| Neurotransmitter receptors are proteins embedded in the postsynaptic plasma membrane. Receptors translate chemical signals into electrical signals by binding neurotransmitter molecules secreted by presynaptic neurons, which leads in turn to opening or closing postsynaptic ion channels. The postsynaptic currents produced by the synchronous opening or closing of the ion channels changes the membrane potential of the postsynaptic cell. Potential changes that increase the probability of firing an action potential are excitatory, whereas those that decrease the probabiity of generating an action potential are inhibitory. Because postsynaptic neurons are usually innervated by many different inputs, the integrated effect of all EPSPs and IPSPs produced in a postsynaptic cell at any moment determines whether or not the cell fires an action potential.
Two broadly different families of neurotransmitter receptors have evolved to carry out the postsynaptic signaling actions of neurotransmitters. Ligand-gated ion channels combine the neurotransmitter receptor and ion channel in one molecular entity and therefore give rise to rapid postsynaptic electrical responses. Metabotropic receptors regulate the activity of postsynaptic ion channels indirectly, via G-proteins, and induce slower and longer-lasting electrical responses. The faster effects of metabotropic receptors—which are still slower than ligant-gated effects—occur when G-proteins themselves activate ion channels. Slower metabotropic effects involve the activation of intracellular effector enzymes that modulate the phosphorylation of target proteins and/or gene transcription.
The postsynaptic response at a given synapse is determined by the combination of receptor subtypes, G-protein subtypes, and ion channels that are expressed in the postsynaptic cell. Because each of these features can very both within and among neurons, a tremendous diversity of transmitter-mediated postsynaptics effects is possible.
Neuroscience, Sinaur Associates (QP355.2.N487 1997) | 
