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Physiological Basis of Olfaction

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Physiological Basis of Olfaction REVIEW ARTICLE PHYSIOLOGICAL BASIS OF OLFACTION Namit Kant Singh1,*, PS Nagpure2 1Assistant Professor, 2Professor & Head, Department of Otolaryngology (ENT), MGIMS, Sevagram, Wardha, India *Corresponding author: Email: [email protected] ABSTRACT The olfactory system represents one of the oldest sensory modalities in the phylogenetic history of mammals. As a chemical sensor, the olfactory system detects food and influences social and sexual behavior. Within the nasal cavity, the turbinates or nasal conchae serve to direct the inspired air toward the olfactory epithelium in the upper posterior region. This area contains more than 100 million olfactory receptor cells. An understanding of the anatomical and physiology of Olfactory System has been given in this review. Key words: Olfaction, olfactory system INTRODUCTION SUPPORTING CELLS The capability to detect various Also called as the sustentacular cells volatile chemicals is one of the primary and function as metabolic and physical function of nose which helps to differentiate support for the olfactory cells. On between thousands of largely organic, low microscopy these are pseudo stratified molecular weight, volatile compounds (1). ciliated columnar epithelium and their nuclei is more apically located. MICROANATOMY BASAL CELLS The olfactory system is one of the oldest sensory modalities in the phylogenetic These cells lie on the basal lamina of history of mammals. the olfactory epithelium and are the stem cells which are capable of division and The olfactory epithelium consists of four differentiation into either supporting cells or distinct cell types. olfactory cells which in turn leads to the 1. Olfactory cells. olfactory epithelium being replaced every 2 2. Supporting cells to 3 weeks (2). The Basal cells can be divided 3. Basal cells on the basis of cellular anatomy histological 4. Brush cells markers into two populations: the horizontal basal cells which line the olfactory Olfactory Cells epithelium and the slightly more superficial globose basal cells (3). These are bipolar neurons which congregate to form the olfactory nerve. The Horizontal basal cells are now olfactory nerves go through the cribriform thought to be the primary stem cell plate and terminate on the dendrites of the population supplying new cells in this mitral cells located in the glomeruli of the system (4). olfactory bulb. The apical poles of these neurons are covered with non-motile cilia, BRUSH CELLS with the plasma membrane containing odorant-binding proteins acting as olfactory A microvilli-bearing columnar cell receptors. The incoming odorants are made with basal surface in contact with afferent soluble by the serous secretion from nerve endings, specialised for transduction Bowman's glands, located in the lamina of general sensation. Nerve fibres are propria of the mucosa. terminal branches of trigeminal nerve Indian Journal of Clinical Anatomy and Physiology, January – March, 2(1), 56-61 56 Singh & Nagpure Physiological Basis of Olfaction... (cranial nerve V), rather than the olfactory determined by airflow direction, velocity and nerve. volume. Normally about 15% of the inhaled air passes to this area which is increased on BOWMAN'S (OLFACTORY) GLANDS sniffing due to increase in turbulence. These are tubuloalveolar serous Olfactory receptors are G-protein secreting glands lying in the lamina propria coupled receptor (GPCR) superfamily and of the mucosa. These glands deliver a have a 7 transmembrane domain (6). These proteinaceous secretion via ducts onto the seven hydrophobic transmembrane domains surface of the mucosa. The role of the differ in their amino acid sequence in the secretions are to trap and dissolve odiferous transmembrane domain III, IV, and V which substances for the bipolar neurons. suggests that these parts are responsible for Constant flow from the Bowman's glands discrimination of odors (9). allows old odors to be constantly washed away (2). Signaling molecules and channels The Sense of Smell: 1. Cyclic adenosine monophosphate formation. The reception of molecules The binding of the odorant to the receptor changes the conformation of its In the 1st Century BC, Epicurean and plasma membrane receptors. The activated atomistic Roman philosopher Lucretius receptors catalyzes the exchange of speculated, different odors are attributed to guanosine 5-diphosphate (GDP) for different shapes and sizes of "atoms" that guanosine 5-triphosphate (GTP) on multiple stimulate the olfactory organ (5). In the trimeric G proteins causing the dissociation present era the theory was supported by the of Gαolf from Gβ and γ (10). Gαolf in cloning of olfactory receptor proteins and activates adenyl cyclase type III that