原文標題：
Neuroscience
Silent synapses
How adult brains learn the new without forgetting the old
神經科學
沉默的突觸
成年人大腦是如何在不忘記舊事物的情況下學習新事物的
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They keep a stock of unused synapses?in reserve, to be activated as needed
它們保留了大量未使用的突觸，以備不時之需

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LEARNING NEW things is hard.Remembering what has already been learned is harder.
學習新事物很難。但記住學過的事物難上加難。
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Any successful learning system, be it a brain or a piece of artificial-intelligence?software, must strike the right balance between stability and flexibility.
任何成功的學習系統(tǒng)，無論是大腦還是人工智能軟件，都必須完美平衡其內部的穩(wěn)定性和靈活性。
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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.
它必須足夠穩(wěn)定，能夠記住重要的舊事物；又必須足夠靈活，能夠在學習新事物的同時，不破壞舊的記憶痕跡--最好能夠一直存在。

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信息以一種被稱為神經遞質的分子形式穿過這些間隙。

Current estimates suggest there are 600 trillion synapses in a human brain.
目前估計人腦中有600萬億個突觸。
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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，如何平衡穩(wěn)定性和靈活性呢?尤其是隨著大腦的老化，如何實現信息的記憶呢?
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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.
麻省理工學院的迪米特拉·瓦達拉基、鐘光洪和馬克·哈內特最近發(fā)表在《自然》雜志上的研究表明，有方法可以將兒童體內的一種形成記憶的突觸保存到成年。
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These are called silent synapses.
這些突觸也叫做沉默突觸。
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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.
沉默突觸，顧名思義，它們不會將信號從一個神經細胞傳遞到另一個神經細胞。人們經常在神經細胞細長的、未成熟的突起末端發(fā)現它，將其稱為絲狀偽足。
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Until now, it had been thought that these disappeared as a brain matured.
人們一直認為，隨著大腦的成熟，這些細胞會消失。
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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.
但瓦爾達拉基博士、鐘博士和哈內特博士已經表明，它們不僅在成年后還存在，而且很常見，至少在老鼠中是這樣。
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Just over a quarter of the connections they sampled in adult mouse visual cortices were silent synapses on filopodia.
他們的采樣證明在成年小鼠視覺皮質中的連接中，有超過四分之一是絲狀足上的沉默突觸。
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And murine and human brains are sufficiently?alike that something similar almost certainly applies to people.
老鼠和人類的大腦非常相似，幾乎可以肯定，某些相似之處也適用于人類。
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To carry out their search for filopodia, the trio used a sensitive?microscopy technique called eMAP.
為了尋找絲狀偽足，三人使用了一種叫做eMAP的敏感顯微鏡技術。
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They studied 2,234 synapses between cortical nerve cells of a type called pyramidal neurons (pictured), which have thousands of synapses each.
他們研究了一種叫做錐體神經元的皮質神經細胞之間的2234個突觸(如圖所示)，每個錐體神經元都有數千個突觸。
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Peering through an eMAP microscope is enough to determine which cellular protrusions are filopodia.
通過eMAP顯微鏡觀察就足以確定哪些細胞突起是絲狀偽足。
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But it cannot show which synapses on them are silent.
但不能確認哪些是沉默突觸。?
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To do that, they needed to test how the filopodia responded to glutamate, the brain’s main excitatory neurotransmitter.
為了確認這一點，他們需要測試絲狀偽足對大腦主要興奮性神經遞質谷氨酸的反應。
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First, they had to deliver a controlled flow of glutamate to the particular synapse they wanted to test.
首先，他們必須向他們想要測試的特定突觸輸送可控的谷氨酸。
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To this end, they poured a soup of “caged” glutamate over the neuron under examination.
為此，他們將谷氨酸“籠”湯倒在被檢查的神經元上。
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This form of the molecule is inert until hit with energy from the intersection of two laser beams.
這種形式的分子是惰性的，直到兩束激光的交叉處的能量將其激活。
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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.
將這些信號對準被研究的突觸，這樣他們能夠釋放神經遞質，并通過使用超細電極測量神經元那部分的電活動，來觀察突觸是否有反應。
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They found that mature pyramidal-neuron protrusions generated electrical activity when exposed to glutamate, as expected.
一如他們所料，成熟的錐體神經元突起在暴露于谷氨酸時產生了電活動。
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Filopodia did not, confirming the silence of their synapses.
絲狀偽足則沒有，這證實了它們突觸的沉默。
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Silent synapses are, however, useless unless they can be switched on at the appropriate?moment.
然而，沉默的突觸是無用的，除非它們能在適當的時刻被打開。
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And the researchers confirmed this is possible.
研究人員證實這是可能的。
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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.
通過模擬谷氨酸的釋放與神經元內隨后的電流激增相匹配，他們能夠誘導絲狀足上的沉默突觸變成成熟的、活躍的突觸。
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This pairing of events caused silent synapses to start, within minutes, displaying receptor molecules characteristic of active synapses.
這種配對事件使沉默突觸在幾分鐘內開始活躍，并顯示出活躍突觸的受體分子特征。
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The same pairing, applied to mature synapses, did nothing.
同樣的配對，應用于成熟的突觸，沒有任何效果。
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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).
研究人員由此表明，很難讓一個成熟的突觸改變其連接的強度從而滿足穩(wěn)定性，但很容易打破沉默的突觸以滿足靈活性。
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The next thing to investigate?is how, why and when new filopodia appear.
接下來要研究的問題是，新的絲狀偽足出現的方式、原因以及時間。
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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”.
哈內特博士表示，所有這些渴望學習的沉默突觸和絲狀偽足的發(fā)現，“是我們理解成年人學習的一個杠桿，以及在衰老或疾病過程中我們的記憶力有多大潛力不會退化?！?/span>?

