Olfactory

Lancet D, Sadovsky E and Seidemann E 1993 Probability model for molecular recognition in biological receptor repertoires significance to the olfactory system Proc. Natl Acad. Sci. USA 90 3715-19... 

Odors are characterized by quaUty and intensity. Descriptive quaUties such as sour, sweet, pungent, fishy, and spicy are commonly used. Intensity is deterrnined by how much the concentration of the odoriferous substance exceeds its detection threshold (the concentration at which most people can detect an odor). Odor intensity is approximately proportional to the logarithm of the concentration. However, several factors affect the abiUty of an individual to detect an odor the sensitivity of a subject s olfactory system, the presence of other masking odors, and olfactory fatigue (ie, reduced olfactory sensitivity during continued exposure to the odorous substance). In addition, the average person s sensitivity to odor decreases with age. 

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). 

Jignosia. InabiUty to recognize sensations may be primarily in one sense, eg, olfactory agnosia. 

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. 

The TLV is set at 0.1 ppm (hydraziae) 0.2 ppm (MMH) and 0.5 ppm (UDMH). The TLV is weU below the olfactory limit of 3—5 ppm (hydraziae). The latter does aot provide adequate warning when exposure exceeds the TLV therefore, monitoring the working environment by suitable means and providing adequate ventilation is necessary. 

The effects of VIP and PACAP are mediated by three GPCR subtypes, VIP, VIP2, and PACAP receptor, coupled to the activation of adenjiate cyclase (54). The VIP subtype is localized ia the lung, Hver, and iatestiae, and the cortex, hippocampus, and olfactory bulb ia the CNS. The VIP2 receptor is most abundant ia the CNS, ia particular ia the thalamus, hippocampus, hypothalamus, and suprachiasmatic nucleus. PACAP receptors have a wide distribution ia the CNS with highest levels ia the olfactory bulb, the dentate gyms, and the cerebellum (84). The receptor is also present ia the pituitary. The VIP and PACAP receptors have been cloned. 

OT receptors are localized ia the brain hypothalamus, limbic system, cortex, striatum, olfactory system, and brain stem. In the periphery, OT is best known for its stimulation of uterine smooth muscle and the milk ejection reflex. Thr , Om ]oxytocin(l—8),... 

Although the nose houses and protects the cells that perceive odor, it does not direcdy participate in odor perception. The primary function of the nose is to direct a stream of air into the respiratory passages. While this function is occurring, a small fraction of the inhaled air passes over the olfactory epithelium, located 5—8 cm inside the nasal passages. This olfactory area occupies about 6.45 cm (one square inch) of surface in each side of the nose. 

The vomeronasal organ (VNO), located in the nose, is a small chemical sensing stmcture associated with odors and behavioral effects. The vomeronasal system, which is made up of the VNO and a portion of the brain s limbic system, is stmcturaHy independent of the olfactory and nervous terminalis systems in the nose. It may, however, interact with these systems in a manner dependent on prior experience or learning, and therefore be direcdy related to the association of smells and experiences. This independent chemosensory system in the nose may prove to open doors to new learning associated with the sense of smell and human behavior. 

According to the chemical theory of olfaction, the mechanism by which olfaction occurs is the emittance of particles by the odorous substances. These particles are conveyed to the olfactory epithelium by convection, diffusion, or both, and dkecdy or indkectly induce chemical changes in the olfactory receptors. 

Researchers at the MoneU Center (Philadelphia, Pennsylvania) are using a variety of electrophysical and biochemical techniques to characterize the ionic currents produced in taste and olfactory receptor cells by chemical stimuli. These studies are concerned with the identification and pharmacology of the active ion channels and mode of production. One of the techniques employed by the MoneU researchers is that of "patch clamp." This method aUows for the study of the electrical properties of smaU patches of the ceU membrane. The program at MoneU has determined that odors stimulate intraceUular enzymes to produce cycUc adenosine 3, 5 -monophosphate (cAMP). This production of cAMP promotes opening of the ion channel, aUowing cations to enter and excite the ceU. MoneU s future studies wiU focus on the connection of cAMP, and the production of the electrical response to the brain. The patch clamp technique also may be a method to study the specificity of receptor ceUs to different odors, as weU as the adaptation to prolonged stimulation (3). 

There are certain weU-estabUshed facts about olfaction (6). AU normal people can smell. People suffering from brain lesions, injured olfactory nerve, or obstmcted nasal passages may be anosmic. Cases of preferential anosmia, ie, abUity to sense certain smells and not others, are not weU estabUshed. Such cases occur, but Utde is known of them. 

