Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are bipolar neurons with dendrites facing the external  surface of the cribriform plate with axons that pass through the cribriform foramina with terminal end at olfactory bulbs.  The ORNs are located in the olfactory epithelium in the nasal cavity.  The cell bodies of the ORNs are distributed among all three of the stratified layers of the olfactory epithelium.
Many tiny hair-like non-motile cilia protrude from the olfactory receptor cell's dendrites. The dendrites extend to the olfactory epithelial surface and each ends in a dendritic knob from which around 20 to 35 cilia protrude. The cilia have a length of up to 100 micrometres and with the cilia from other dendrites form a meshwork in the olfactory mucus.  The surface of the cilia is covered with olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell expresses only one type of olfactory receptor (OR), but many separate olfactory receptor cells express ORs which bind the same set of odors.  The axons of olfactory receptor cells which express the same OR converge to form glomeruli in the olfactory bulb.