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The Molecular Logic of Smell

Organisms possess a remarkable ability to detect smells in the environment through the olfactory system. This system originates in the olfactory epithelium, in the nasal cavity. It is made up of three main cell types: the olfactory receptor cells, supporting cells, and basal cells.
Olfactory receptor cells are neurons, and send axons directly into the central nervous system. The supporting cells do not pass information – they produce mucus. Basal cells are the source of new olfactory receptor cells, which grow, die and regenerate over a period of 4 to 8 weeks.
Small molecules present in the air become dissolved in the mucus lining the nasal cavity. Olfactory receptor cells send dendrites up to the mucus layer. Cilia extend into the mucus, where odorants can bind to the surface and activate the olfactory receptor cells. Olfactory receptor cells send unmyelinated axons by thefirst cranial nerve into the olfactory bulb. The molecules bind to cilia and activate olfactory receptor cells. Olfactory receptor nerve cells connect with nerves in the olfactory bulb. More nerves connect the olfactory tract to the thalamus. Thalamic pathways then extend to the frontal lobe of the cerebral cortex. Direct connections are also made between the olfactory bulb and the olfactory cortex.
The mammalian genome contains a family of about 1000 related but separate genes encoding different odor receptors. (But only 40% of these are functional in humans.) The olfactory epithelium of rats expresses several hundred genes not expressed in other tissues. Gene probes for a single type of receptor bind to only 1 in a 1000 sensory neurons in a normal olfactory epithelium. However, rats that express a single type of receptor in large numbers of their olfactory neurons tend to respond much more vigorously to a single type of odorant than to any of the others tested.
This article continues to leave the questions in mind. How does the olfactory cortex decode t

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