Neuroscientists from MIT discovered that there are millions of silent synapses that abound within the adult brain. These silent synapses are immature neural links that remain inactive until they are necessary for the creation of fresh memories.
Silent Synapses Thought To Be Present Only During Early Development
SciTechDaily reports that it was previously thought that such silent synapses were only present in early development. These neural links helped the brain take in new information that it came across during these phases. However, a recent study conducted by MIT neuroscientists found that among adult mice, around 30% of synapses within the cortex were found to be silent.
The presence of such silent synapses could shed light on the capacity of the human brain to keep on creating new memories and learning different things even without the modification of current typical synapses.
Lead author and MIT graduate student Dimitra Vardalaki notes how the silent synapses search for new connections. Moreover, when vital new data is presented, the relevant neural links get strengthened. This helps the brain come up with fresh memories even without having to overwrite other vital memories kept within mature links, which are more difficult to alter.
Laboratory Equipment notes that when silent synapses were first found, they were observed in young mice and animal brains. However, they disappeared as the organisms aged. However, there were neuroscientists who proposed that these silent synapses may stay all the way until adulthood.
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Silent Synapses in Mature Brains
The researchers attempted to gauge the receptors for neurotransmitters across various branches of dendrites. They utilized a method known as eMAP which enabled them to physically stretch a tissue sample and point out the particular proteins present within it. This made it possible for them to come up with extremely high-resolution photos.
As they performed the imaging, they discovered something quite surprising. They noticed that filopodia were everywhere.
These filopodia are thin protrusions of membranes that extend out of dendrites. While they were previously observed, neuroscientists did not know what to do about them. This is partially because these filopodia are minute and hard to see using usual imaging methodologies.
After observing these, the team used eMAP to try to spot filopodia across other brain regions. Surprisingly, they were able to spot filopodia in the visual cortex and other areas. These filopodia levels were 10 times higher than previously observed. The scientists also observed that there were neurotransmitter receptors in filopodia known as NMDA receptors. AMPA receptors, on the other hand, were not present.
Usual synapses have both receptor types. When AMPA ones are not there, these synapses that are only filled with NMDA receptors are dubbed silent synapses. The researchers investigated this further using a modified patch clamping methodology. By doing so, they discovered that there is no electrical signal in the filopodia that receives input that will be generated from glutamate, unless the receptors get unblocked experimentally.
Such findings support the theory that filopodia represent brain silent synapses. The researchers also demonstrated that these synapses can be unsilenced by mixing glutamate release and neural electrical current. They also discovered that turning the synapses active is easier than changing mature ones.
Could the Adult Human Brain Be Filled With Silent Synapses, Too?
The neuroscientists are now searching for proof that silent synapses are also present within tissues of the human brain. They plan to study if the function or number of the synapses is affected by neurodegenerative conditions or aging.
Senior author and associate professor Mark Harnett notes the possibility of how altering flexibility in the memory system could affect difficulty levels when it comes to altering habits or behaviors and learning new information. He notes that one may imagine spotting molecule forerunners that are involved within filopodia and that attempt to manipulate things in order for flexible memory to be restored as aging continues.
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