists always believe.
B、It shows that memory comes from chemicals in the spaces between old ones.
C、It shows that memory comes from new nerve cells.
D、It tells the relationship between memory, new nerve cells and chemicals in the spaces between old ones.
答案B
解析 细节题。本题可以定位到第四段。本题是有关Rakic研究的。根据第四段Rakic’s study squares with the idea that memory comes not from new nerve cells but from chemicals in the spaces between old ones．说明人们一直认为记忆并非产生于新的神经元细胞，而是由于旧神经元细胞之间化学物质的变化，在这点上，Rakic的研究结果是与之一致的。
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