Industry has standardized procedures for the quantitative sensory assessment of the perceived olfactory intensity of indoor malodors and their relationship to the deodorant efficacy of air freshener products. Synthetic malodors are used for these evaluation purposes. These malodors should be hedonicaHy associated to the "real" malodor, and must be readily available and of consistent odor quaUty. These malodors should be tested in various concentrations and be representative of intensities experienced under normal domestic conditions. 

Masking. Masking can be defined as the reduction of olfactory perception of a defined odor stimulus by means of presentation of another odorous substance without the physical removal or chemical alteration of the defined stimulus from the environment. Masking is therefore hyperadditive it raises the total odor level, possibly creating an overpowering sensation, and maybe defined as a reodorant, rather than a deodorant. Its end result can be explained by the simple equation of 1 + 1 = >2 (Fig. 2a). 

The future in research will certainly lead to a better understanding of how odors are recognized, sorted, and classified. Studies promise, among other things, to determine whether perceptually similar, but stmcturaHy different, odors share the same class of receptor proteins, whether responses to odors can be modified, and possibly why olfactory neurons regenerate but other neurons do not. 

Olfactory receptors have been a subject of great interest (9). Much that has been postulated was done by analogy to the sense of sight in which there are a limited number of receptor types and, as a consequence, only three primary colors. Thus attempts have been made to recognize primary odors that can combine to produce all of the odors that can be perceived. Evidence for this includes rough correlations of odors with chemical stmctural types and the existence in some individuals having specific anosmias. Cross-adaptation studies, in which exposure to one odorant temporarily reduces the perception of a chemically related one, also fit into this hypothetical framework. Implicit in this theory is the idea that there is a small number of well-defined odor receptors, so that eventually the shape and charge distribution of a specific receptor can be learned and the kinds of molecular stmctures for a specified odor can be deduced. 

Pseudohypoparathyroidism is characterized by end-organ resistance to parathyroid hormone (98,108). This disease takes various forms, including Albright s hereditary osteodystrophy, which has unusual physical features and a generalized resistance to G-protein-linked hormones that function through cAMP as a second messenger. This defect is associated with a deficiency in the levels of the a-subunit of (109). Because this defect may be generalized, such patients also have olfactory dysfunction (110). 

Many attempts have been made to reduce the ammoniacal and sulfurous odor of the standard thioglycolate formulations. As the cosmetics market is very sensitive to the presence of impurities, odor, and color, various treatments of purification have been claimed to improve the olfactory properties of thioglycolic acid and its salts, such as distillation (33), stabilization against the formation of H2S using active ingredients (34), extraction with solvents (35), active carbon (36), and chelate resin treatments (37). 

M. Devos, F. Patte, J. Rouault, P. Laffort, and L. J. Van Gemert, eds., Standardi edHuman Olfactory Thresholds, IRE Press, New York, 1990. 

The odor threshold of carbon disulfide is about 1 ppm in air but varies widely depending on individual sensitivity and purity of the carbon disulfide. However, using the sense of smell to detect excessive concentrations of carbon disulfide is unreHable because of the frequent co-presence of hydrogen sulfide that dulls the olfactory sense. 

The distinctive odor of trichloroethylene may not necessarily provide adequate warning of exposure, because it quickly desensitizes olfactory responses. EataUties have occurred when unprotected workers have entered unventilated areas with high vapor concentrations of trichloroethylene or other chlorinated solvents. Eor a complete description of proper entry to vessels containing any chlorinated solvent, see ASTM D4276-84, Standard Practice for Confined Area Entry (34). 

AHyl chloride has a disagreeable, pungent odor. The odor threshold has been estimated at approximately 3—6 ppm (51). Olfactory detection of odor is thus not an adequate warning of overexposure. 

Residual monomers exhibit a characteristic sharp odor even in subtoxic concentration, due to the very low olfactory threshold. Modem requirements in terms of environmental safeguard have led to significant improvements in the control of polymerization effluents, driving off gases, and residual monomer in the raw polymer. Consequendy, the acryUc elastomers of the 1990s are practically odor-free, and represent a significant improvement over the products of the past. 

The design of these devices is based on biological principles of organization of human olfactory and tasting abilities. 

Ammonia (aqueous) [7664-41-7] M 17.0 + H2O, d 0.90 (satd, 27% w/v, 14.3 N), pK 9.25. Obtained metal-free by saturating distilled water, in a cooling bath, with ammonia (from tank) gas. Alternatively, can use isothermal distn by placing a dish of cone aq ammonia and a dish of pure water in an empty desiccator and leaving for several days. AMMONIA (gas, liquid or aq soln) is very irritating and should not be inhaled in large volumes as it can lead to olfactory paralysis (temporary and partially permanent). 

Air pollution principally affects the respiratory, circulatory, and olfactory systems. The respiratory system is the principal route of entry for air pollutants, some of which may alter the function of the lungs. 

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