Synapses are sites of interneuronal communication where neurotransmitters released by the presynaptic cell elicit a response on the postsynaptic cell. There are 2 main kinds of synapses, based on the effect that they elicit. The excitatory synapse consists of a multitude of different components, ranging from receptors and channels to neurotransmitters and vesicle-associated proteins.
Neurotransmitter release in an excitatory synapse results in the depolarization of the postsynaptic membrane, whereas the corresponding result in an inhibitory synapse is hyperpolarization. Glutamate is the most important excitatory neurotransmitter.
Neurotransmitters are synthesized in the neuronal cell body and trafficked in vesicles to the presynaptic membrane along the cytoskeleton. These vesicles associate with synapsins, calcium dependent proteins that regulate their release into the synaptic cleft.
When a depolarizing signal reaches the presynaptic terminal, it causes the opening of voltage-gated Ca2+ channels. The resulting influx of Ca2+ from the synaptic cleft triggers the fusion of the synaptic vesicle to plasma membrane and neurotransmitter release. Excess or unbound neurotransmitter is continually recycled via clathrin-mediated endocytosis.
Once the excitatory neurotransmitter reaches the postsynaptic membrane it can bind to either an ionotropic (ligand-gated ion channel) or a metabotropic (G-protein couple receptor) receptor. In both cases, but through different mechanisms, neurotransmitter binding results in the opening of ion channels which raises the membrane potential and increases the probability of the postsynaptic neuron firing an action potential.
This pathway card presents the basic anatomy of an excitatory synapse as well as the cascade of events that occur up to and after neurotransmitter release.