How Many Smell Receptors Do Humans Have?

Noam Sobel says humans have about 7 million smell receptors lining the olfactory epithelium, and probably about six or seven million receptors, of about 350 different kinds. A meaningful percentage of the genome is devoted just to the kinds of olfactory receptor subtypes in the nose.

Primary olfaction is orthonasal olfaction, smell through the nose by sniffing. Airborne molecules travel about six or seven centimeters up the nose to a convoluted surface, not a flat sheet. Odorants dock at a receptor and turn into a neural signal made of action potentials, not gradient potentials. The signal propagates via the olfactory nerve through the thinnest part of the skull, which is perforated and has a lot of holes, and synapses at the olfactory bulb. All the receptors of one subtype converge to one location in the bulb. The location is a glomerulus, and in fact two glomeruli, in the most extreme case of convergence in the mammalian nervous system. Every receptor subtype responds to a small subset of different molecular shapes, and each shape activates a different subset of receptors. Convergence leaves a map on the bulb that reflects receptor identity. Coffee that activates receptors of type one, three, and seven lights up the glomeruli of receptors one, three, and seven.

A smell shows up as a pattern of glomeruli, matched to the receptor subtypes the odorants activate.