**3. Olfactory receptor cells and transduction**

Olfactory receptor cells have a distinct dendritic process that extends to the surface of the epithelium where its tip is expanded into a club-shaped prominence, the olfactory vesicle. This bears cilia, which have the typical 9 + 2 microtubule arrangement for some of their length, but there is a long distal portion, which contains only the two central microtubule fibers. In contrast to cilia in the respiratory epithelium, the olfactory cilia (5–20) are almost immotile, and they are inserted into basal bodies in the olfactory vesicle. It is in the cilia of olfactory receptor cells where olfactory transduction takes place, that is, the conversion of an odor signal into an electrical signal. Odorant molecules bind to olfactory receptor proteins and trigger a signaling cascade that involves G-proteins and leads to the generation of action potentials (nerve impulses). These nerve impulses are sent to the brain, specifically, to the olfactory bulb.

**5**

*Introductory Chapter: Dysfunction of the Olfactory System and Nasal Disorders*

All olfactory receptor proteins are part of a family of G-protein-coupled receptors that are expressed in the olfactory epithelium [11–13]. There are many different odor receptor proteins and genes that encode them, with more than 1000 in the mammalian genome. Not all of these potential odor receptor genes are expressed and functional. The olfactory receptor multigene family consists of around 400 genes in humans and 1400 genes in mice [14–17]*.* According to an analysis of data derived from the Human Genome Project, humans have approximately 400 functional genes coding for olfactory receptors, and about 600 candidates are pseudogenes [14]. It implies that a human nose has around 400 types of scent receptors or ~ 400 different functional olfactory receptors. This is a large number since the entire human genome has only ~20,000–25,000 genes. It implies that ~2% of our genes are coding for olfactory receptors. Each olfactory receptor cell in the olfactory epithelium expresses only one of these 1000 olfactory receptor genes [18]. The expression of olfactory receptor genes is confined to four different zones of the olfactory epithelium [19–21]. Olfactory receptor cells that express the same olfactory receptor are found in only one of the four zones and they project to the same glomerulus in the olfactory bulb. The cells are randomly distributed in a given zone. Olfactory receptor cells respond to several different odor-causing chemicals. Each receptor cell type can respond to more than one odorant. A given odorant can activate one or more receptor cells. Different types of receptor cells can respond not only to the same but also to different odorants. When an odorant ligand binds to a receptor protein, these proteins initiate a G-protein mechanism, which uses cyclic AMP (cAMP) or inositol triphosphate (IP3) as a second messenger [22]. The intracellular messengers open sodium and calcium channels, which results in depolarization of the receptor membrane that then triggers an action potential.

The olfactory pathway starts with olfactory receptor neurons in the olfactory epithelium that send their axons to the ipsilateral olfactory bulb. There, they make synaptic contacts with central neurons in spherical structures, the olfactory glomeruli (2000 per bulb in the mouse). In the olfactory bulb, sensory information is processed in olfactory glomeruli. Each glomerulus is a discrete anatomical and functional unit and serves as an anatomical address dedicated to collecting and processing of specific molecular features about the olfactory environment conveyed to it by olfactory receptor cell axons expressing specific olfactory receptor proteins [23–25]. Thus, the glomeruli in the olfactory bulbs are organized chemotopically [26, 27], analogous to visuotopy, in visual systems, and tonotopy, in auditory systems. Olfactory information is extensively processed at the level of the glomeruli through feedforward and feedback inhibition and modulation provided by centrifugal neurons. Information is subsequently conveyed to higher-order olfactory center such as the olfactory cortex in vertebrates. Olfactory receptor cells in the olfactory epithelium that have the same type of olfactory receptor, that is, they express the same olfactory receptor gene (1 of ~1000), send their axons to the same glomerulus (1 of 2000) in the olfactory bulb. This is an example of sensory axons converging on one glomerulus in the brain. In the olfactory bulb, olfactory receptor cell axons synapse on mitral/tufted cells. Glomerular mitral/tufted cells process odor signals coming from the nasal olfactory epithelium. The central neurons in the olfactory bulb, such as the mitral and tufted cells, project to higher olfactory centers. Twenty to 50 neurons output neurons (mitral/tufted cells), innervate each glomerulus, and project out of the olfactory bulb. Mitral cells that innervate different glomeruli typically respond to different types of odorants. A given odorant can activate mitral cells in several or many glomeruli. Odorant identity

*DOI: http://dx.doi.org/10.5772/intechopen.93596*

**4. The olfactory pathway**

#### *Introductory Chapter: Dysfunction of the Olfactory System and Nasal Disorders DOI: http://dx.doi.org/10.5772/intechopen.93596*

All olfactory receptor proteins are part of a family of G-protein-coupled receptors that are expressed in the olfactory epithelium [11–13]. There are many different odor receptor proteins and genes that encode them, with more than 1000 in the mammalian genome. Not all of these potential odor receptor genes are expressed and functional. The olfactory receptor multigene family consists of around 400 genes in humans and 1400 genes in mice [14–17]*.* According to an analysis of data derived from the Human Genome Project, humans have approximately 400 functional genes coding for olfactory receptors, and about 600 candidates are pseudogenes [14]. It implies that a human nose has around 400 types of scent receptors or ~ 400 different functional olfactory receptors. This is a large number since the entire human genome has only ~20,000–25,000 genes. It implies that ~2% of our genes are coding for olfactory receptors. Each olfactory receptor cell in the olfactory epithelium expresses only one of these 1000 olfactory receptor genes [18]. The expression of olfactory receptor genes is confined to four different zones of the olfactory epithelium [19–21]. Olfactory receptor cells that express the same olfactory receptor are found in only one of the four zones and they project to the same glomerulus in the olfactory bulb. The cells are randomly distributed in a given zone.

Olfactory receptor cells respond to several different odor-causing chemicals. Each receptor cell type can respond to more than one odorant. A given odorant can activate one or more receptor cells. Different types of receptor cells can respond not only to the same but also to different odorants. When an odorant ligand binds to a receptor protein, these proteins initiate a G-protein mechanism, which uses cyclic AMP (cAMP) or inositol triphosphate (IP3) as a second messenger [22]. The intracellular messengers open sodium and calcium channels, which results in depolarization of the receptor membrane that then triggers an action potential.
