The term accessory olfactory receptor neuron (ORNa) refers to one of two kinds of olfactory receptor (ORN) in the olfactory system (OSY). The other is the main olfactory receptor neuron (ORNm). OLRa's are found on the dendrites of accessory olfactory sensory neurons (OSNa's) located primarily in the vomeronasal organ (VNO), a small circumscribed depression in the olfactory epithelium (OLE) of the nasal septum rostral and far ventral to the OSNm's. ORNa's detect pheromones, that is, social oderants that are age- and sex-specific chemicals secreted by individuals of a given species. The VNO locus of ORNa's is quite prominent in the rodent ( Wirsig-Wiechmann-2001 ) but largely absent in the human and macaque ( Zhang-2003, Francia-2014 ). For more on species differences and function of ORNa's, see olfactory system (OSY). Final 18 Jan 2025.