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A recent study has uncovered an abundance of silent synapses in the adult brain

By: April Carson

Astonishingly, Neuroscientists from MIT have uncovered that the adult human brain is filled with millions of “silent synapses” — dormant neuronal connections which are only activated to establish new memories when necessary. This finding has huge implications for the way we think about memory formation in adults, suggesting that adult brains are far more plastic than previously thought.

An MIT study recently revealed a game-changing discovery - about 30% of synapses in the brain's cortex remain silent even in adult mice. This goes against the prior belief that these types of neurons are only active when learning new information during early life stages. The findings from this research have opened up numerous pathways for further studies into understanding and influencing neurological behavior.

The presence of silent synapses may have important implications for the way we think about memory formation and recall processes in adults. This could mean that adult brains are far more capable of forming new memories than previously thought, as these synapses can be activated and strengthened if needed.

According to the researchers, these unidentified synapses might be responsible for allowing adults' brains to continuously learn new skills and remember recent information without having to change already existing structures. In other words, thanks to these silent synapses our brains are constantly adapting and expanding.

This study could help us understand how to better use our brain's capacity for learning and memory formation.

Lead author of a new study and MIT graduate student, Dimitra Vardalaki, explains that the brain's silent synapses actively seek relationships when presented with important information. This allows for fresh memories to be formed without disturbing pre-existing ones which are more difficult to alter.

An unexpected revelation

Decades ago, scientists uncovered the presence of silent synapses in the brains of juvenile mice and other creatures. These synapses are presumed to be instrumental during early growth, granting young animals a plethora of knowledge that they need to comprehend their surroundings as well as how to interact with them. Research indicates that these brain connections reach maturity by around 12 days old in mice (which is analogous to when human babies start developing).

Neuroscientists suggest that silent synapses can last into adulthood and contribute to memory formation. This idea has been backed up by evidence from animal studies of addiction, which is believed to stem from abnormal learning patterns.

Pioneering research from Stefano Fusi and Larry Abbott of Columbia University has suggested that the brain's ability to learn new information and store it in long-term memory is dependent upon neurons exhibiting diverse plasticity mechanisms. In their model, some synapses must be modified or created readily for short-term memories to form; however, others should remain reliable so existing long-term memories are preserved.

To investigate an elusive finding from their previous experiment, the MIT team set out to study silent synapses. Contrary to what was expected, they observed that within a single neuron all its dendrites - which are like antennae extending outward - process synaptic input differently depending on where in the neuron it is located. This discovery has been instrumental in understanding neuronal networks better and could potentially lead us closer to elucidating some of our most perplexing medical mysteries.

To investigate the discrepancies between behavior, scientists employed eMAP (epitope-preserving Magnified Analysis of the Proteome), a method developed by Chung to evaluate neurotransmitter receptors in distinct dendritic branches. This procedure enables researchers to physically extend samples and distinguish particular proteins in them, providing amazingly detailed images with remarkable accuracy.

To their surprise, while they were gathering the imaging data, Harnett and his team noticed filopodia scattered throughout. “It was very strange - we didn’t anticipate that at all!” he exclaims.

Although filopodia, delicate outgrowths along dendrites, have been observed in the past, scientists were dumbfounded as to their function. This is largely due to these slim projections being difficult to capture with conventional imaging methods because of their minuscule size.

With this realization in hand, the MIT team employed eMAP to investigate filopodia's presence within other sections of an adult mouse brain. To their astonishment, they discovered these thin projections were 10 times more abundant than previously thought and had NMDA receptors but no AMPA receptors! Not only did they find them in the visual cortex, but also distributed throughout various regions of the brain.

Synapses that contain both AMPA and NMDA receptors are considered active, as the two work together to bind glutamate neurotransmitters. However, thanks to magnesium ions blocking access at typical neurons' resting potentials, NMDA receptors alone typically cannot transmit electrical signals - leaving them known as 'silent'.

To determine if these filopodia constituted silent synapses, the researchers administered a modified version of patch clamping to monitor electrical activity in individual filopodia. This process enabled them to evaluate how the stimulation of mimicking glutamate release from neighboring neurons affected each one's behavior.

Applying this method, the researchers observed that glutamate would not produce any electrical signal in the filopodium receiving input unless NMDA receptors were deliberately deactivated. This robustly confirms their theory suggesting filopodia serve as silent synapses within the brain, they explain.

The scientists additionally demonstrated that they could reinstate these synapses by uniting glutamate release with an electrical current from the neuron's body. This combined activation results in a pile-up of AMPA receptors at the silent synapse, permitting it to forge a robust link with the proximate axon releasing glutamate.

According to the researchers, it was significantly simpler to convert silent synapses into active ones as compared with changing mature synapses.

Harnett explains that plasticity protocols are ineffective when starting with a fully functioning synapse since the adult brain has an elevated threshold for memories to be more durable. In contrast, filopodia can be captured to build new reminiscences. Consequently, you do not have to worry about your memories being overwritten too often or quickly.

Silent synapses are widely distributed in a variety of brain regions, which could suggest that they are involved in more than just memory formation and storage. Through the study's findings, scientists are better able to understand how memories may be manipulated in the adult brain. The study was published in the journal Nature Neuroscience and could offer new possibilities for therapeutic approaches to memory-related disorders.

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April Carson is the daughter of Billy Carson. She received her bachelor's degree in Social Sciences from Jacksonville University, where she was also on the Women's Basketball team. She now has a successful clothing company that specializes in organic baby clothes and other items. Take a look at their most popular fall fashions on

To read more of April's blogs, check out her website! She publishes new blogs on a daily basis, including the most helpful mommy advice and baby care tips! Follow on IG @bossbabymav



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