Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Serotonin (pronounced / s r to n n/) or 5-Hydroxytryptamine (5-HT) is a monoamine neurotransmitter that is primarily found in the gastrointestinal (GI) tract and central nervous system (CNS) of humans and animals. Approximately 80 percent of the human body''s total serotonin is located in the enterochromaffin cells in the gut, where it is used to regulate intestinal movements. The remainder is synthesized in serotonergic neurons in the CNS where it has various functions, including the regulation of mood, appetite, sleep, muscle contraction, and some cognitive functions including memory and learning. Modulation of serotonin at synapses is thought to be a major action of several classes of pharmacological antidepressants.