Sites located on the surface membranes of cells in the central nervous system (CNS) and peripheral nervous system to which the neurotransmitter serotonin (5-hydroxytryptamine [5-HT]) binds, thereby activating serotonin neurons and downstream pathways. Serotonin receptors (seven classes [5-HT1–5-HT7] have been identified, many of which have multiple subtypes) are involved in many biological and neurological processes, including aggression, anxiety, appetite, cognition, learning, memory, mood, nausea, sleep, and thermoregulation.