Smell odor olfaction

For that matter, in my search through Bartlett s and other quotation encyclopedias for a suitable epigraph to this Chapter 5, looking under smell, odor, olfaction, perfume, and the like, I found very few quotations at all, and those I did find were really about something else (e.g., "A rose by any other name would smell as sweet"). Somewhat spitefully, I have made do with Bienfang. For the reader s delectation, I have appended a brief selection from my own more extensive catalogue of specifically olfactory quotations. 

The sense of smell, or olfaction, is remarkable in its specificity—it can, for example, discern stereoisomers of small organic compounds as distinct aromas. The 7TM receptors that detect these odorants operate in conjunction with a G protein that activates a cAMP cascade resulting in the opening of an ion channel and the generation of a nerve impulse. An outstanding feature of the olfactory system is its ability to detect a vast array of odorants. Each olfactory neuron expresses only one type of receptor and connects to a particular region of the olfactory bulb. Odors are decoded by a combinatorial mechanism—each odorant activates a number of receptors, each to a different extent, and most receptors are activated by more than one odorant. 

In 1986, the National Geographic Society, in cooperation with the MoneU Center, conducted a worldwide survey of the sense of smell. Over 10 million survey forms were sent to readers of the Society s journal, of which close to 1.5 million forms were completed and returned. With responses to 40 demographic and 42 odor-related questions, the results constitute the largest set of data on human olfaction (4). 

Odor travels downwind. Many animals have a keener sense of olfaction than humans. Insects have such extraordinary keenness of smell that it may be a different modaUty of the chemical sense from that known to humans. 

Odors play a much greater role in human behavior than previously thought. The sense of smell provides a direct link with the function of the brain therefore, the further study of olfaction can only advance the learning of causes and effects of stimuli to the brain. 

Moulton D., Celebi G. and Fink R. (1970). Olfaction in Mammals — two aspects proliferation of cells in the olfactory epithelium and sensitivity to odors. In Taste and Smell in Vertebrates (Wolstenholme G. and Knight J., eds.). Ciba, London, pp. 227-250. 

Beyond that, the sense of olfaction does not depend on the concentration of the odorant concentration invariance. If you are exposed to jasmine at very low concentration, it smells like jasmine if the concentration is significantly raised, it still smells like jasmine. Perhaps more to the point is the concentration invariance of complex aromas such as that of coffee. The brain forms a single perception from complex inputs, regardless of the intensity of the signal. Olfaction has this property in common with taste. 

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. 

The gravitational field affects olfaction as it does vision, audition, or vestibular function. It could impair detection of dangerous fumes or burning electrical equipment in airplanes or space vehicles. Astronauts also report altered perception of food flavors under weightless conditions. Men and women tested with four scratch-and-sniff odor samples of the University of Pennsylvania Smell Identification Test identified odors more poorly when in an upside-down position (Mester eta/., 1988). 

An interaction between main olfaction and the trigeminal somatosensory system has been proposed to facilitate directional smelling. For carbon dioxide and menthol, human subjects were able to tell the odor direction in 96% of cases, while the results for hydrogen sulfide and vanillin were random (Kobal et ah, 1989). Rats can discriminate odor direction in one sniff (Rajan et ah, 2006). 

The sense of smell is called olfaction. In order to have a scent a compound must be volatile and exert a sufficient vapour pressure at room temperature. A typical scent molecule, called an odorant, usually has a molecular weight of less than 300. This includes the majority of molecules found in essential oils. 

Vertebrates possess three primary chemosensory systems gustation ( taste ), trigeminal, and olfaction ( smell ) but only one of these, the olfactory system, mediates responses to pheromones. Chemicals that stimulate the olfactory system are known as odorants and comprise one type of biological cue (any entity that stimulates a sensory system). Bouquets of odorants that can be discriminated as specific entities are termed odors. The olfactory system contains olfactory receptor neurons (ORNs) that comprise cranial nerve I and project directly to the forebrain. ORNs are now known to express only one to a few olfactory receptor proteins ( receptors ), which means that the chemoreceptive range of each neuron can be very narrow. The olfactory system also has several subcomponents including the vomeronasal organ, which is described below. 

Clinically, AD patients consistently show deficits in odor identification compared to controls (Doty et al., 1991). These deficits have been shown to be a true decline in odor identification ability that cannot be explained by lexical difficulty in interpreting written words in the multiple choice test formats. The University of Pennsylvania has developed a smell identification test (UPSIT), with ranges 0-40, which is widely used in clinical settings. Recent studies have shown that odor identification deficits predict conversion fi-om normal to MCI, particularly decline of verbal memory (Wilson et al., 2007). Other series (Devanand et al., 2008a) of studies have demonstrated that olfaction has a strong predictive power of MCI to AD in both types of MCI. Many other biomarkers have been used to predict AD... 

