Smell receptors

Smelling salts Smell receptors Smelters Smelting... 

Flavor has been defined as a memory and an experience (1). These definitions have always included as part of the explanation at least two phenomena, ie, taste and smell (2). It is suggested that in defining flavor too much emphasis is put on the olfactory (smell) and gustatory (taste) aspects (3), and that vision, hearing, and tactile senses also contribute to the total flavor impression. Flavor is viewed as a division between physical sense, eg, appearance, texture, and consistency, and chemical sense, ie, smell, taste, and feeling (4). The Society of Flavor Chemists, Inc, defines flavor as "the sum total of those characteristics of any material taken in the mouth, perceived principally by the senses of taste and smell and also the general senses of pain and tactile receptors in the mouth, as perceived by the brain" (5). 

Taste-active chemicals react with receptors on the surface of sensory cells in the papillae causing electrical depolarization, ie, drop in the voltage across the sensory cell membrane. The collection of biochemical events that are involved in this process is called transduction (15,16). Not all the chemical steps involved in transduction are known however, it is clear that different transduction mechanisms are involved in different taste quaUties different transduction mechanisms exist for the same chemical in different species (15). Thus the specificity of chemosensory processes, ie, taste and smell, to different chemicals is caused by differences in the sensory cell membrane, the transduction mechanisms, and the central nervous system (14). 

Flavor. The sensation produced by a material taken into the mouth, perceived principally by the senses of taste and smell, but also by the common chemical sense produced by pain, tactile, and temperature receptors in the mouth. 

Odor and Odorant. That which is smelled. Odor may refer to the odorant or to the sensation resulting from the stimulation of olfactory receptors in the nasal cavity by gaseous material. 

Synthesis of steroidal diazirines has led to some biologically active compounds (65JA2665). The observation that diazirines and their parent ketones are identical in smell (B-67MI50800), points to the possibility that a diazirine group may stand for a keto group vis-d-vis a receptor. 

G proteins are molecular amplifiers for a large number of seven-trans-membrane helix receptors that regulate responses like vision, smell and stress response. They are heterotrimeric molecules, Gap, that dissociate into membrane-bound Ga and Gpy signal transmitters upon activation of the receptor. 

The epithelium covering the nasal cavity. This epithelium contains numerous cell types including the specialized olfactory sensory neurons which detect the chemical stimuli derived from smells by a specific family of G protein-coupled receptors known as olfactory receptors. 

Taste and smell, unlike sight and sound, are not induced by any known waves in the air, but are caused by the presence of certain specific, chemical compounds by interaction with certain receptor-sites. The power of reaction to chemical stimuli has evolved from the diffused, and confused, chemotactic sensitivity of the primitive protozoa through to man, where specialized organs have been developed. 

In the foregoing we loosely talked about the intensity of a sensory attribute for a given sample, as if the assessors perceive a single (scalar) response. In reality, perception is a dynamic process, and a very complex one. For example, when a food product is taken in the mouth, the product disintegrates, emulsions are broken, flavours are released and transported from the mouth to the olfactory (smell) receptors in the nose. The measurement of these processes, analyzing and interpreting the results and, eventually, their control is of importance to the food... 

Buck L. (1993). Receptor diversity and spatial patterning in the mammalian olfactory system. In The Molecular Basis of Smell and Taste Transduction (Chadwick D., et al., eds). John Wiley, London, Ciba Symposium 179, pp. 51-67. 

Motion sickness is caused by stimulation of the vestibular system. This area contains many histaminic (Hj) and muscarinic cholinergic receptors. The higher brain (i.e., cerebral cortex) is affected by sensory input such as sights, smells, or emotions that can lead to vomiting. This area is involved in anticipatory nausea and vomiting associated with chemotherapy. 

Chemoreceptor A sensory receptor responding to a chemical stimulus (e.g., smell or taste) or change in the concentration of a chemical (e.g., H+ ions in the blood or PH). 

