Olfactory transduction. (A) ORNs are embedded in the olfactory epithelium together with sustentacular and basal cells. They are bipolar neurons, sending an axon to the olfactory bulb and extending a single dendrite to the epithelial border. The dendrite terminates in the dendritic knob, from which cilia radiate into the mucus that lines the nasal cavity. (B) Cross section through the tip of an olfactory cilium that contains the olfactory G protein–coupled transduction cascade. “R” and “G” denote the seven-transmembrane OR and the olfactory G protein, G_olf. Green and red arrows indicate transduction mechanisms involved in activating or terminating/adapting the odorant-induced response.