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MIT scientists discover millions of “silent synapses” in the adult brain

Your brain may be packed with hidden backup connections, waiting to turn new experiences into lasting memories.

Date:

May 6, 2026

Source:

Massachusetts Institute of Technology

Summary:

MIT neuroscientists have uncovered a surprising secret hidden in the adult brain: millions of “silent synapses,” dormant connections that lie in wait until new learning calls them into action. Once thought to exist only in early development, these inactive links make up about 30% of synapses in the adult cortex and can be rapidly activated to form fresh memories.

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The adult brain contains millions of hidden “silent synapses” that can be activated to form new memories on demand. This built-in reserve helps us learn new things without erasing what we already know. Credit: Shutterstock

MIT neuroscientists have uncovered a surprising feature of the adult brain. It contains millions of "silent synapses," which are immature connections between neurons that remain inactive until they are needed to help form new memories.

For many years, scientists believed these silent synapses existed only during early development, when the brain is rapidly learning about the world. But the MIT team found that in adult mice, roughly 30 percent of synapses in the brain's cortex are still silent. This suggests the adult brain holds a large reserve of unused connections that can be activated when new information arrives.

Researchers say this hidden pool of synapses may explain how the brain continues to learn throughout life without disrupting existing memories.

"These silent synapses are looking for new connections, and when important new information is presented, connections between the relevant neurons are strengthened. This lets the brain create new memories without overwriting the important memories stored in mature synapses, which are harder to change," says Dimitra Vardalaki, an MIT graduate student and the lead author of the study.

Mark Harnett, an associate professor of brain and cognitive sciences, is the senior author of the paper, published in Nature. Kwanghun Chung, an associate professor of chemical engineering at MIT, is also an author.

Rethinking How Memory Works in the Adult Brain

Silent synapses were first identified decades ago, mostly in young animals. During early development, they are thought to help the brain absorb large amounts of new information about the environment. In mice, scientists believed these synapses disappeared by about 12 days of age (equivalent to the first months of human life).

However, some researchers suspected they might persist into adulthood. Clues came from studies of addiction, which is often considered a form of maladaptive learning. These studies hinted that silent synapses could reappear or remain in the adult brain.

Theoretical work by neuroscientists Stefano Fusi and Larry Abbott also suggested the brain needs a mix of flexible and stable synapses. Some connections must be easy to change to support new learning, while others must remain steady to preserve long-term memories.

A Chance Discovery Using Advanced Imaging

The MIT team was not initially searching for silent synapses. They were following up on earlier work showing that dendrites, the branch-like extensions of neurons, process signals differently depending on their location.

To explore this further, the researchers measured neurotransmitter receptors along dendrites using a technique called eMAP (epitope-preserving Magnified Analysis of the Proteome). This method physically expands brain tissue, allowing scientists to label proteins and view them in extremely high detail.

During this imaging, the researchers noticed something unexpected.

"The first thing we saw, which was super bizarre and we didn't expect, was that there were filopodia everywhere," Harnett says.

Filopodia are tiny protrusions that extend from dendrites. Although they had been observed before, their function was unclear because they are so small and difficult to study with traditional tools.

Filopodia and the Signature of Silent Synapses

Using the eMAP technique, the team found filopodia across multiple regions of the adult mouse brain, including the visual cortex, at levels far higher than previously reported. These structures contained NMDA receptors but lacked AMPA receptors.

This detail is crucial. Active synapses typically have both receptor types, which work together to transmit signals using the neurotransmitter glutamate. NMDA receptors alone cannot pass electrical signals under normal conditions because they are blocked by magnesium ions. Without AMPA receptors, these connections remain electrically inactive, which is why they are called "silent."

Turning Silent Synapses On

To test whether these filopodia function as silent synapses, the researchers used a mo