Editing Silent synapse (section)

== Synaptic transmission ==
[[Image:Silentsynapse.png|thumb|right|300px|Silent synapse having NMDA but no AMPA receptors.]]
Normal [[synaptic transmission|transmission]] across a glutamatergic synapse relies on the [[neurotransmitter]] [[glutamate]], the glutamate-specific [[AMPA receptor]] (AMPAR), and [[calcium]] [[ion]]s. 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]]: [[AMPAR|AMPA receptors]] (AMPARs) (formerly known as quisqualate receptors), [[NMDAR|NMDA receptors]] (NMDARs), [[kainate receptors]], and [[metabotropic glutamate receptor]]s (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<sup>+</sup> and K<sup>+</sup> 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. The NMDAR is functionally similar to AMPAR except for two major differences: NMDARs carry ion currents composed of Na<sup>+</sup>, K<sup>+</sup>, but also (unlike most AMPAR) Ca<sup>2+</sup>; NMDARs also have a site inside their ion channel that binds magnesium ions (Mg<sup>2+</sup>). This magnesium binding site is located in the pore of the channel, at a place within the electrical field generated by the membrane potential. Normally, current will not flow through the NMDAR channel, even when it has bound glutamate. This is because the ion channel associated with this receptor is plugged by magnesium, acting like a cork in a bottle. However, since the Mg<sup>2+</sup> is charged and is bound within the membrane's electric field, depolarization of the membrane potential above threshold can dislodge the magnesium, allowing current flow through the NMDAR channel. This gives the NMDAR the property of being voltage-dependent, in that it requires strong postsynaptic [[depolarization]] to allow ion flux.