《经济学人》双语：MIT研究发现“沉默突触”有利于提高成年人记忆力？

原文标题：
Neuroscience
Silent synapses
How adult brains learn the new without forgetting the old
神经科学
沉默的突触
成年人大脑是如何在不忘记旧事物的情况下学习新事物的
 
They keep a stock of unused synapses in reserve, to be activated as needed
它们保留了大量未使用的突触，以备不时之需

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LEARNING NEW things is hard.Remembering what has already been learned is harder.
学习新事物很难。但记住学过的事物难上加难。
 
Any successful learning system, be it a brain or a piece of artificial-intelligence software, must strike the right balance between stability and flexibility.
任何成功的学习系统，无论是大脑还是人工智能软件，都必须完美平衡其内部的稳定性和灵活性。
 
It must be stable enough to remember important old things yet flexible enough to learn new ones without destroying old memory traces—preferably for as long as it exists.
它必须足够稳定，能够记住重要的旧事物；又必须足够灵活，能够在学习新事物的同时，不破坏旧的记忆痕迹--最好能够一直存在。

 

信息以一种被称为神经递质的分子形式穿过这些间隙。

Current estimates suggest there are 600 trillion synapses in a human brain.
目前估计人脑中有600万亿个突触。
 
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How, then, to deal with the stability-plasticity dilemma—particularly as brains age and, as it were, fill up?
那么，如何平衡稳定性和灵活性呢?尤其是随着大脑的老化，如何实现信息的记忆呢?
 
Research by Dimitra Vardalaki, Kwanghun Chung and Mark Harnett at the Massachusetts Institute of Technology, just published in Nature, suggests one way is to preserve into adulthood a type of memory-forming synapse found in children.
麻省理工学院的迪米特拉·瓦达拉基、钟光洪和马克·哈内特最近发表在《自然》杂志上的研究表明，有方法可以将儿童体内的一种形成记忆的突触保存到成年。
 
These are called silent synapses.
这些突触也叫做沉默突触。
 
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Silent synapses—which, as their name suggests, transmit no signal from one nerve cell to another—are often found on the ends of slender, immature protrusions from nerve cells, called filopodia.
沉默突触，顾名思义，它们不会将信号从一个神经细胞传递到另一个神经细胞。人们经常在神经细胞细长的、未成熟的突起末端发现它，将其称为丝状伪足。
 
Until now, it had been thought that these disappeared as a brain matured.
人们一直认为，随着大脑的成熟，这些细胞会消失。
 
But Drs Vardalaki, Chung and Harnett have shown not only that they are present in adulthood, but also that they are common, at least in mice.
但瓦尔达拉基博士、钟博士和哈内特博士已经表明，它们不仅在成年后还存在，而且很常见，至少在老鼠中是这样。
 
Just over a quarter of the connections they sampled in adult mouse visual cortices were silent synapses on filopodia.
他们的采样证明在成年小鼠视觉皮质中的连接中，有超过四分之一是丝状足上的沉默突触。
 
And murine and human brains are sufficiently alike that something similar almost certainly applies to people.
老鼠和人类的大脑非常相似，几乎可以肯定，某些相似之处也适用于人类。
 
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To carry out their search for filopodia, the trio used a sensitive microscopy technique called eMAP.
为了寻找丝状伪足，三人使用了一种叫做eMAP的敏感显微镜技术。
 
They studied 2,234 synapses between cortical nerve cells of a type called pyramidal neurons (pictured), which have thousands of synapses each.
他们研究了一种叫做锥体神经元的皮质神经细胞之间的2234个突触(如图所示)，每个锥体神经元都有数千个突触。
 
Peering through an eMAP microscope is enough to determine which cellular protrusions are filopodia.
通过eMAP显微镜观察就足以确定哪些细胞突起是丝状伪足。
 
But it cannot show which synapses on them are silent.
但不能确认哪些是沉默突触。 
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To do that, they needed to test how the filopodia responded to glutamate, the brain’s main excitatory neurotransmitter.
为了确认这一点，他们需要测试丝状伪足对大脑主要兴奋性神经递质谷氨酸的反应。
 
First, they had to deliver a controlled flow of glutamate to the particular synapse they wanted to test.
首先，他们必须向他们想要测试的特定突触输送可控的谷氨酸。
 
To this end, they poured a soup of “caged” glutamate over the neuron under examination.
为此，他们将谷氨酸“笼”汤倒在被检查的神经元上。
 
