Watch this 2-Minute Neuroscience video to learn more about receptors and ligands.

Allosterism:

also known as neuromodulation, allosterism occurs when a ligand binds to a receptor at a site near the binding site where the neurotransmitter normally binds. Binding produces an allosteric effect, which increases or decreases the likelihood that neurotransmitter will bind to its typical binding site, or modifies the effect it has when it does bind. Thus, allosterism indirectly affects neural transmission.