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Our Sense of Smell
Dr. Matsunami is assistant professor, Department of Molecular Genetics and Microbiology, Duke University Medical Center. Dr. Zhuang is assistant professor, Department of Pathophysiology, Shanghai Jiao Tong University School of Medicine.
The sense of smell is vital to humans and other animals, and it makes life more interesting. It forms most of what we think of as our sense of taste. It helps in identifying food, predators, mating partners and poisonous substances. In humans in particular, the sense of smell allows us to enjoy meals, detect gas leaks and even analyze the body odor of others. Studies indicate that it plays a strong role in sexual attraction. The mammalian olfactory system is a wonder; it can detect and distinguish between many thousands of individual odors.
All mammals, including humans, have evolved the ability to distinguish extremely small differences in the structure of various chemicals. In addition, the olfactory system may perceive the same molecules as having a different smell at different concentrations. For example, some perfume ingredients are perceived as very pleasant at low concentrations but very unpleasant at higher concentrations. An interesting feature of the sense of smell is that it is the only one of the five senses in which there is direct contact between cells that receive the stimuli and nerve cells in the brain. This may well be part of the reason why smells produce such vivid reactions in the brain and why, for example, smells seem to have such a great power to stimulate memory.
How Smell WorksSmell, or olfaction, begins in what are called odorant receptors (ORs); these are located in the olfactory sensory neurons (OSNs) of the main olfactory epithelium. The olfactory epithelium is an area of tissue inside the nasal cavity. In humans, it measures about 1 inch wide by 2 inches and is located about 3 inches above and behind the nostrils. The number of ORs in a given species is genetically determined. There are approximately 400 OR genes in human. There are more than 1000 OR genes in mice. This is probably the reason why mice can distinguish more odors in the environment than humans.(1)(2)(3)(4)(5) ORs operate in a way similar to the specialized plugs at the ends of electronic cables. When certain substances from outside the body encounter an OR that they can "plug into," a reaction begins that results in the brain perceiving a certain smell. If no matching OR exists, then the molecule will not register as a smell. Mice, with the 1000 OR genes, would have many more sites for outside substances to plug into to register a smell.
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