The term accessory olfactory system (OSYa) refers to one of two types of olfactory system (OSY). The other is the main olfactory system (OSYm). The OSYa consists of the vomeronasal organ (VNO), accessory olfactory receptor neurons (OLRa), vomeronasal-terminal nerve complex (vtnc), and accessory olfactory bulb (AOB). Projections beyond the AOB join those of the main olfactory bulb (MOB) in the olfactory tract (oltr) and continue to the medial amygdalar nucleus (MEA) of the amygdala (AMG). That is in contrast to the OSYm where the MOB projects to a number of central structures, but not to the (MEA) ( Buck-2013 ). The OSYa is found in rodents but not primates. For more on species differences and function of the OSYa, see olfactory system (OSY). Final 18 Jan 2025.