Olfaction system

Arrays were introduced in the mid-eighties as a method to counteract the cross-selectivity of gas sensors. Their use has since become a common practice in sensor applications [1], The great advantage of this technique is that once arrays are matched with proper multivariate data analysis, the use of non-selective sensors for practical applications becomes possible. Again in the eighties, Persaud and Dodds argued that such arrays has a very close connection with mammalian olfaction systems. This conjecture opened the way to the advent of electronic noses [2], a popular name for chemical sensor arrays used for qualitative analysis of complex samples. 

Keywords electronic nose principal component analysis pattern recognition chemical sensors sensor arrays olfaction system multivariate data analysis. 

Pheromones are powerful modulators of insect behavior. Since the isolation and identification of the first pheromone, (10E, 12Z)-hexadec-10,12-dien-l-ol, the sex attractant of the silk moth Bombyx mori, thousands of other insect pheromones have been identified. Our understanding of the sensory apparatus required for pheromone detection has also increased significantly. Coincidentally, B. mori was instrumental in many of these advances (see below). Volatile pheromones are detected by a specialized olfactory system localized on the antennae. The precise recognition of species-specific nuances in the structure and composition of pheromone components is essential for effective pheromone-based communication. The pheromone olfactory system of species studied so far exhibits remarkable selectivity towards the species-specific pheromone blend. Pheromones are emitted in low (fg-pg) quantities and are dispersed and greatly diluted in air plumes. Thus, pheromone olfaction systems are among the most sensitive chemosensory systems known. (Schneider et al., 1968). This chapter summarizes efforts (particularly over the past 10 years) to understand the molecular basis for the remarkable selectivity and sensitivity of the pheromone olfactory system in insects. The chapter will also outline efforts to design compounds that interfere with one or more of the early events in olfaction. 

To develop an artificial olfaction system and to build the pattern recognition and the odour regression model for this kind of application, we use a stepwise methodology. 

Insect Sex Pheromone Olfaction Systems Flux Detection and Mixtures... 

The amino acid sequences of OBPs show little homology between orders and no homology to analogous proteins found in vertebrate olfaction systems, implying convergent evolutionary processes. However, there is conservation of six cysteine residues in all insect OBPs,... 

The most important element of an artificial olfaction system is the ensemble of sensors translating the primary stimuli into a measurable signal, usually electric. 

The previously mentioned quantities are completely general, and their importance holds for any kind of sensor. For chemical sensors an additional parameter of great importance is the selectivity. The selectivity defines the capability of a sensor to be sensitive only to one quantity rejecting all the others. In case of physical sensors, the number of quantities is limited to a dozen and the selectivity can be achieved in many practical applications. For chemical sensors, it is important to consider that the number of chemical compounds is of millions and that the structural differences among them may be extremely subtle. With these conditions the selectivity of chemical sensor can be obtained only in very limited conditions. Lack of selectivity means that the sensor responds with comparable intensity to different species and with such a sensor it is not possible to deduce any reliable information about the chemical composition of the measured sample. Selectivity is a straightforward requisite for analytical systems where sensors and its related measurement technique are addressed to the detection of individual compounds. As mentioned in the previous section, selectivity is not found in olfactory receptors. As a consequence, artificial olfaction systems are not based on individual selective sensors, but on sensors whose selectivity can be oriented towards molecular families, or better, towards interaction mechanisms. Figure 22.5 shows a typical selectivity map related to an array of quartz microbalances (see next section) coated with different metalloporphyrins based on the same macrocycle (tetraphenyl-porphyrin) but with different metal atoms. Figure 22.5 depicts well the concept of combinatorial selectivity, namely each compounds is identified by a unique sensitivity pattern that makes possible the identification. 

The methodology and currently developed devices are inspired by the natural olfaction system, where odours are sampled, detected and analysed. In the natural olfaction system of mammalians, odours are sampled by aspiration and condnced into the nasal cavity where they are detected by a series of olfaction cells that provide inpnt signals to the brain. The brain stores the signals in its memory and provides a comparison when a new odour is detected. 

The central point in electronic noses is that the individual gas sensors are completely unspecific, as are the olfactory cells in the natural olfaction system. This means, not only that an individual gas sensor may provide exactly the same electrical signal in the presence of two different pure gases, but also for two very different odours, hence one individual gas sensor is not able to discriminate between two complex odours (except in very simple cases which are not of interest). For example, an individual sensor may provide the same signal for someone smoking very close to the sensor and for something burning nearby hence, the sensor cannot discriminate between both situations. 

The oscillation of membrane current or membrane potential is well-known to occur in biomembranes of neurons and heart cells, and a great number of experimental and theoretical studies on oscillations in biomembranes as well as artificial membranes [1,2] have been carried out from the viewpoint of their biological importance. The oscillation in the membrane system is also related to the sensing and signal transmission of taste and olfaction. Artificial oscillation systems with high sensitivity and selectivity have been pursued in order to develop new sensors [3-8]. 

The remaining reptiles are monosmic, i.e. they are MOS-dependent with no functional accessory system. They derive from a secondarily aquatic group of Mesozoic dinosaurs, whose survivals are now represented by Crocodiles, Alligators and Caimans (Howes, 1891 Saint Girons, 1976). In these, the loss of accessory olfaction may have been part of a pre-adaptive trend. Genomic comparisons with the avian OR repertoires could provide some clues on AOS history in their living relatives. 

The developmental processes which produce the AOS parallel to those of the MOS and, although they become separate systems as soon as the organisation of the forebrain begins, do not differ except in the details specific to each system. The differentiation of cell types within the organ, synapse formation in the AOB and the establishment of more central tertiary connections occur in a similar but not identical sequence. In general, the differences amount to alterations in the timing of ontogenetic events, with primary olfaction usually in advance of the... 

Pfaff D.W., ed. (1985). Taste, Olfaction, and the Central Nervous System. Rockefeller University Press, New York, p. 346. 

Nef S., Allaman I., Fiumelli H., De Castro E., et al. (1996). Olfaction in birds differential embryonic expression of 9 putative OR genes in avian olfactory system. Mech Dev 55, 65-77. 

Pfeiffer C. and Johnston R.E. (1994). Hormonal and behavioral responses of male hamsters to females and female odors roles of olfaction, the vomeronasal system, and sexual experience. Physiol Behav 55, 129-138. 

Sanchez Criado J.E. (1982). Involvement of the vomeronasal system in the reproductive physiology of the rat. In Olfaction and Endocrine Regulation (Breipohl W., ed.). IRL Press, London, pp. 209-222. 

Tucker D. (1971). Non-olfactory responses from nasal cavity Jacobson s Organ and trigeminal system. In Handbook of Sensory Physiology Chemical Senses, 1. Olfaction (Biedler L., ed.). Springer, Berlin, pp. 151-181. 

Using the principles of biological olfaction, electronic nose systems contain arrays of different types of cross-reactive vapor-sensitive sensors. While it is difficult to discriminate analytes entirely by their responses to a single type of sensor, using an array of sensors yields response patterns that can readily distinguish many different vapors. Ideally, the response mechanisms of the sensors are highly varied and encompass both physical and chemical phenomena1. 

After considering the evolutionary origins of the olfactory system and some basic principles of olfaction, this brief review examines one of the most extensively studied examples of neural processing of semiochemical information the sex pheromone-specific olfactory subsystem in male moths. This male-specific subsystem can be viewed as representing an exaggeration of organizational principles and functional mechanisms that are characteristic of olfactory systems in general. 

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