What Is an Olfactory Receptor?

Regarding olfactory production, many researchers have put forward some theories from different angles. What they can explain is mainly the first stage of the olfactory process, that is, the changes caused by the olfactory substances and the nasal mucosa. As for the relationship between the stimulus transmission and the olfactory sensation in the next stage, it needs to be further studied. These olfactory theories include three-dimensional structure theory, molecular vibration theory, adsorption theory, enzyme theory, and Sumner theory.

Regarding the mechanism of olfactory production, many researchers have put forward some theories to explain from different angles, but they are not perfect. At present, the basic conditions for generating olfactory are generally accepted. These conditions include: the odor-generating substance itself can be volatilized, so that it can reach the olfactory region in the nasal cavity under breathing; the odor substance can be dissolved in the olfactory mucosa It can also dissolve in the fat or lipid end of olfactory cells; if the odor substance is dissolved in the olfactory sensation area, it will trigger some chemical reactions, and the stimulus generated by the reaction will be sent to the brain to generate olfactory sensation.

There are many theories about the production of olfactory, so far there are no less than 50 kinds. ^[1]

In 1946, Linus Pauling, the Nobel laureate, described the particular smell as related to the shape and size of molecules. In the book " Molecular Basis of Odor " by Jone Amoore, he extended "theory of three-dimensional structure". The theory was first developed by R. W. Moncrieff proposed in 1949. He believes that the chemicals in the air can be smelled because these molecules are suitable for the location of specific sensory receptors on the olfactory nerve. Such a "lock-key" approach is actually derived from the kinetics of the enzyme reaction. After studying the aroma characteristics of 600 organic compounds, Amoore found that 7 kinds of original odor appeared more frequently. In 1962, he proposed the original odor concept (ether odor, camphor odor, musk, floral, mint odor, irritating odor and malodor), and compared the volume and shape of various molecules with their odor. ^[2]

As early as the 1920s and 1930s, Malcolm Dyson boldly guessed that olfactory sensed different chemical substances through the vibration of odor molecules, and proposed that infrared resonance (IR) of molecules may be different from odors. Related, called odor vibration theory. According to the molecular vibration theory, the aroma is generated by the electronic vibration of the aroma molecules, and the odor characteristics are related to the vibration frequency of the odor molecules, which is within the oral temperature range. The vibration energy level of odor molecules is in the infrared or Raman spectral region, and human olfactory receptors sense the vibration energy of the molecules and generate signals. This theory became popular with Wright in the 1950s.

Many aromas vibrate in the infrared region. Is the infrared vibration of molecules related to smell? The male moth rushed to the candle because the infrared light emitted by the candle was the same as the biological pheromone of the female moth. Different frequencies of IR may produce different odors. In the 1960s and early 1970s, heated debates arose over the classification of chemical odorants. By the mid-1970s, after rigorous practical tests, Wright's theory failed because the optical isomers of menthol and carvone had significantly different odors, and the corresponding infrared spectra were the same. ^[2]

The adsorption theory believes that olfactory mucosa adsorbs odorous substances that enter the olfactory region, and this adsorption causes the position of the mucosa to change, which in turn induces nerve pulses that produce olfactory sensations. In this process, the degree to which the odor is perceived depends mainly on the adsorption capacity of the odor molecules from the gas phase to the liquid phase mucosa, and the membrane translocation caused by the adsorption of these molecules, which is mainly affected by the shape and size of the molecules. ^[4]

Enzymes are found in all parts of the body, and the olfactory area is no exception. Enzyme theory emphasizes the role of enzymes in the process of olfactory production. The theory assumes that before entering the olfactory region, odorous substances can inhibit the activity of one or more types of enzymes in the region. This selective inhibition changes the relative concentrations of various compounds on the olfactory receptor, thereby triggering the production of Olfactory nerve impulses.

In enzyme theory, lipids located on the olfactory mucosa are the most important when feeling the odor. Tests have confirmed that the alkaline phosphatase activity found on the olfactory and taste mucosa can be affected by substances such as vanilla and coffee. Inhibition, but sugar, salt and quinine can not inhibit the activity of this enzyme, quinine can inhibit the activity of lipase, and sugar, salt can not. The above facts show that during the formation of olfactory and taste sensations, there are changes in enzyme activity and the concentration of certain substances. However, some researchers object to this explanation, because enzymes are rarely inhibited by a specific substance, and many odorless substances also inhibit enzyme activity. Even if the assertion that odor substances inhibit enzyme activity produces an olfactory sense, it cannot be fulfilled. Explain that some odorous substances can be sensed at very low concentrations, because the concentration of odorous substances at this time is theoretically not an inhibitory effect on the enzyme. Although the enzyme theory can explain this phenomenon produced by the sense of smell, it is not a theory that fully explains the mechanism of smell formation. ^[4]

Sumner summarized the previous theoretical explanations of olfactory, such as adsorption theory, enzyme theory, absorption theory, and physiological interpretation of smell. He proposed that it is very difficult to form a complete theory based on the experimental techniques currently available and the data that has been obtained. Only on this basis, the rough outline of the olfactory theory and the process of olfactory production can be captured and drawn. Sumner's theory can be summarized as follows: first, the odorous chemical I will react with the olfactory receptor A, and molecules can be released from A. These released molecules stimulate the olfactory sensory nerve to generate a pulse; The scented chemical II reacts with olfactory receptor B, and the molecules released from B generate another kind of stimulation pulse. Substance II can also react with susceptor c, releasing molecules from c. The number of molecules released from the susceptor is directly proportional to the amount of odorous substance. Substance I reacts quickly with olfactory receptor A, and only a small amount reacts with receptors B and C. When odorous substances react with olfactory receptors for a period of time, due to the continuous release of molecules by olfactory receptors, the lack of olfactory receptors Effective molecules, at which point olfactory fatigue occurs until such molecules are replenished. Because each person has a different number of olfactory receptors A, B, or C in the olfactory sensation area, some people are very sensitive to some specific odors, while others are relatively dull. ^[4]