Wednesday, October 19, 2011
The synaptic transmission is essentially characterized by three processes: 1) conversion of the action potential arriving at the axon terminal into a chemical signal; depolarization results in the opening of calcium channels and in the influx of calcium, which, mediated by certain proteins, causes fusion of synaptic vesicles with the presynaptic membrane and release of the neurotransmitter into the cleft. 2) The released transmitter binds to specific receptors. 3) In the case of ligand-gated ion channels, this results in their opening for certain ions; in the case of glutamatergic receptors, the influx of Na+ or Ca2 + causes depolarization of the postsynaptic membrane (excitatory postsynaptic potential, EPSP). In the case of GABA and glycine receptors, the influx of Cl– causes hyperpolarization of the postsynaptic membrane (inhibitory postsynaptic potential, IPSP). The activation of G protein-coupled receptors results in a long-lasting response that may finally lead to a change in gene expression in the postsynaptic neuron.