The olfactory epithelium

The supporting cells of the olfactory epithelium separate and partially wrap the ORNs. Their apical surface, in humans and some other vertebrates, is covered with microvilli, which project along with the olfactory cilia into the mucous layer. A third cell type, the microvillar cells, present at about one-tenth the number of the ORNs in 

Olfactory receptor cells and axons contain olfactory marker protein (OMP), which is unique to olfactory neurons (Fig. 3A). OMP is found in a number of mammalian species, including humans, this protein, whose function is unknown, is expressed in all mature ORNs and accounts for 1% of the total protein content of these cells. 

However, the solution of these fundamental problems will not bring a complete understanding of the olfactory code any more than the elucidation of retinal receptor transduction events has been able to clarify the neural mechanisms that underlie visual perception. The ORN is but the first element in a complex neural network. The operations of central olfactory networks are also poorly understood and the anatomical organization of the olfactory system appears in some respects to be fundamentally different from the familiar topographically organized circuits of the other major sensory systems. Thus, much remains to be discovered before we will approach the kind of understanding we currently have of visual, auditory and somatosensory neural network function. 

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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. 

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. 

Odors are perceived via the olfactory system, which is composed of two organs in the nose the olfactory epithelium, a very small area in the nasal system, and the trigeminal nerve endings, which are much more widely distributed in the nasal cavity (11). The olfactory epithelium is extremely sensitive, and humans often sniff to bring more odorant in contact with this area. The trigeminal nerves initiate protective reflexes, such as sneezing or interruption of irrhalation, with exposure to noxious odorants. 

Liberies SD, Buck LB (2006) A second class of chemosensory receptors in the olfactory epithelium. Nature 442 645-650... 

Hansen A., Zeiske E. and Reutter K. (1994). Microvillous and ciliated receptor cells in the olfactory epithelium of the Australian Lungfish, Neoceratodus forsteri. In Advances in Biosciences 93 (Apfelbach R., et al., eds.). Elsevier, Oxford, pp. 43-51. 

Kolnberger I. (1971). Comparative studies of the olfactory epithelium especially the Vomeronasal (Jacobson s) Organ in Amphibia, Reptiles and Mammals. Z Zellf Mikrosk Anat 122, 53-67. 

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. 

Nef P., Hermans-Borgmeyer I., Artieres-Pin H Beasley L., et al. (1992). Spatial pattern of receptor expression in the olfactory epithelium. Proc Natl Acad Sci 89, 8948-8952. 

Ressler K.J., Sullivan S.L. and Buck L. (1993). Zonal organisation of odorant receptor gene expression in the olfactory epithelium. Cell 73, 597-609. 

Suzuki Y., Takeda M. and Obara N. (1998). Colchicine-induced cell death and proliferation in the olfactory epithelium and vomeronasal organ of the mouse. Anat Embryol 198, 43-51. 

Taniguchi K., Toshima Y. and Saito T. (1996). Development of the olfactory epithelium and vomeronasal organ in the Japanese reddish frog. Rana japonica. J Vet Med Sci 58, 7-15. 

The first G-protein a subunit to be identified was Gs. The a subunit of Gs (as) is responsible for stimulating adenylate cyclase (hence, the subscript s ) and is ADP-ribosylated and activated by CTx. Gs has at least four molecular variants. Some evidence exists that as can also enhance the activity of cardiac L-type Ca2+ channels, independently of their phosphorylation by cAMP-stimu-lated protein kinase A. Golf is a cyclase-stimulating homolog in the olfactory epithelium, activated by the large family of olfactory receptors. 

FIGURE 50-1 A schematic diagram of the olfactory epithelium. The initial events in odor perception occur in the olfactory epithelium of the nasal cavity. Odorants interact with specific odorant receptors on the lumenal cilia of olfactory sensory neurons. The signals generated by the initial binding events are transmitted along olfactory neuron axons to the olfactory bulb of the brain. 

These results suggest that an initial organization of olfactory sensory information occurs in the olfactory epithelium, and that this organization is maintained in the patterns of signals transmitted to the olfactory bulb. 

Whereas the olfactory epithelium only detects airborne volatiles, Wysocki, Wellington and Beauchamp (1980) demonstrated that the VN organ detects involatile chemicals, which are actively pumped into it (vomeronasal pumping Meredith, Marques, O Connell and Stern, 1980). Although in vitro some volatiles are able... 