The fact that the enantiomers of a compound do not smell the same suggests that the receptor sites in the nose for these compounds are chiral, and only the correct enantiomer will fit its particular site (just as a hand requires a glove of the ocrrect chirality for a proper fit). 

The goal of chemical analysis of odorous compounds in air is to determine all substances, which interact with odour perception cells in our nose, both qualitatively and quantitatively. However, with a few exceptions all compounds with certain vapor pressure have an odour, meaning that their volatilized molecules react with the membrane of odour receptor cells. As will be shown, always hundreds of compounds are present in air this means that the analysis would be very complex. However as was said before, our sense of smell is selective for some products it is very sensitive for other compounds it is much less sensitive. 

The olfactory epithelium of mammals contains many types of olfactory neurons, each expressing a specific odorant receptor. Linda Buck has shown that an odorant can activate multiple distinct receptors and that a receptor can be activated by multiple odorants. Thus, there must exist a combinatorial mechanism for odor detection some sort of pattern recognition. The axons of olfactory neurons converge on glomeruli in the olfactory bulb. There, incoming signals are integrated and the sense of smell is created. 

Here is a bit of a complication there is a lot of individual variation in the sense of human olfaction. Not everything smells the same to everyone. This holds both for the intensity of the perceived smeU as well as for its quality pleasant, floral, skunky, sweaty, or no odor at all. Andreas Keller has recently demonstrated that some significant part of this individual variation in the sense of smell derives from genetic variation in human odorant genes. Specifically, two single nucleotide polymorphisms (SNPs), leading to two amino acid substitutions in an odorant receptor, have dramatic affects on the perception of the odor of androstenone, a steroid derived from testosterone. 

At the cellular level, the various types of receptor, transporter, enzyme and ion charmel are all chiral in form. Thus although the enantiomers of a drug may have identical physicochemical properties, the way in which they may interact with chiral targets at the level of the cell will give rise to different pharmacod)mamic and pharmacokinetic properties. A few simple examples will illustrate how taste and olfactory receptors can differentiate between enantiomers. Thus R-carvone tastes like spearmint whereas the S-isomer tastes like caraway. Similarly, R-limolene smells like lemon whereas the S-enantiomer tastes of orange. 

In fish, both taste and olfactory stimuli are waterborne. However, taste involves the seventh, ninth or tenth cranial nerves, in contrast to the first cranial nerve for smell. Elasmobranchs have their taste buds in the mouth and pharynx, but in bony fish they occur around the gills, on barbels and pectoral fins, and also scattered over the rest of the body surface. They crowd particularly in the roof of the mouth, forming the palatal organ. The taste receptor cells are arranged as a bundle to form a taste bud. Like other vertebrates, fish have receptors for sweet, sour, salty, and bitter. For instance, goldfish reject quinine-treated food pellets (Jobling, 1995). Many fish species are particularly sensitive to acidic taste characteristics. The responses of fish to amino acids will be discussed in Chapter 12. 

Smell receptors in the marine carnivorous Hawaiian goatfish, Parupeneus por-phyreus (Mullidae), located on the chin barbels, mediate both arousal and food searching in response to prey homogenate and rinse of intact live prey. The... 

The major constituent of caraway oil is (+)-carvone, and the typical caraway odour is mainly due to this component. On the other hand, the typical minty smell of spearmint oil is due to its major component, (—)-carvone. These enantiomers are unusual in having quite different smells, i.e. they interact with nasal receptors quite differently. The two enantiomeric forms are shown here in their half-chair conformations. 

In addition, physical stimuli such as light signals are registered and converted into intracellular signals by G-protein coupled receptors they are also involved in perception of taste and smell. 

FIGURE 1-23 Stereoisomers distinguishable by smell and taste in humans, (a) Two stereoisomers of carvone R) carvone (isolated from spearmint oil) has the characteristic fragrance of spearmint (S)-carvone (from caraway seed oil) smells like caraway, (b) Aspartame, the artificial sweetener sold under the trade name NutraSweet, is easily distinguishable by taste receptors from its bitter-tasting stereoisomer, although the two differ only in the configuration at one of the two chiral carbon atoms. 

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