Smell & Taste theory updated on 2021 BY PANDIAN M.

10.13 & 10.14 Describe and discuss perception of smell and taste sensation
At the end of the session, the first phase MBBS student should be able to
1] Describe the location, structure, and afferent pathways of taste receptors.
2]Describe the location, structure, and afferent pathways of smell receptors.
3]Name the basic taste sensations, identify the five distinct gustatory modalities.
4] describe the cells of a taste bud.
5] explain how taste receptors are activated and explain the mechanism of taste transduction for each taste quality.

6] explain how olfactory receptors are activated and explain the mechanism of olfactory transduction.
7] identify the three cranial nerves that transmit taste information to the cerebral cortex.

1. Theory part Smell & Taste Pandian M Dept. of Physiology DYPMCKOP

2. 10.13 & 10.14 Describe and discuss perception of smell and taste sensation At the end of the session, the first phase MBBS student should be able to   1 ] Describe the location, structure, and afferent pathways of taste receptors . 2 ] Describe the location, structure, and afferent pathways of smell receptors .   3 ] Name the basic taste sensations, identify the five distinct gustatory modalities .  

3. 4 ] describe the cells of a taste bud . 5 ] explain how taste receptors are activated and explain the mechanism of taste transduction for each taste quality . 6 ] explain how olfactory receptors are activated and explain the mechanism of olfactory transduction . 7 ] identify the three cranial nerves that transmit taste information to the cerebral cortex .

4. Describe and discuss patho - physiology of altered smell and taste sensation At the end of the session, the first phase MBBS student should be able to   1 ] Enumerate and discuss the abnormalities of Taste sensation 2 ] Name and discuss the abnormalities of Smell sensation 37) Examine the taste sensation of the subject from anterior 2/3 rd and posterior 1/3 rd of the tongue

5. Essential Questions How do smell and taste work? (Background info) What is the nature of energy transduction? What are some important anatomical structures involved in smell and taste? What causes loss of smell and taste? What are common smell and taste disorders? What are the four basic tastes? What are the differences between smell, taste, and flavor? What are the similarities between smell and taste?

6. Taste and Smell

7. O B J E C T I V E S After studying this chapter, you should be able to: Describe the basic features of the neural elements in the olfactory epithelium and olfactory bulb. Describe signal transduction in odorant receptors. Outline the pathway by which impulses generated in the olfactory epithelium reach the olfactory cortex. Describe the location and cellular composition of taste buds. Name the five major taste receptors and signal transduction mechanisms in these receptors. Outline the pathways by which impulses generated in taste receptors reach the insular cortex.

8. INTRODUCTION Smell ( olfaction ) and taste ( gustation ) are generally classified as visceral senses Because of their close association with gastrointestinal function. Physiologically, they are related to each other. The flavors of various foods are in large part a combination of their taste and smell . Consequently, food may taste “ different ” if one has a cold that depresses the sense of Smell . Both smell and taste receptors are chemoreceptors that are stimulated by molecules in solution in mucus in the nose and saliva in the mouth.

9. Cont …… Because stimuli arrive from an external source , they are also classified as exteroceptors . The sensations of smell and taste allow individuals to distinguish between estimates of up to 30 million compounds that are present in Food, predators, and mates and to convert the information received into appropriate behaviors.

10. THE SENSE OF SMELL The sense of smell or olfaction is well developed in animals like dog and rabbit to give warning of the environmental dangers. Such animals are called macrosmatics . In humans, apes and monkeys ( primates ), the sense of smell is comparatively less developed, but still it is important for pleasure and for enjoying the taste of food. Therefore, the humans And primates are called microsmatics .

11. SITE OF OLFACTION The olfactory stimuli are detected by the specialized receptors. Located on the free nerve endings of the olfactory nerves, which are located in the: Olfactory mucosa of nose in human beings and Vomeronasal organ in reptiles and certain mammals.

