北京 2017 七选五

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Every animal sleeps, but the reason for this has remained foggy. When lab rats are not allowed to sleep, they die within a month. 71 C. Similarly, when people go for a few days without sleeping, they get sick.

One idea is that sleep helps us strengthen new memories. 72 E. That’s why students do better in tests if they get a chance to sleep after learning. We know that, while awake, fresh memories are recorded by reinforcing (加强) connections between brain cells, but the memory processes that take place while we sleep have been unclear.

Support is growing for a theory that sleep evolved so that connections between neurons(神经元) in the brain can be weakened overnight, making room for fresh memories to form the next day. 73 F. “Sleep is the price we pay for learning,”says Giulio Tononi, who developed the idea.

Now we have the most direct evidence yet that he is right. 74 G. Tononi’s team measured the size of these connections, or synapses, in the brains of 12 mice.

The synapses in the mice taken at the end of a period of sleep were 18 per cent smaller than those taken before sleep, showing that the connections between neurons weaken while sleeping.

If Tononi’s theory is right, it would explain why, when we miss a night's, we find it harder the next day to concentrate and learn new information — our brains may have smaller room for new experiences.

Their research also suggests how we may build lasting memories over time even though the synapses become thinner. The team discovered that some synapses seem to be protected and stayed the same size. 75 B. It’s as if the brain is preserving its most important memories.

“You keep what matters,” Tononi says.

A. We should also try to sleep well the night before.

D. The processes take place to stop our brains becoming loaded with memories.

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每种动物都要睡觉，但其中的原因一直模糊不清。如果不让实验室的老鼠睡觉，它们会在一个月内死亡。同样，如果人几天不睡觉就会生病。

有一种观点认为，睡眠有助于我们强化新的记忆。这就是为什么学生在学习后如果有机会睡觉，在考试中就能表现得更好。我们知道，在清醒的时候，新的记忆是通过加强脑细胞之间的联系来记录的，但我们睡觉时发生的记忆过程却一直不明确。

越来越多的人支持这样一种理论：睡眠的进化是为了让大脑中的神经元之间的联系在夜间得以减弱，从而为第二天形成新的记忆腾出空间。“睡眠是我们为学习付出的代价，”提出这一观点的朱利奥・托诺尼说道。

现在我们有了最直接的证据证明他是正确的。托诺尼的团队测量了 12 只老鼠大脑中这些连接，即突触的大小。 在睡眠一段时间结束时获取的老鼠的突触，比睡眠前获取的突触小 18%，这表明神经元之间的连接在睡眠时会减弱。

如果托诺尼的理论是正确的，这就能解释为什么当我们一晚没睡时，第二天会发现更难集中精力和学习新信息 —— 我们的大脑可能为新体验留出的空间变小了。

他们的研究还表明，尽管突触会变弱，但随着时间推移，我们是如何建立起持久的记忆的。研究小组发现，一些突触似乎受到了保护，大小保持不变。就好像大脑在保留它最重要的记忆。

“你保留了重要的东西，”托诺尼说。