Olfactory receptors Subject

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. 

Each human uses only about 350-400 different types of olfactory receptors and there is variation between individuals as to which 350-400 of the 1000 are active genes and which are P-genes (Lancet, 2003). Thus, the subjectivity of olfaction starts at the most basic level in that statistics indicate that each of us probably detects odours with a unique combination of receptors. An example of individual variation is given by Lasker and Teubner (1999). They investigated the ability of subjects to discriminate between stereoisomers in 10 pairs of enantiomers and found that the degree of such ability was time stable for each individual but that there were big differences between individuals. 

Figure 2. A schematic representation of the influences of intrinsic factors on spying. The top fish in each pair is the donor, shown releasing an array of hormonal products (A-E). The bottom fish in each pair is the receiver. Above the receiver, a horizontal dashed line indicates no olfactory receptor is present for a hormonal product, a filled circle indicates a receptor is present, and an empty circle indicates an olfactory receptor has been recently expressed. Spying evolves when a receiver comes to detect a hormonal product (now termed a pheromonal cue) released by a conspecific donor and benefits from doing so. Most likely, the process starts with development of sensitivity to a single cue but may rapidly expand to include a mixture of compounds, the phenomenon we term, cue elaboration. Pheromonal mixtures will be subject to modification by extrinsic factors. Also they may come to serve a communicatory function (and thus be susceptible to a different suite of pressures) if the pheromone donor comes to benefit directly from releasing the cue.

This is subject to physiological regulation and has a marked effect on the composition of the saliva. The flow is increased not only by direct stimulation of taste and olfactory receptors but also by other forms of oral stimulation, such as those experienced during dental treatment. The exact nature of the stimulus also affects the composition. 

Olfactory sensation results when olfactory receptors in the nose are stimulated by a particular substance in gaseous form called an odorant, which is capable of being translated into the subjective responses of neural brain stimulation that we term odour, smell, aroma, flavour or scent. Gustatory sensation that determines the taste is a sensation ehcited by substances acting on taste receptors in taste cells in the mouth. 

Recently, studies were reported measuring the kinetics of stimulation of both cAMP and 3 using a mixing device and rapid quenching, in the millisecond range, in rat olfactory cilia (67). The response to a mixture of < orants peaks within 25 to 50 milliseconds, the time frame expected for receptors, with both cAMP and IP3. Similar measurements of the change in concentration of cAMP or IP3 were also done in the taste cell. Here mice, which were bred as bitter tasters and nontasters , were used as subjects. The bitter stimuli, sucrose octaacetate, strychnine and denatonium benzoate, were shown to increase IP3 levels in a membrane preparation from "taster" mice in the presence of GTP-protein and Ca but not in membranes from "nontaster" mice (68). 

The mechanism of olfaction has many theories but is not fully understood and is still the subject of research. The nose is the human organ that detects smell (Fig. 5.9). It extends from the face to the end of the palate. In its simplest explanation the two nasal cavities are lined with a mucous membrane, kept moist by the secreted substance mucus. Chemicals in the air entering the nose must dissolve in this mucus before they can be detected. A small area - about the size of a small postage stamp - in the upper part of the nasal cavity contains olfactory cells, which are sensitive to the chemicals in the mucus solution. For a molecule to be detected it must bind specifically to the sensitive cells that act as sensory receptors. The sensory receptors situated in the olfactory epithelium (epithelium is the name given to the outer layer of covering cells) are believed to bind specifically with substances according to the shape of their molecules. 

For a terrestrial animal, flavor may be defined as the composite sensation resulting from placing something in the mouth. Therefore, flavor may Include taste, olfactory, vomeronasal, trigeminal and other chemical sense Inputs as well as tactile, temperature and proprioceptive cues. Thus, the subjective sensation we call flavor is the result of interactions of a complex of receptors. The bulk of experimental work in this field has focussed upon one class of receptors and associated CNS processes, the taste system. There are powerful arguments that the taste system is a uniquely important component in regulating flavor perception and food Intake (e.g.,, but other sensory components significantly influence flavor perception (e.g., ). 

Again Vinnikov (1 ) presents a review on this subject at the molecular level. Function of the olfactory organ is suggested as a simple diffusion of odorant molecules — "wafted around by air currents" — as they are volatile. Interaction with receptors in the olfactory region transmits an impulse to the central nervous system. 

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