12. 12 Detecting Odour - continued Rats are 8 to 50 times more sensitive to odours than humans Dogs are 300 to 10,000 times more sensitive The difference lies in the number of receptors they each have Humans have 10 million and dogs have 1 billion olfactory receptors

13. Olfactory mucosa In humans, the olfactory mucosa is confined to upper one-third of nasal cavity. It includes the roof of nasal cavity and the adjoining areas on the medial wall (septum) and superior nasal concha on the lateral wall The olfactory neuroepithelium is a patch of thin and dull yellow mucosa about 5.0 cm 2 in area. A mucous layer covers the entire epithelium .

14. Histological structure Histologically, the olfactory mucosa consists of three types of cells 1. Receptor cells. 2. Supporting cells, 3. Basal cells

15. 1. Receptor cells. About 10–20 million receptor cells are present in the olfactory mucosa. These cells are bipolar neurons , Lie between the supporting ( sustentacular ) cells. The dendrites of the receptor cells terminate in a rod from which 10–20 fine cilia project This form a dense mat into the Mucous layer of the olfactory mucosa. The cilia are about 2 μm in length and 0.1 μm in diameter. Their axons are fine Unmyelinated fibres , which form the olfactory nerves.

16. Characteristic features of olfactory receptor cells, which differentiate it from other sensory neurons, are: These are the only sensory neurons whose cell bodies are closest to the external environment. These cells have a short life span of about 60 days and get replaced by the proliferation of basal cells . This natural Turnover is a unique feature of these sensory neurons. Note. Bone Morphogenic Protein (BMP) inhibits the renewal Turnover. BMP is a growth factor that promotes bone growth But also acts on other tissues.

17. 2. Supporting cells, also known as sustentacular cells , are Columnar in shape . Microvilli extend from the surface of these cells into the mucous layer covering the olfactory Mucosa. These cells secrete mucus . The bowman’s glands lying just under the basement membrane also secrete mucus . 3. Basal cells are stem cells from which new receptor cells are formed. As mentioned above, there is a continuous replacement of receptor cells by mitosis of the basal cells.

18. Distinguishing features of olfactory mucosa from the surrounding respiratory mucosa of nasal cavity are: Presence of receptor cells, Presence of bowman’s glands, Absence of rhythmic ciliary beating (which is a characteristic Feature of respiratory mucosa) and Presence of a distinctive yellow-brown pigment.

19. Nerve supply of olfactory mucosa Special sensory nerves innervating the olfactory mucosa are 15–20 bundles of olfactory nerve fibres (first cranial Nerve) which convey sense of smell. General sensory nerves supplying the olfactory mucosa are branches of trigeminal nerve (fifth cranial nerve). The irritative character of some odorants results from Stimulation of free nerve endings of the trigeminal nerve.

20. Diagram of the olfactory pathway.

21. Information is transmitted from the olfactory bulb by axons of mitral and tufted relay neurons in the lateral olfactory tract. Mitral cells project to five regions of the olfactory cortex: anterior olfactory nucleus, olfactory tubercle, piriform cortex, and parts of the amygdala and entorhinal Cortex. Tufted cells project to anterior olfactory nucleus and Olfactory tubercle; mitral cells in the accessory olfactory bulb project Only to the amygdala. Conscious discrimination of odor depends On the neocortex (orbitofrontal and frontal cortices). Emotive Aspects of olfaction derive from limbic projections (amygdala and Hypothalamus). (From kandel ER, schwartz JH, jessell TM [editors]: principles of Neural science , 4th ed. Mcgraw-hill , 2000.)

22. Transmission of smell signals to CNS

23. IMPORTANT NOTE Vapours of ammonia are never used to test the sense of smell as they stimulate the fibres of trigeminal nerve and cause irritation in the nose rather than stimulating the olfactory receptors.

24. CLASSIFICATION OF ODOR Odor is classified into various types. Substances producing different types of odor are: 1. Aromatic or resinous odor: camphor, lavender, Clove and bitter almonds 2. Ambrosial odor: musk 3. Burning odor: burning feathers, tobacco, roasted Coffee and meat 4. Ethereal odor: fruits, ethers and beeswax 5. Fragrant or balsamic odor: flowers and perfumes 6. Garlic odor: garlic, onion and sulfur 7. Goat odor: caproic acid and sweet cheese 8. Nauseating odor: decayed vegetables and feces 9. Repulsive odor: bed bug.

