How adult brains learn the new without forgetting the old.

成年人的大脑如何在不忘记旧内容的情况下学习新内容。

Learning new things is hard.

学习新事物很难。

Remembering what has already been learned is harder.

记住已经学过的东西更难。

To carry out their search for filopodia, the trio used a sensitive microscopy technique called eMAP.

为了寻找树突丝，三位研究人员使用了一种名为“蛋白质组表位保存放大分析”（eMAP）的灵敏显微镜成像技术。

They studied 2,234 synapses between cortical nerve cells of a type called pyramidal neurons, which have thousands of synapses each.

他们研究了2234个皮质神经细胞之间的突触，这种突触被称为锥体神经元，每个锥体神经元有数千个突触。

Peering through an emap microscope is enough to determine which cellular protrusions are filopodia.

通过eMAP显微镜观察足以确定哪些细胞突起是树突丝。

But it cannot show which synapses on them are silent.

但无法确定树突丝上的哪些突触是沉默突触。

To do that, they needed to test how the filopodia responded to glutamate, the brain's main excitatory neurotransmitter.

要做到这一点，他们需要测试树突丝对谷氨酸的反应，谷氨酸也就是大脑主要的兴奋性神经递质。

First, they had to deliver a controlled flow of glutamate to the particular synapse they wanted to test.

首先，他们必须向想要测试的特定突触输送一定量的谷氨酸。

To this end, they poured a soup of "caged" glutamate over the neuron under examination.

为此，他们在想要测试的神经元上倾倒“被装起来”的谷氨酸。

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分子，需要两种电流交汇，在分子处对其进行刺激。

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 ultrafine electrode, whether the synapse responded.

将研究的突触作为目标能够让这些突触释放神经递质，并让研究人员能够使用超细电极测量神经元这一部分的电活动，以此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.

他们发现，正如预期的那样，成熟的锥体神经元暴露在谷氨酸中时，其突起会产生电活动。

Filopodia did not, confirming the silence of their synapses.

而树突丝的突触没有反应，证明它们的突触是沉默突触。

Silent synapses are, however, useless unless they can be switched on at the appropriate moment.

然而，如果不能在适当的时候被激活，沉默突触就是无用的。

And the researchers confirmed this is possible.

研究人员证实，激活沉默突触是有可能的。

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.

他们能够模拟谷氨酸释放，结合神经元内的电流，将树突丝上的沉默突触变为成熟、活跃的突触。

This pairing of events caused silent synapses to start, within minutes, displaying receptor molecules characteristic of active synapses.

这种激活能够让沉默突触在几分钟内开始显示活跃突触特有的受体分子。

The same pairing, applied to mature synapses, did nothing.

而在成熟突触上进行的相同实验没有起到任何作用。

The researchers thereby show that 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).

研究人员由此确定，改变成熟突触连接的强度很难（因此能够实现平衡性中稳定性的要求），但激活沉默突触是很容易的（因此满足平衡性中可塑性的要求）。

The next thing to investigate is how, why and when new filopodia appear.

下一步要调查的是新的树突丝是如何、为什么以及何时出现的。

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".

哈尼特博士说，所有这些“渴望学习”的沉默突触和树突丝的发现“是一种杠杆，能够帮助我们理解成年人的学习过程，以及如何才能让人类大脑不在衰老或疾病过程中退化”。

来源：经济学人