Olfactory neuron

Function

In vertebrate ORNs, odorant receptors are G protein-coupled receptors located mainly in the membrane of the olfactory cilia. There are approximately 1000 different genes that code for the ORs in the mouse, making them the largest gene family in the mammalian genome; humans have roughly 400 functional OR genes, the remainder having become pseudogenes. An odorant dissolves into the mucus of the olfactory epithelium and binds to an OR. Individual ORs can respond to multiple odorants, and a single odorant can activate multiple ORs, producing a combinatorial receptor code. Odorant binding activates the G protein G_olf; its α-subunit, encoded by GNAL, stimulates adenylyl cyclase type III (ACIII) to produce cyclic AMP (cAMP). cAMP opens cyclic nucleotide-gated (CNG) channels, admitting Na and Ca. The rise in intracellular Ca opens calcium-activated chloride channels, including TMEM16B/ANO2, and because olfactory neurons maintain a high internal chloride concentration, Cl flows out of the cell, amplifying the depolarization and promoting action potential firing.