25. ODORANT RECEPTORS AND SIGNAL TRANSDUCTION Biologic question of how a simple sense organ such as the olfactory epithelium and its Brain representation, which apparently lacks a high degree of complexity, can mediate discrimination of more than 10,000 different odors. One part of the answer to this question is that there are many different odorant receptors .

26. Stimulation of Olfactory cells G- PROTEIN is stimulated -triggers activation of Adenyl cyclase (enzyme speeds up the conversion of ATP to cAMP – cAMP then binds to action channels in membrane of cilia- this causes channels to open and Ca ions to enter cilia – influx of Ca ions activates Cl channels to open and Cl leaves. Membrane becomes depolarized and AP is created. The action potential travels down the axon of olfactory receptor cell eventually meets with the other axons and forms the olfactory nerve (CN I)

27. Cont …… There are approximately 500 functional olfactory genes in Humans. Accounting for about 2% of the human genome. The Amino acid sequences of odorant receptors are very diverse, But all the odorant receptors are G protein coupled receptors ( Gpcr ). When an odorant molecule binds to its receptor, the G Protein subunits ( α, β, γ) dissociate the α- subunit. Activates adenylate cyclase to catalyze the production of cAMP , Which acts as a second messenger to open cation channels ,

28. Increasing the permeability to Na + , K + , and Ca 2+ the net effect is an inward-directed Ca 2+ current which produces the graded Receptor potential . This then opens Ca 2+ -activated Cl - channels, Further depolarizing the cell due to the high intracellular Cl – levels in olfactory sensory neurons. If the stimulus is sufficient for the receptor potential to exceed its threshold, an action potential in the olfactory nerve (first cranial nerve) is triggered.

29. A second part of the answer to the question of how 10,000 different Odors can be detected lies in the neural organization of the Olfactory pathway. Although there are millions of olfactory sensory Neurons, each expresses only one of the 500 olfactory genes. Each neuron projects to one or two glomeruli. This Provides a distinct two-dimensional map in the olfactory bulb that is unique to the odorant. The mitral cells with their glomeruli project to different parts of the olfactory cortex.

30. Smell is the only sensory input not routed through Thalamus

31. ADAPTATION – 30sec to 15 min when one is continuously exposed to even the most disagreeable odor, perception of the odor decreases and eventually ceases. This sometimes beneficent Phenomenon is due to the fairly rapid adaptation , or desensitization , that occurs in the olfactory system. Adaptation in the Olfactory system occurs in several stages. The first step may be Mediated by a calcium-binding protein (calcium/ calmodulin )

32. That binds to the receptor channel protein to lower its affinity for cyclic nucleotides . The next step is called short-term adaptation , which occurs in response to cAMP and implicates a feedback pathway involving Calcium/ calmodulin -dependent protein kinase II acting on adenylyl cyclase. The next step is called long-term adaptation , which includes activation of guanylate cyclase and cGMP production. A Na + /Ca 2+ exchanger to restore ion balance also contributes to long-term adaptation.

33. The very old, the less old, and the newer olfactory pathways into the central nervous system The olfactory tract enters the brain at the anterior junction between the mesencephalon and cerebrum ; There, the tract divides into two pathways, one Passing medially into the medial olfactory area of the brain Stem , and The other passing laterally into the lateral olfactory Area . The medial olfactory area represents a very old olfactory System , whereas the lateral olfactory area is the input to (1) a less old olfactory system and (2) a newer system.

34. The Very Old Olfactory System—The Medial Olfactory Area. located in the midbasal portions of the brain immediately anterior to the hypothalamus . Most conspicuous are the septal nuclei , that feed into the hypothalamus and other primitive portions of the brain’s limbic system .

35. Lateral olfactory areas on both sides of the brain are removed and only the medial system remains. Primitive responses to Olfaction, such as licking the lips, salivation, and other Feeding responses caused by the smell of food or by primitive Emotional drives associated with smell . Conversely , Removal of the lateral areas abolishes the more complicated Olfactory conditioned reflexes .

