physiology of smell and applied aspects of smell

Physiology of smell. Applied aspects

1. PHYSIOLOGY OF SMELL BY SHAMA PHYSIOLOGY TUTOR

2. CONTENTS 1 2 3 4 OLFACTORY RECEPTORS OLFACTORY PATHWAYS PHYSIOLOGY OF OLFACTION APPLIED ASPECTS 5 INTRODUCTION

3. Click here to add content of the text，and briefly explain your point of view Click here to add content of the text，and briefly explain your point of view Smell (olfaction) and taste (gustation) are examples of visceral senses because of their close association with gastrointestinal function. Physiologically, they are related to each other as the flavor of food is a combination of its taste and smell. This is why food may taste “different” if one has a cold that depresses the sense of smell. The olfactory epithelium is a specialized portion of the nasal mucosa. It contains three types of neurons that are important for olfaction: olfactory sensory neurons, supporting cells, and basal stem cells The bipolar olfactory sensory neurons (also called olfactory receptor cells) are responsible for olfactory transduction. The axons of the olfactory sensory neurons ( ie , olfactory nerve) pass through the cribriform plate of the ethmoid bone to enter the olfactory bulbs. INTRODUCTION

4. Click here to add content of the text，and briefly explain your point of view Click here to add content of the text，and briefly explain your point of view The supporting cells secrete the mucus that provides the appropriate molecular and ionic environment for odor detection in the olfactory epithelium. Odor-producing molecules (odorants) dissolve in the mucus and bind to odorant receptors on the cilia of the olfactory sensory neurons. Odorant binding proteins in the mucus may facilitate the diffusion of odorants to and from the odorant receptor. Basal stem cells undergo mitosis to generate new olfactory sensory neurons as needed to replace those damaged by exposure to the environment; olfactory sensory neurons generally survive for only 1–2 months.

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6. OLFACTORY RECEPTORS The olfactory sense is highly developed in rabbit and dog which are called microsmatic but is much less in human, monkeys which are called microsmatic. sense arises from stimulation of receptors in the yellowish brown olfactory mucous membrane. it contains 10-20 million special sensory cells called receptor cells lie between supporting cell and their dendrites extend as naked processes called olfactory rods. supporting cells end in microvilli which secrete mucus. it contains of phospholipids, lecithin and their auto-oxidation products. it is necessary for the sense of smell. Click here to add content of the text，and briefly explain your point of view Click here to add content of the text，and briefly explain your point of view

7. The receptors cells lies between the supporting cells And their dendrites extend as naked processes called olfactory rods Which end in fine cilia. The supporting cells end in microvilli which secrete mucus. The Mucus is also secreted by Bowman’s gland located just under the basal lamina of the olfactory mucous membrane. It consists of Phospholipids, lecithin and their auto- oxidation products. It is Necessary for the sense of smell. These cells also provide physical support, Nourishment, and electrical stimulation for the olfactory receptors. OLFACTORY RECEPTORS Click here to add content of the text，and briefly explain your point of view Click here to add content of the text，and briefly explain your point of view

8. OLFACTORY PATHWAY The axons from receptor cells are fine unmyelinated fiberswhich pierce the cribriform plate of ethmoid bone and enter the olfactory bulb. within olfactory bulb the axons of olfactory with dendrites from mitral and tufted cells. the glomeruli also contains periglomerular short axon cells which are inhibitory neurons connecting one glomerulus to another and mediate lateral inhibition. each glomerulus receives impulses from 26000 receptors and passes this infomation through 24 mitral cells and 68 tufted cells. the arrangement is excellent for spatial summation. the axons of the mitral cells from the lateral olfactory stria and run to the olfactory cortex on the same side which lies between anterior perforated substance and the uncus. the olfactory cortex includes the part of the limbic system viz. anterior olfactory nucleus, piriform cortex, olfactory tubercle, amygdala and entorhinal cortex. the axons of tufted cells run in the medial olfactory stria and cross the midline in the anterior commissure to form synapses with granule cells in the opposite olfactory bulb. there is pt. to pt. representation of olfactory mucous membrane in the olfactory bulb.

9. OLFACTORY PATHWAY INHIBITORY PATHWAY: t here are efferent fibers in the olfactory striae and stimulation of these fibers can depress the electrical activity in the olfactory bulb. these effects mediate by granule cells releasing GABA that make reciprocal synaptic connections with dendrites of mitral cells.

