Olfactory receptor neurons

Each olfactory receptor neuron (ORN) has a single dendrite extending to the epithelial surface, where it swells into an olfactory knob bearing 10–30 long, non-motile cilia (they have a 9+2 axoneme but lack dynein arms) that spread in the mucus. The cilia carry odorant receptors — G-protein-coupled receptors encoded by the largest gene family in the genome: ≈850 OR genes in humans, of which only ≈350–400 are functional (the rest are pseudogenes). By the "one neuron – one receptor" rule, each neuron expresses a single receptor allele.

Transduction: odorant binds the receptor → activates G_olf → adenylyl cyclase type III → ↑cAMP → opens cyclic nucleotide-gated (CNG) channels → Na⁺ and Ca²⁺ influx → Ca²⁺ opens Ca²⁺-activated Cl⁻ channels (ANO2); because ORNs accumulate Cl⁻ (via NKCC1), Cl⁻ efflux further depolarises and amplifies the signal → generator potential → action potentials in the unmyelinated axon. Ca²⁺–calmodulin feedback on the CNG channel and phosphodiesterase activity contribute to rapid adaptation.

Combinatorial coding: each receptor binds several odorants and each odorant activates several receptors, so an odour is encoded by a unique combination of activated receptor types — allowing discrimination of a vast number of odours. All neurons expressing a given receptor are scattered in the epithelium but their axons converge on a few glomeruli (usually 1–2 per bulb half) — forming a chemotopic odour map. ORNs live ≈1–2 months and are replaced from basal cells, the new axons finding their way to the correct glomerulus guided by the receptor itself.