36. Functions of The Very Old Olfactory System—The Medial Olfactory Area. Primitive response to olfaction Licking the lips Salivation

37. The Less Old Olfactory System—The Lateral Olfactory Area. The lateral olfactory area is composed mainly of the prepyriform and pyriform cortex plus the cortical Portion of the amygdaloid nuclei . From these areas, Signal pathways pass into almost all portions of the limbic System, Especially into less primitive portions such as the Hippocampus, most important for learning to like or dislike certain foods depending on one’s experiences with them. Lateral Olfactory area and its many connections with the limbic behavioral system cause a person to develop an absolute aversion to foods that have caused nausea and vomiting .

38. An important feature of the lateral olfactory area is that many signal pathways from this area also feed directly Into an older part of the cerebral cortex called the paleocortex in the anteromedial portion of the temporal lobe . This is the only area of the entire cerebral cortex where Sensory signals pass directly to the cortex without passing first through the thalamus.

39. The Newer Pathway. A newer olfactory pathway that passes through the thalamus, It passing to the dorsomedial thalamic nucleus and then to the lateroposterior quadrant of the orbitofrontale cortex , has been found. On the basis of studies in monkeys, this newer system probably helps in the conscious analysis of odor .

40. Diagram of the olfactory pathway.

41. Role of Pain Fibers in the Nose Many trigeminal pain fibers are found in olfactory membrane They are stimulated by irritating substances Are responsible for initiating sneezing, lacrimation and other reflex responses.

42. APPLIED PHYSIOLOGY – ABNORMALITIES OF OLFACTORY SENSATION Anosmia Anosmia refers to total loss of sensation of smell, i.e. Inability to recognize or detect any odor. It may be temporary or permanent. Temporary anosmia i s due to obstruction of nose, which occurs during Common cold, nasal sinus and allergic conditions. Permanent anosmia occurs during lesion in olfactory Tract, meningitis and degenerative conditions such as Parkinson disease and alzheimer disease.

43. HYPOSMIA Hyposmia is the reduced ability to recognize and to Detect any odor. The odors can be detected only at Higher concentrations. It is the most common disorder of Smell. Hyposmia also may be temporary or permanent. It occurs due to same causes of anosmia.

44. Hyperosmia Hyperosmia is the increased or exaggerated olfactory Sensation. It is also called olfactory hyperesthesia. It Occurs in brain injury, epilepsy and neurotic conditions.

45. Applied Smell Anosmia : inability to detect odors at all. Hyposmia : reduced ability to detect odors. Hyperosmia : increased or exaggerated olfactory sensation. Also called olfactory hyperesthesia Dysosmia : disturbed sense of smell. Parosmia : tumor of olfactory cortex result abnormal smell sensation. In adrenocortical deficiency sensitivity of smell is ↑sed. Kallman’s syndrome: hypogonadism with partial or complete loss of sense of smell. People who experience smell disorders either have a loss in their ability to smell or changes in the way they perceive odors.

46. Sensation of Taste

47. Taste Buds Figure 15.1

48. 52 Tongue Papillae: Filiform - shaped like cones and located over entire surface Fungiform - shaped like mushrooms and found on sides and tip Foliate - series of folds on back and sides Circumvallate - shaped like flat mounds in a trench located at back circumvllate foliate filiform fungiform

49. ch 15 53 Structure of the Taste System - continued Taste buds are located in papallae except for filiform Tongue contains approximately 10,000 taste buds Each taste bud has taste cells with tips that extend into the taste pore Transduction occurs when chemicals contact the receptor sites on the tips

50. How tasting works Taste sensory cells (found in taste buds): odor and food molecules activate membrane receptors  taste signals go to the limbic system and cerebral cortex  patterns of nerve activity encode taste sensations  sensory processing allows us to interpret flavors Genes determine the kinds of taste receptors we have, and experiences shape our perceptions Taste disorders may be genetic, or may result from illness or injury Taste preference : infants have heightened taste sensitivity while elders have decreased ability to taste

