Cracking the Code: How Does Our Sense of Smell Work?

Most humans enjoy puzzles and making and breaking codes. Equally, most of us are intrigued by the mechanisms by which our bodies function. All of our information about the wonderful universe in which we live, is obtained through our five senses. Therefore, it is not in the least surprising that much research and speculation has been invested into trying to understand how we smell and how that sense decodes the olfactory messages of molecules.

Lock and Key Models of Olfaction

The first modern theories to attempt to break the olfactory code were based on the lock and key principle. Enzymes recognize their substrates by the close fit of the substrate into the active site of the enzyme. It therefore seemed reasonable to postulate that the olfactory receptors might behave like enzymes and recognize specifi c substrates. A simple schematic of this is shown in Figure 1. In this model the receptors are tuned to odor types. The top receptor detects rose, the next musk, the next lily, and so on. There would be one receptor type for each primary odor. In Figure 1, for example, a green odorant approaches the array of receptor cells and docks into the green receptor. The brain then receives a message that receptor Type 5 is fi ring and therefore concludes that a green odorant is present. The best known example of this type of model is that of John Amoore. He fi rst postulated the existence of primary odors similar to the three primary colors of vision. Thus, complex odors would be produced by combinations of the primary odors, similar to the combination of red and yellow to produce orange. Based on this concept, he postulated a set of seven receptor types based on the most commonly used odor descriptors. He later refi ned the model based on Guillot’s suggestion that, in the case of anosmia, a given receptor type is missing. For example, an individual who did not possess receptor Type 2, would be unable to detect musk odorants. This approach led him to postulate about 30 primary odors.

However, when we look at the list of puzzling features of the sense of smell, there are several that cannot be accounted for by such models. Musk, for example, is one of the most common anosmias. Therefore, proponents of the lock and key model would argue for the existence of a musk receptor. So, any individual without this receptor would not be able to smell musks. This would mean that if an individual cannot smell one musk, then he/she would not be able to smell any musks. The model does not account for those who can smell all but one specifi c musk material. Furthermore, if the musk receptor were missing, how would an anosmic individual be able to detect the presence of a musk in a compounded fragrance?

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