Sensitization is a simple form of nonassociative learning that involves the enhancement of the response to a weak stimulus that occurs after the presentation of a strong or noxious stimulus. Sensitization usually occurs in two forms that differ in their duration and underlying mechanisms. Short-term sensitization lasts seconds to minutes and involves the modification of neuronal membrane properties and synaptic efficacy, often through the alteration of the phosphorylation state of existing proteins. Long-term sensitization lasts from days to weeks, depending on the training protocol. Unlike the short-term version, long-term sensitization requires synthesis of new macromolecules—the inhibition of either gene transcription into mRNA or translation of mRNA into protein blocks long-term sensitization. In its most persistent form, long-term sensitization involves morphological changes and neuronal growth.

The marine mollusk Aplysia has proved a useful model for gaining insights into the underlying...