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Concept
Silent synapse
Summary
In neuroscience, a silent synapse is an excitatory glutamatergic synapse whose postsynaptic membrane contains NMDA-type glutamate receptors but no AMPA-type glutamate receptors. These synapses are named "silent" because normal AMPA receptor-mediated signaling is not present, rendering the synapse inactive under typical conditions. Silent synapses are typically considered to be immature glutamatergic synapses. As the brain matures, the relative number of silent synapses decreases. However, recent research on hippocampal silent synapses shows that while they may indeed be a developmental landmark in the formation of a synapse, that synapses can be "silenced" by activity, even once they have acquired AMPA receptors. Thus, silence may be a state that synapses can visit many times during their lifetimes.
Normal transmission across a glutamatergic synapse relies on the neurotransmitter glutamate, the glutamate-specific AMPA receptor (AMPAR), and calcium ions. Calcium ion entry into the presynaptic terminal causes the presynaptic release of glutamate, which diffuses across the synaptic cleft, binding to glutamate receptors on the postsynaptic membrane. There are four subtypes of glutamate receptors: AMPA receptors (AMPARs) (formerly known as quisqualate receptors), NMDA receptors (NMDARs), kainate receptors, and metabotropic glutamate receptors (mGluRs). Most research has been focused on the AMPARs and the NMDARs. When glutamate binds to AMPARs located on the postsynaptic membrane, they permit a mixed flow of Na+ and K+ to cross the cells membrane, causing a depolarization of the postsynaptic membrane. This localized depolarization is called an excitatory postsynaptic potential (EPSP).
Silent synapses release glutamate as do prototypical glutamatergic synapses, but their postsynaptic membranes contain only NMDA—and possibly mGlu—receptors able to bind glutamate. Though AMPA receptors are not expressed in the postsynaptic membranes of silent synapses, they are stored in vesicles inside the postsynaptic cells, where they cannot detect extracellular glutamate, but can be quickly inserted into the postsynaptic cell membrane in response to a tetanizing stimulus.
Official source
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