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About the primary biological mechanism of odour-perception

Some important publications have appeared about the primary biological mechanism of odour perception. The problems concerning this mechanism are more and more solved during the last decade. A short review will be given of some important publications in this area.

In 1991 Buck and Axel published their studies about a novel multigene family, which may encode odorant receptors as a molecular basis for odor perception. Their work is summarized as follows: "The mammalian olfactory system can recognize and discriminate a large number of different odorant molecules. The detection of chemically distinct odorants presumably results from the association of odorous ligands with specific receptors on olfactory sensory neurons. To adress the problem of olfactory perception at a molecular level, we have cloned and characterized 18 different members of an extremely large multigene family that encodes seven transmembrane domain proteins whose expression is restricted to the olfactory epithelium. The members of this novel gene family are likely to encode a diverse family of odorant receptors".

Following these findings Axel wrote an article in 1995 called: "The molecular Logic of Smell" with a subtitle: "Mammals can recognize thousands of odors, some of which prompt powerful responses. Recent experiments illuminate how the nose and brain may perceive scents". He reviewed the primary process of olfaction as: "Scent of a flower is translated from a sniff to a smile by the olfactory sensory system. An odor is first detected in the upper region of the nose, at the olfactory epithelium. Within this area, odor molecules bind to receptors on hairlike projections, or cilia. The receptors are part of neurons that can extend three to four centimeters from the inside of the nose to the brain. Structures known as axons run from the neuronal cell body to the olfactory bulb in the brain. In the bulb, axons converge at sites called glomeruli; from these signals are relayed to other regions of the brain, including the olfactory cortex. The vomeronasal organ is part of a separate sensory system that governs innate responses in some mammals. Its role in human behavior is not well known." He finished with a few questions: "How does the cortex prompt the range of emotional or behavioral responses that smell often provoke? To what extent is the recognition of odors in humans conscious or nonconscious, and how much of behavior or mood is governed by the perception of odors in our environment? We have only begun to explore the logic of smell and how it can evoke the vast structure of recollection".

Finally in 1999 Malnic et al. published their work on combinatorial receptor codes for odors resulting from recent investigations and the former publications from Buck and Axel. They summarized their work as follows: "The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. Here we used a combination of calcium imaging and single-cell RT-PCR (reverse transcriptase-polymerase chain reaction) to identify odorant receptors (ORs) for odorants with related structures but varied odors. We found that one OR recognizes multiple odorants and that one odorant is recognized by multiple ORs, but that different odorants are recognized by different combination of ORs. Thus, the olfactory system used a combinatorial receptor coding scheme to encode odor identities. Our studies also indicate that slight alterations in an odorant, or a change in concentration, can change its "code", potentially explaining how such changes can alter perceived odor quality".

Concluding remarks
One can imagine that with the existence of about thousand of different receptor-types and thousands of different odorant molecules the primary process of odour perception becomes rather complicated. Although the primary biological mechanism of odour perception is well understood, the final behavioral responses during olfaction by humans are still subjective and often irreproducible. The verbal responses during human olfaction depend on various psychological factors, such as: more or less intellectual means, learning processes, mood, mental attitude and physical conditions (sex, age, occupation).

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