How Do the G-Protein-Linked Receptor and an Ion Channel Differ From Each Other?
Receptors and ligands come in many forms, but they all have one thing in common: they come in closely matched pairs, with a receptor recognizing just one (or a few) specific ligands, and a ligand binding to just one (or a few) target receptors. Binding of a ligand to a receptor changes its shape or activity, allowing it to transmit a signal or directly produce a change inside of the cell.
Among these membrane proteins is the GPCRs Superfamily. They form the largest and most diverse family of cell surface receptors and proteins. For example, there are about 1000 GPCRs in neurons alone. The diversity of this superfamily is a result of the large number of members comprising this family, their ability to form different dimer combinations and their ability to respond to a multitude of stimuli, as well as by the large number of intracellular signaling pathways they activate. Despite their structural and functional diversity, all GPCRs share a similar molecular architecture. They consist of seven transmembrane domains, linked by alternating intracellular and extracellular loops, an extracellular N-terminus and an intracellular C-terminus.
Importantly, a given transmitter may activate both metabotropic receptors and ligand-gated ion channels to produce both fast and slow PSPs at the same synapse. Perhaps the most important principle to keep in mind is that the response elicited by a given neurotransmitter is determined by the postsynaptic complement of receptors and their associated channels. Exactly how postsynaptic responses are produced by some especially important examples of neurotransmitter receptors is considered in the following sections.
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