51. 5. Sensory interaction : taste receptors easily damaged by alcohol, smoke, acids, or hot foods but gustatory receptors are frequently replaced 6. Supertasters are those ppl with taste buds for bitter flavors, experiences sense of taste with far greater intensity. Nontasters : person unable to taste the chemical phenylthiocarbamide 7. Sensations of flavor and aroma often work together, especially during eating

52. Four Basic Tastes The sense of taste (gustation) have been isolated in laboratory experiments to show these four qualities Sweet Sour Bitter Salty Recently researchers have found a fifth taste quality called umami that is associated with monosodium glutamate.

53. Receptor for sweet – GPCR ( G protein coupled receptor) Receptor for salt – ENaC ( epithelial sodium channel) Receptor for sour – same ENaC & HCN (hyperpolarization – activated cyclic nucleotide gate cation channel. For bitter - GPCR & sour subs activate phospholipase c through g ptn’s . Umami – mGLuR4 ( metabotropic glutamate receptor)

54. ch 15 59 Functions of Taste Sweetness is usually associated with substances that have nutritive value Bitter is usually associated with substances that are potentially harmful Salty taste indicates the presence of sodium However, there is not a perfect connection between tastes and function of substances

55. Sense of Taste Mechanism of Stimulation Receptor potential- substance causes the taste hair to depolarize For salty and sour, the receptor opens specific ion channels For sweet and bitter, a second messenger is activated b. Generation of nerve impulses by the taste bud

56. ch 15 61 Structure of the Taste System - continued Signals from taste cells travel along a set of pathways: Chorda tympani nerve from front and sides of tongue Glossopharyngeal nerve from back of tongue Vagus nerve from mouth and throat Superficial petronasal nerve from soft palate

57. ch 15 62 Structure of the Taste System - continued These pathways make connections in the nucleus of solitary tract in the spinal cord Then they travel to the thalamus Followed by areas in the frontal lobe: Insula Frontal opervulum cortex Orbital frontal cortex

58. Sense of Taste Transmission of Taste Signals into the CNS Fig. 53.2

59. GUSTATORY PATHWAY

60. Taste pathway Taste buds Tractus solitarius Nucleus tractus solitarius Medial lemniscus VPM nucleus of thalamus Inferolateral part of post central gyrus ( area 43)

61. Taste Signals in the Limbic system and cerebral cortex

62. ch 15 67 Neural Coding for Taste - continued Evidence exists for both specificity and distributed coding. Some researchers suggest that the neural system for taste may function like the visual system for color. Currently there is no agreed upon explanation for the neural system for taste.

63. Similarities/Connections between taste and smell The complicated process of smelling and tasting begins when molecules released by the substances around us stimulate special nerve cells in the nose, mouth, or throat. These cells transmit messages to the brain, where specific smells or tastes are identified. Both olfaction (smell) and gustation (taste) depend upon a dissolved sample of chemical compound fitting into a receptor cell, like a key fits into a lock.

64. Many flavors are recognized through the sense of smell. Taste and smell cells are the only cells in the nervous system that are replaced when they become old or damaged.

65. Common sensory disorders Phantom taste perception: most common, it’s a lingering, often unpleasant taste even though you have nothing in your mouth. Hypogeusia : reduced ability to taste sweet, sour, bitter, salty, and umami Ageusia : inability to detect any tastes (rare) Dysgeusia : a foul, salty, rancid, or metallic taste sensation will persist in the mouth. In adrenocortical deficiency sensitivity to taste is enhanced . Taste blindness : inherited as an autosomal recessive trait. Familial dysautonomia : this is rare congenital disorder even saturated solution of nacl , sucrose, urea . Most often, people are experiencing a loss of smell as opposed to a loss of taste.

66. Reference Text book of Medical Physiology Guyton & Hall, Ganong Human Physiology Vander Text book of Medical Physiology Indukurana . A.K.Jain , Sembu Principles of Anatomy and Physiology Totora Hutchinson Clinical Methods Net source

67. THANK YOU . . .