（恭喜讀完，本篇英語詞匯量646左右）
原文出自：2022年12月10日《The Economist》Science&technology版塊。
精讀筆記來源于：自由英語之路

本文翻譯整理: Maree

本文編輯校對: Irene
僅供個人英語學習交流使用。

【補充資料】（來自于網絡）
11 月 30 日，一個來自麻省理工學院的研究團隊在 Nature 發(fā)表了最新論文， 發(fā)現在成年小鼠的大腦中依然包含數百萬個“沉默突觸”，也就是神經元之間未被“啟用”的連接 ，這些“沉默突觸”約占成年小鼠大腦皮層中所有突觸的 30%。研究人員認為，可能正是這些“沉默突觸”的存在為成人大腦的記憶和學習能力提供了持續(xù)動力，而不是通過修改那些已被使用的常規(guī)突觸。絲狀偽足上具有被稱為 NMDA 受體的神經遞質受體，但卻缺乏 AMPA 受體。正常情況下，活躍突觸需同時具有這兩種類型的受體，它們之間形成合作并結合中樞神經系統(tǒng)的主要神經遞質谷氨酸。而只有 NMDA 受體而沒有 AMPA 受體時，突觸就不能傳遞電流， 這正是一種典型的沉默突觸模式。通過釋放谷氨酸并結合來自神經元體的電流刺激能夠“解除”這些突觸的沉默。這種聯(lián)合刺激能夠導致 AMPA 受體在沉默突觸中積累，使其轉換為有功能活性的突觸，與附近釋放谷氨酸的神經軸突形成牢固的神經連接。

【重點句子】（3 個）
LEARNING NEW things is hard. Remembering what has already been learned is harder.
學習新事物很難。但記住學過的事物難上加難。
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Learning is a result of changes in the pattern of neural?connectivity in the brain.
學習是大腦中神經連接模式改變的結果。
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And murine and human brains are sufficiently alike that something similar almost certainly applies to people.
老鼠和人類的大腦非常相似，幾乎可以肯定，某些相似之處也適用于人類。