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.

1. SMELL & TASTE
PANDIAN M
DEPT OF PHYSIOLOGY
DYPMCKOP

2. 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?

3. TASTE AND SMELL

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 9
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

10. 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 cm2 in area. A mucous layer covers the entire epithelium.

11. HISTOLOGICAL STRUCTURE
• Histologically, the olfactory mucosa consists of three types
of cells
1. Receptor cells.
2. Supporting cells,
3. Basal cells

12. 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.

13. • 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.

14. • 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.

17. 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.

18. 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.

19. Diagram of the olfactory pathway.

20. • 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. 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.

23. 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.

24. 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.

25. 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,

26. • 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.

27. • 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.

28. SMELL IS THE ONLY SENSORY
INPUT NOT ROUTED THROUGH
THALAMUS

30. 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)

31. • 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 + /Ca2+ exchanger to restore ion balance also contributes to long-
term adaptation.

32. 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.

33. 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.

34. • 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.

35. FUNCTIONS OF THE VERY OLD OLFACTORY
SYSTEM—THE MEDIAL
OLFACTORY AREA.
•Primitive response to olfaction
•Licking the lips
•Salivation

36. 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.

37. • 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.

38. 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.

39. Diagram of the olfactory pathway.

40. 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 is 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.

41. 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.

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

43. 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.

44. SENSATION OF TASTE

45. Taste Buds
Figure 15.1

46. 46
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

47. ch 15 47
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

48. HOW TASTING WORKS
1. 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
2. Genes determine the kinds of taste receptors we have, and experiences
shape our perceptions
3. Taste disorders may be genetic, or may result from illness or injury
4. Taste preference: infants have heightened taste sensitivity while elders
have decreased ability to taste

49. 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

50. 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.

52. • 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)

53. ch 15 53
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

54. SENSE OF TASTE
• Mechanism of Stimulation
a. Receptor potential- substance causes the taste hair
to depolarize
1. For salty and sour, the receptor opens specific ion
channels
2. For sweet and bitter, a second messenger is
activated
b. Generation of nerve impulses by the taste bud

55. ch 15 55
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

56. ch 15 56
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

57. SENSE OF TASTE
• Transmission of Taste
• Signals into the CNS
Fig. 53.2

58. ch 15 58
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.

59. 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.

60. • 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.

61. 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.

62. 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
• NET SOURCE

63. THANK YOU . . .