Pioneering efforts to understand the nature of olfactory coding were reported by Adrian (24-27). His work introduced the ideas that different odors activate ORCs in different regions of the olfactory epithelium and that spatiotemporal patterns of ORC firing would suffice to encode different odors. Subsequent studies by many investigators and involving various recording methods (reviewed in refs. 13 and 28) led to the conclusion that, at various levels of the pathway, the olfactory system uses distributed neural activity to encode information about olfactory stimuli. 

Different odor substances stimulate different patterns of ORCs in the olfactory epithelium, owing to the different sensitivity spectra of the ORCs (28). The pattern of activity in the epithelium evoked by a particular odor substance constitutes the first molecular image of that stimulus, which represents the determinants of the stimulating molecules (13). Thus, although olfaction is not a spatial sensory modality, in contrast, for example, to vision and somatosensation, the initial representation of an odor stimulus in the olfactory pathway does have spatial structure. 

After intratracheal instillation of nickel chloride or nickel sulphate in rats, a modest inflammatory response with increased number of macrophages and polynuclear leucocytes was obtained, together with increased activities of lactate dehydrogenase and -glucuronidase in bronchoalveolar fluid [351]. More severe lesions were characterized by type II cell hyperplasia with epithelialization of alveoli, and in some animals, fibroplasia of the pulmonary interstitium. By inhalation in rats, the nickel salts produced chronic inflammation and degeneration of the bronchiolar epithelium [352, 353]. There was also atrophy of the olfactory epithelium and hyperplasia of the bronchial and mediastinal lymph nodes. Nickel sulphate also produced a low incidence of emphysema and fibrosis [353]. 

Hurtt ME, Working PK, Morgan KT. 1987b. Degeneration and regeneration of the olfactory epithelium following inhalation exposure to methyl bromide [Abstract]. Toxicologist 7 195. 

Fischer- 344) (GO) Hepatic 34 F periosteal hypercellularity degeneration of the olfactory epithelium and superficial Bowman s glands) 400 F (mild centrolobular ... 

The sense of smell in humans is not limited to detection of those volatile molecules inhaled through the nose, termed orthonasal olfaction. Molecules released at the back of the mouth, particularly in the chewing of food, can make their way up through the nasopharynx to the olfactory epithelium, termed retronasal olfaction. This system is activated when air is exhaled. Orthonasal olfaction is used to detect the scent of flowers and perfumes, food aromas, the presence of skunks, and the like. Retronasal olfaction detects the volatile molecules released from food. It is retronasal olfaction that makes a major olfactory contribution to the taste of food. And it is retronasal olfaction that helped to elicit Proust s profound reaction to a madeleine dipped in tea. 

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. 

It is not always clear how pheromone signals are detected in mammals. Most vertebrates, mice for example, have a VNO in addition to the main olfactory system. The VNO has two separate families of olfactory receptors Vlr, 137 functional receptors in mice V2r, 60 functional receptors in mice. The genes for these are only distantly related to those for the main olfactory receptors, suggesting that these systems evolved independently. As a general rule, it is the VNO and not the olfactory epithelium that is responsible for detecting pheromone molecules. However, it has been demonstrated that mice whose VNO has been surgically removed can discriminate MHC-determined odor types. This finding clearly implicates the main olfactory system in the detection of pheromones. 

The hnman genome includes about 350 genes that encode receptors for odors. These are expressed in the olfactory epithelium. Some other species contain even more odorant genes rats have about 1200. 

In the olfactory epithelium, each olfactory neuron expresses a specific odorant receptor. Each may be activated by multiple odorants. 

Although pheromones can be considered as a special form of odorants (scents), their actions, effects and functions have similarities to those of hormones. They bind to a specific receptor which then activates an effector system, which initiates an action potential. They bind to specific sensory cells, the neurones, in the olfactory epithelium, which is located on the roof of the nasal cavities. The epithelium consists of three types of cells, basal, supporting and sensory cells (neurones). The neurones are bipolar, that is they possess a single dendrite, which extends from the cell body to the surface of the olfactory epithelium, and an axon that forms a synapse with a nerve that transfers information to the olfactory centre in the brain. The epithelium is covered with a thick layer of mucus, in which the pheromones dissolve. The mucus contains proteins that bind the pheromone(s) for delivery to the olfactory receptors and then to remove them once they have been detected. 

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