Anosmia, the inability to smell, can be divided into two classes. General anosmia, the inability to smell any odors at all, usually is the result of disease or accident. More common is specific anosmia, in which an individual either cannot detect a specific chemical substance that most people can detect or displays a threshold of detection for it which is significantly above the normal range. At one time, specific anosmias were finked to the concept of primary odors (13), but confirmation of the combinatorial mechanism of olfaction has put paid to this concept. Interestingly, it has been demonstrated that exposure to the substance can affect anosmia and individuals can begin to smell materials to which they were previously anosmic. This effect has been demonstrated for androstenone, amyl acetate, geranyl nitrile, and isoborneol (14—18). 

The inability to taste food is a common complaint when nasal congestion reduces the sense of smell. Thus, smell greatly augments our sense of taste (also known as gustation), and taste is, in many ways, the sister sense to olfaction. Nevertheless, the two senses differ from each other in several important ways. First, we are able to sense several classes of compounds by taste that we are unable to detect by smell salt and sugar have very little odor, yet they are primary stimuli of the gustatory system. Second, whereas we are able to discriminate thousands of odorants, discrimination by taste is much more modest. Five primary tastes are perceived bitter, sweet, sour, salty, and umami (the taste of glutamate from the Japanese word for "deliciousness"). These five tastes serve to classify compounds into potentially... 

Olfaction, the sense of smell, is an important neural system in various animal species, including fish, for their life. Fish can detect a variety of odorants emitted from objects and dissolved in the water, such as amino acids, bile salts, nucleotides, polyamines, prostaglandins, and steroids. The fish olfactory system is extensively developed to receive and discriminate these odorant molecules, to transmit their signals to the brain, and to mediate fundamental behaviors such as food finding, alarm response, predator avoidance, social communication, reproductive activity, and spawning migration (Sorensen and Caprio 1998 Zielinski and Hara 2007). 

Surely the science of chemistry will some day reconcile the sense of smell with a reductionist approach in terms of pure compounds. In the interim, this chapter seeks to raise two questions related to the one posed in the opening paragraph what can another organism detect by means of olfaction How might an odor be archived so as to convey it to posterity These questions address the issue of reproducibility of sensation. Unless we take the position that all sensory experiences are unique, some criterion must be advanced to assess whether two stimuli are perceived as similar. Ideally, this criterion should apply to other air-breathing vertebrates as well as ourselves, par-... 

This list embraces much of what I believe to be true of olfaction in human beings, and is put forth as a set of necessary conditions for the analogous chemical sense in other vertebrate species. Given the range of biological diversity, it probably does not represent a set of conditions sufficient to define olfaction. In discussing these four guidelines I shall make reference not only to my own perceptions of odors, but also to published reports about people who display peculiarities in their sense of smell. Some are anosmic (i.e., "odor blind"), while at least one may be said to be "odor deaf."... 

H. M. clearly understands how to discriminate chemical stimuli. His impairment limits only his recognition of odors, but no other aspect of his sense of smell. He is "odor deaf," by analogy to stroke victims who can read and write and retain an intact sense of hearing but cannot recognize words aurally (and are said to be "word deaf ) (Takahashi et al., 1992). Based on the evidence discussed so far, H. M. s behavior does not satisfy the fourth of the guidelines for olfaction. 

If the odors of specific objects translate into unitary percepts, which constitute the basic entities in linguistic descriptions of olfaction, then the question follows as to whether these unitary percepts take shape at the level of the receptor neurons or in the olfactory bulb or elsewhere in the brain. That question remains unanswered, as of this writing. Because the sense of smell does not correlate perfectly with externally monitored patterns of electrical response from the receptor neurons or the olfactory bulb, the nature of olfactory coding remains unknown. Outside the laboratory unitary percepts rarely equate to pure compounds. Two vocabularies coexist, one of smells (which varies from individual to individual, and which refers to other inputs besides olfaction) and the other of chemical structures. 

To these species the structure of air currents is not invisible. A considerable portion of their brains has evolved to process information from their whiskers (or vibrissae), and one may plausibly suggest that their olfactory sensitivity derives, at least in part, from the ability to monitor the structure of air and to situate a scent within the currents that eddy about their snouts (Cain, et al., 1985). If, even in the absence of vibrissae, human olfaction can, nevertheless, sense the heterogeneity of odors, this adds another complexity to our experience of smell. As argued above, diffusion through mucus limits the temporal resolution of olfaction to about 0.1 second. If humans have the capacity... 

The receptors used to detect odorants are similar to those used to detect hormones and, indeed, to those used in vision. Therefore, any increase in our knowledge of smell and taste receptors could also benefit our understanding in other fields, even though this is more advanced than our understanding of olfaction at present. Apart from the academic interest, there are obvious commercial reasons for increasing our understanding. For example, it is of particular interest... 

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