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Chemical Reaction: Nose News

By: Steve Herman
Posted: January 8, 2007, from the January 2007 issue of GCI Magazine.

“Smell is a potent wizard that transports you across thousands of miles and all the years you have lived.”—Helen Keller

The exciting thing about the sense of smell is that its mysteries are still being unraveled. Of course, more is known than in the Dark Ages, before Linda B. Buck and Richard Axel discovered the odor receptor gene family in 1991. They revolutionized the study of olfaction by enabling the tools of genetic research to be applied to the odor receptors. They were rewarded with the 2004 Nobel Prize in Physiology or Medicine, but the quest for deeper understanding continues. Not only has genetics provided new insights, but the understanding of the brain has taken the exploration of scent into the inner depths of the human mind.

The classic simplistic text book description of odor reception refers to hair-like cilia in the top of the nasal cavity. A signal is sent from the cilia to the olfactory bulb and on to the brain. Correct as far as it goes, yet it explains nothing about the detailed mechanism required to decode an odor. The generally accepted structure/odor theories stress either the shape of the odorant molecule or alternatively emphasize molecular vibrations.1

An airborne odorant comes in contact with a receptor, a 7-transmembrane protein (7TM) in the cilia in the top of the nasal cavity. The odorant molecule must be volatile, rarely bigger than a molecular weight of 300 and slightly polar. A signal is triggered, either due to conformation change or molecular vibration. The signal activates a G-protein, the discovery of which was recognized by the 1992 Nobel Prize.

The G-protein is both an on/off switch and signal amplifier. The cascade it generates opens an ion channel that travels through an axon to a ball of nerves called the glumerili. The glumerili reside in the olfactory bulb. From here, an impulse travels to several areas in the limbic system of the brain. Since the limbic system is the emotive part of the brain, it becomes clear why odors can have such profound effects on memories and feelings.