This form of the molecule is inert until hit with energy from the intersection of two laser beams.
这种形式的分子是惰性的，直到两束激光的交叉处的能量将其激活。
 
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Aiming those at the synapse under study enabled them to uncage the neurotransmitter and see, by measuring the electrical activity in that part of the neuron using an ultra fine electrode, whether the synapse responded.
将这些信号对准被研究的突触，这样他们能够释放神经递质，并通过使用超细电极测量神经元那部分的电活动，来观察突触是否有反应。
 
They found that mature pyramidal-neuron protrusions generated electrical activity when exposed to glutamate, as expected.
一如他们所料，成熟的锥体神经元突起在暴露于谷氨酸时产生了电活动。
 
Filopodia did not, confirming the silence of their synapses.
丝状伪足则没有，这证实了它们突触的沉默。
 
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Silent synapses are, however, useless unless they can be switched on at the appropriate moment.
然而，沉默的突触是无用的，除非它们能在适当的时刻被打开。
 
And the researchers confirmed this is possible.
研究人员证实这是可能的。
 
They were able to induce the silent versions on filopodia to turn into mature, active synapses by pairing the simulated release of glutamate with a subsequent surge of electricity inside the neuron.
通过模拟谷氨酸的释放与神经元内随后的电流激增相匹配，他们能够诱导丝状足上的沉默突触变成成熟的、活跃的突触。
 
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This pairing of events caused silent synapses to start, within minutes, displaying receptor molecules characteristic of active synapses.
这种配对事件使沉默突触在几分钟内开始活跃，并显示出活跃突触的受体分子特征。
 
The same pairing, applied to mature synapses, did nothing.
同样的配对，应用于成熟的突触，没有任何效果。
 
The researchers thereby show it is hard to get a mature synapse to change the strength of its connection (thus satisfying the stability side of the dilemma), but easy to unsilence a silent one (satisfying the plasticity side).
研究人员由此表明，很难让一个成熟的突触改变其连接的强度从而满足稳定性，但很容易打破沉默的突触以满足灵活性。
 
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The next thing to investigate is how, why and when new filopodia appear.
接下来要研究的问题是，新的丝状伪足出现的方式、原因以及时间。
 
The discovery of all these eager-to-learn silent synapses and filopodia, Dr Harnett says, “is a lever for us to get into understanding learning in adults and how potentially we can get access to make it not degrade over the course of ageing or disease”.
哈内特博士表示，所有这些渴望学习的沉默突触和丝状伪足的发现，“是我们理解成年人学习的一个杠杆，以及在衰老或疾病过程中我们的记忆力有多大潜力不会退化。” 

（恭喜读完，本篇英语词汇量646左右）
原文出自：2022年12月10日《The Economist》Science&technology版块。
精读笔记来源于：自由英语之路

本文翻译整理: Maree

本文编辑校对: Irene
仅供个人英语学习交流使用。

【补充资料】（来自于网络）
11 月 30 日，一个来自麻省理工学院的研究团队在 Nature 发表了最新论文， 发现在成年小鼠的大脑中依然包含数百万个“沉默突触”，也就是神经元之间未被“启用”的连接 ，这些“沉默突触”约占成年小鼠大脑皮层中所有突触的 30%。研究人员认为，可能正是这些“沉默突触”的存在为成人大脑的记忆和学习能力提供了持续动力，而不是通过修改那些已被使用的常规突触。丝状伪足上具有被称为 NMDA 受体的神经递质受体，但却缺乏 AMPA 受体。正常情况下，活跃突触需同时具有这两种类型的受体，它们之间形成合作并结合中枢神经系统的主要神经递质谷氨酸。而只有 NMDA 受体而没有 AMPA 受体时，突触就不能传递电流， 这正是一种典型的沉默突触模式。通过释放谷氨酸并结合来自神经元体的电流刺激能够“解除”这些突触的沉默。这种联合刺激能够导致 AMPA 受体在沉默突触中积累，使其转换为有功能活性的突触，与附近释放谷氨酸的神经轴突形成牢固的神经连接。

【重点句子】（3 个）
LEARNING NEW things is hard. Remembering what has already been learned is harder.
学习新事物很难。但记住学过的事物难上加难。
 
Learning is a result of changes in the pattern of neural connectivity in the brain.
学习是大脑中神经连接模式改变的结果。
 
And murine and human brains are sufficiently alike that something similar almost certainly applies to people.
老鼠和人类的大脑非常相似，几乎可以肯定，某些相似之处也适用于人类。