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Insufficient AMPA-type glutamate receptors prevent freshly formed excitatory synapses in the mammalian cortex from mediating neurotransmission. As a result, these synapses are functionally quiescent or silent and must develop through activity-dependent plasticity. Silent synapses are assumed to be few in adults, but they are common throughout early development when they mediate circuit creation and refinement. But adults still have the ability for flexible learning and brain plasticity, which shows that the development of new connections is still common.
Silent synapses aid in the creation of new memories.
Many years ago, when the idea of a “silent synapse” was discovered, they were mainly seen in the brains of young mice and other animals. These synapses are thought to aid in the brain’s ability to absorb the enormous amounts of information that infants require in order to learn about and interact with their surroundings. These synapses were thought to vanish in mice by the time they were 12 days old, which corresponds to the first day of human life. Silent synapses, on the other hand, may last into adulthood and aid in the creation of new memories, according to some neuroscientists. Animal addiction models, which are assumed to be essentially a condition of abnormal learning, provide evidence for this.
A team of neuroscientists at MIT has demonstrated that dendrites can handle synaptic information differently depending on where they are located inside a single neuron. Dendrites, branched protoplasmic extensions of a nerve cell, transmit the electrochemical stimulus that the neuron’s cell body receives from other neural cells. In several dendritic branches, they examined neurotransmitter receptors using a technique called eMAP (epitope-preserving Magnified Analysis of the Proteome). This technique allowed them to physically enlarge tissue samples before labeling particular proteins in the samples to get ultra-high-resolution images. The tips of the dendritic protrusions contained filopodia, which made up roughly 30% of all dendritic protrusions and were where these alleged silent synapses were discovered. Laboratory mice were used for the procedure, and they discovered filopodia at a level 10 times greater than previously observed in the mouse visual cortex and other regions of the brain. Filopodia displayed NMDA-receptor-mediated synaptic transmission, but the experiment showed they lacked AMPA-receptor-mediated transmission. It was also demonstrated that Hebbian plasticity, which involves bringing new active connections into a neuron’s input matrix, can cause functionally silent synapses on filopodia to become active.
The search for these silent synapses in human brain tissue is currently underway. Additionally, they want to investigate how aging and neurodegenerative diseases, as well as other variables, may influence the number or functionality of these synapses.
The existence of these silent synapses may contribute to understanding how the adult brain can continuously create new memories and learn new information without having to alter its already-existing conventional synapses. These connections are inactive until they are called upon to support the formation of new memories.