• Neurotransmitters convey electrical signals from neuron to neuron or from neuron to target cell. • Many neurotransmitters function as ligands—molecules that bind to a specific site on a receptor molecule—that bind to ligand-gated ion channels. • A single neurotransmitter may have more than a dozen different receptors • Some other neurotransmitters bind to metabotropic receptors that activates enzymes for production of a second messenger that may trigger changes in gene expression, enzyme activity, or membrane potential. • Compared to ligand-gated channels, the effects of second-messenger systems have a slower onset but last longer