Abstract
A large number of substances including amines, amino acids, and peptides have been assigned neurotransmitter roles in both vertebrate and invertebrate nervous systems. Although explanations for this large number of neurotransmitters have been proposed, it is still far from understood how all these neurotransmitter systems function and interact within the nervous system. Some clues to the answer to this question should come from current work on the neurotransmitter systems involved in the modulation of the output of a “relatively simple system,” the stomatogastric ganglion (STG) of decapod crustaceans.