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11. PHYSIOLOGY OF OLFACTION Enter title Odoriferous substance must come into contact with the receptors cells of the olfactory epithelium for sensing of smell. Deep breathing or act of sniffing more effectively brings substance to the olfactory mucous membrane by producing turbulence in the airflow in the nasal cavity. the odoriferous substance now combines with the receptors on the surface of cilia. the pattern of induced activity depends on physical & chemical properties of stimulant molecules and of the molecules of the receptive substance. when odour producing substances become attached to the olfactory mucous membrane there develops a generator potential which lasts 4-6 seconds. Action potentials are then set up in the olfactory receptors and are conducted along the axons to the olfactory bulb. the receptors in the olfactory mucous membrane are coupled to G-proteins. Some act via adenyl cyclase and cAMP and others act via phospholipase C. all of them cation channels causing Na+ and Ca++ influx.

12. Step 1: The odorant binds to a specific olfactory receptor protein in the cell membrane of a cilium of an olfactory receptor cell. Step 2: Receptor activation stimulates a heterotrimeric G protein called Golf Step 3: The α subunit of Golf in turn activates an adenylyl cyclase (specifically, ACIII), which produces cAMP. Step 4: The cAMP binds to a CNG cyclic nucleotide–gated cation channel Step 5: Opening of this channel increases permeability to Na+ , K+ , and Ca2+ . Step 6: The net inward current leads to membrane depolarization and increased [Ca2+ ] i . Step 7: The increased [Ca2+ ] i opens Ca2+ -activated Cl− channels called anoctamin2 Opening of these channels produces more depolarization because of the relatively high [Cl− ] i of olfactory receptor neurons. Step 8: If the receptor potential exceeds the threshold, it triggers action potentials in the soma that travel down the axon and into the brain. PHYSIOLOGY OF OLFACTION Enter title

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14. Humans can distinguish between 4000 and 10000 different odours . this is possible due to presence of different of odorant receptors and frequency of action potentials in the afferent nerve reaching the brain, Weber Fechner Law. localization of smell: direction from which a smell comes depend on the difference of time between the arrival of odoriferous molecules in the two nostrils. Role of higher centres : the olfactory cortex also involves the parts of the limbic cortex. anterior olfactory nucleus is concerned with coordination of input from contralateral olfactory cortex thus help in transfer of olfactory memories from one side to another. piriform cortex is concerned with olfactory discrimination and conscious perception. amygdala is concerned with emotional responses to olfactory stimuli. entorhinal cortex is concerned with olfactory memories.

15. individuals variation in response: human beings vary in their sensitivity to odoriferous substances. Relation to sexual activity: in women, sense of smell is more acute at the time of ovulation. Role of Pain fibers: Naked endings of trigeminal pain fibers found in the olfactory mucous membraneare stimulated by irritant substances such as menthol, chlorine, peppermint and ammonia. Adaptation: adaptation to smell is called olfactory fatigue

16. Olfactory fatigue : Adaptation to the smell of odoriferous substances is called olfactory fatigue . An odor which at first is quite clearly perceptible, gradually disappears and becomes imperceptible. It develops within seconds or minutes, depending on the nature of the substances. It s mediated by Ca2+. The degree of adaptation can be measured by the rise in threshold concentration required to excite the sense of smell. Adaptation also occurs to irritants which excite the common chemical sense. Mill workers adapt to the presence of irritants in the atmosphere and tolerate concentrations which can cause marked sneezing and watering of the eyes in unexposed individuals.

17. the nasal cavity contains another patch of olfactory epithelium located along the nasal septum in the vomeronasal organ that is concerned with the perception of pheromones . Vomeronasal sensory neurons project to the accessory olfactory bulb and from there to regions of the amygdala and hypothalamus that are concerned with reproduction and ingestive behavior. The vomeronasal organ has about 100 G-protein-coupled odorant receptors that differ in structure from those in the rest of the olfactory epithelium.

18. ANOSMIA: complete absence of sense of smell PAROSMIA: alteration in character of smell HYPOSMIA: reduction of sense of smell. develops due to damage of olfactory mucosa or the olfactory pathways by trauma or diseases. Patients with adrenal insufficiency have greatly increased sensitivity for smell. APPLIED ASPECTS

19. Quite often anosmia is a temporary condition due to sinus infection or a common cold, but it can be permanent if caused by nasal polyps or trauma. Antibiotics can be prescribed to reduce the inflammation caused by polyps and improve the ability to smell. In some cases, surgery is performed to remove the nasal polyps. Topical corticosteroids have also been shown to be effective in reversing the loss of smell due to nasal and sinus diseases Treatment

20. thank you