subsequent pairing of odor molecules to converts adenosine triphosphate (ATP) into specific receptor proteins. Each odor Cyclic Adenosine Monophosphate (cAMP) receptor molecule recognizes only a (11, 12). particular molecular feature or class of odor molecules. Mammals have about a thousand The rise in ciliary cAMP opens a genes that code for odor reception. This cyclic neucleotide gated cation channel, theory was postulated by Linda B. Buck and allowing the influx of Na+ and Ca++, and Richard Axel (who were awarded the Nobel release of K+. This results in depolarization Prize in 2004) (6). Each olfactory receptor of the cell and the firing of action potentials neuron expresses only one functional odor along the axon to the glomureli, which are receptor (7). These odor receptor nerve cells globose structures located in the outer part work like a lock and key system i.e. if the of the olfactory bulb (13). This provides volatile molecule of a chemical can fit into amplification of odor binding events as the the lock then the nerve cell will respond. activation of one olfactory receptor activate Humans have far fewer active odor receptor multiple G- proteins and a subsequent genes than other primates and other cascade of cAMP production. mammals (8). 2. Cyclic Neuclotide gated channels: Olfactory Perception These channels play a significant role The perception of odors starts when in chemo electrical conversion in olfactory odors molecules activate olfactory receptors cilia. cAMP binds to the CNG channels in the present in the olfactory sensory neurons of OSN membrane which opens them and the the olfactory epithelium. Odour molecules cell becomes highly permeable to Ca2+ are transported across the mucus that which causes depolarization. CNG channels covers the olfactory epithelium. Regulation also allow Na+ to flow into the cell. The of the olfactory mucus occurs through increased Ca2+ concentration inside the cell adrenergic, cholinergic and peptidergic activates Ca2+-dependent chloride (Cl−) stimulation which may affect the degree of channels, which causes intracellular Cl− odor perception. The level of stimulation is ions to flow out of the cell augmenting the Indian Journal of Clinical Anatomy and Physiology, January – March, 2(1), 56-61 57 Singh & Nagpure Physiological Basis of Olfaction... depolarization event. This depolarization dependent protein kinase CaMK II (32). stimulates an action potential that Moreover, CaM desensitizes the cAMP-gated ultimately signals the reception of the channel to its ligand and thereby promotes odorant. In addition to cAMP gated ion channel closure and rapid adaptation.(33) channels, a small subset of OSNs also has The inhibitory effects of Ca-CaM restrict the cGMP-selective CNG channels (14). activity of the cAMP-gated channels to a short time period. The transient Ca influx 3. Calcium activated chloride channels: during this period causes a prolonged elevation of the ciliary Ca concentration and, The Ca2+-activated Cl– current was hence, the opening of multiple Cl channels, found to be part of the odorant-induced as there are eightfold more Cl channels in current in amphibian (15) and mammalian the ciliary membrane than cation channels. OSN (16). This finding provided an Consequently, the initial small Ca influx explanation for the observation that the triggers a much larger Cl efflux and olfactory response persisted even in the generates an excitatory receptor current absence of external Na+ (17, 18). Ca- with Cl as the main charge carrier and this activated Cl channels are present in the causes anion based signal amplification. ciliary membrane (15, 19). The increase in Hence calmodulin controls both the Ca2+ concentration inside the cilia activates excitatory and inhibitory processes and a Cl- current (15, 16, 19, 20) serves as a link between the two types of transduction channels (34). OSNs maintain an unusually high internal concentration of Cl that is in the Olfactory Transduction and adaptation. same range of the Cl concentration present in the mucus at the external side of the cilia The Olfactory (OLFR) gene family is (21, 22, 23, 24). In physiological conditions, one of the largest in the mammalian genome, the opening of Ca-activated Cl channels in comprising about 900 genes in human (~3% the ciliary membrane causes an efflux of Cl of the total genome). Buck and Axel has ions from the cilia, corresponding to an converged on a set of transduction inward current that further contributes to mechanisms, involving G-protein-coupled the depolarization of OSNs (15, 16, 19, 20) second-messenger systems, and neural processing mechanisms, involving modules During olfactory transduction, the called glomeruli, that appear to be adapted secondary Ca-activated Cl current plays the for the requirements of different species. important role
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