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Adult memory: Many silent synapses present

Scientists show for the first time that the adult brain can also create countless new nerve connections. So-called “silent synapses” are waiting to be activated by learning processes.

Enlarged process of a nerve cell with active and silent synapses.

Dimitra Vardalaki, Mark Harnett

Healthy adults learn new things every day. Neuroscience has not yet been able to explain what sounds obvious. Why are old memories not overwritten by new things and thus forgotten?

The explanation for this is known in childhood. When learning, millions of new nerve connections are created, which are broken down again when they are not used. But in the adult brain, scientists only knew up to now that existing nerve connections, so-called synapses, are strengthened or weakened during the learning process.

Researchers at the Massachusetts Institute of Technology (MIT) in Boston, USA, are now finding for the first time that many new nerve connections may also be formed in the adult brain. The researchers discovered many “silent synapses” in the brains of adult mice.

When silent synapses are activated through learning processes, they begin to send signals to the surrounding brain cells like other nerve connections. Researchers report this on Wednesday in the journal “Nature”.

New method revealed previously unknown details

A lot has to come together for groundbreaking research like this. A scientist’s unorthodox view of his research object is just as much a part of this as new technical possibilities.

In this case, the prerequisite for the discovery was a new imaging method. Another group of scientists at MIT developed a method called eMAP in 2016 that reveals the smallest details of brain cells.

The electron microscope used in the past had already delivered very high-resolution images of the brain cells. However, many details remained hidden from the researchers. The images lacked contrast. With the new method, protein compounds in the order of micrometers have now become visible for the first time.

Bulges in the membrane are silent synapses

Actually, the researchers were interested in something else when they examined the nerve cells of the brains of adult mice with eMAP. But then they discovered a large number of bulges in the membrane of the nerve cells – so-called filopodia – and subsequently began to examine them.

Dozens of researchers may have already examined nerve cells using the new method. But neuroscientists Dimitra Vardalaki and Mark Harnett at MIT thought a step further. What if the many filopodia have a function? What if they are “silent” synapses? It has long been suspected that the adult brain must also have silent synapses. So far, no one has found her.

Evidence in the electrophysiological experiment

In contrast to active synapses, silent synapses cannot transmit nerve signals. The necessary docking points for messenger substances are missing. If the filopodia are silent synapses, it should be possible to activate them by simulating a learning process in the laboratory.

This is exactly what the researchers achieved in the electrophysiological experiment. When two neighboring nerve cells were activated simultaneously in the laboratory, the shape of the filopodia changed and they began to transmit an electrical signal.

Neuroscientist Igor Delvendahl, who researches neuronal signal transmission at the University of Zurich, explains: “The next step is to investigate whether the activated synapses remain active over the long term.” It would be proof that new nerve connections can also develop in the adult brain.

Memory is a balance of flexibility and stability

The newly discovered silent synapses may explain how, even in adulthood, memory can be flexible enough to learn new things and stable enough not to forget old ones.

Because a relatively small activation of the neurons in the laboratory was already enough to activate the silent synapses. In contrast, the existing synapses were hardly affected by this neuronal activity. So it is the silent synapses with which the brain reacts particularly sensitively to new stimuli.

Large number of silent synapses

For scientists, the new discovery of silent synapses is a step towards explaining human memory. The layperson, on the other hand, is more likely to be impressed by the sheer number of these silent synapses in the brain. Instead of the previously assumed 2 percent, a full 30 percent of the synapses in the brain are silent synapses – per brain cell. Extrapolated to the number of brain cells, billions of silent synapses are waiting to store new memory content.

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