The chemical senses of olfaction and gustation are involved in detecting chemicals in the environment and food. Odor and taste receptors generate signals upon binding chemicals and signal the presence of nutrients to seek or toxins to avoid. The olfactory system involves odorant receptors in the nose binding smells and sending signals through the olfactory bulb and tract to areas of the brain involved in perception and behavior. Olfaction provides important functions but can also exhibit abnormalities in conditions affecting the olfactory system.

1. CHEMICAL SENSES
Olfaction & Gustation
Dr. Misbah-ul-Qamar

2.  The chemical senses provide a “quality-control”
checkpoint for substances available for ingestion
 These are also classified as visceral senses because
of their close association with gastrointestinal function
(flow of digestive juices, effect on appetite)
Dr. Misbah-ul-Qamar

3.  Receptors for both smell & taste are chemoreceptors (
generate neural signals on binding with particular chemicals)
 Also classified as exteroceptors because stimuli arrive from
an external source
 Stimulation of these receptors induces pleasureable or
objectionable sensations & signals the presence of:
 Something to seek (nutritionally useful, good tasting food)
 Something to avoid (a potentially toxic, bad tasting substance)
Dr. Misbah-ul-Qamar

4. Development of chemical
senses
Dr. Misbah-ul-Qamar

5. Olfaction
Poorly developed in humans.
Human olfaction is capable of
distinguishing between roughly 10,000
unique odors.
Sense of smell can trigger memory as
the smell analyzing region of brain is
closely connected to amygdala &
hippocampus that handle memory &
emotion.
Dr. Misbah-ul-Qamar

6. Importance of olfaction
 It is important for the enjoyment & selection of food
 Flavours are a combination of taste & smell (smell
contribution is about 80%)
 Gives warning of harmful substances or places
 In lower animals, smell also plays a major role in
 Finding direction (seeking prey or avoiding predators)
 Sexual attraction to a mate
Dr. Misbah-ul-Qamar

7. To be smelled
A substance must be:
1. Sufficiently volatile (easily vaporized) for entry in nose
with inspired air
2. Sufficiently water soluble to be dissolved in mucus
3. atleast slightly lipid soluble . Lipid constituents of
cilium itself are a weak barrier to non lipid soluble
odourants
Dr. Misbah-ul-Qamar

8. OLFACTORY MEMBRANE/
MUCOSA
 Location: Located in the upper part of the nasal cavity.
 Area: 2.4-3 sq.cm.
 Cell population: Inhabited by 3 CELL TYPES
 Olfactory receptor cells– replaced every 2 months,decline
with age; approximately 1% of these sensory receptor
cells are not replaced each year
 Supporting cells—secrete mucous
 Basal cells– precursors for new receptor cells.
 Mucous is present on top of membrane.
Dr. Misbah-ul-Qamar

9. Reception for Olfaction
 Olfactory receptor cells are bipolar neurons derived from CNS.
 About 100 million of 1000 different types in each individual. A given
receptor can respond a particular odor component.
 Although there are millions of olfactory sensory neurons, each
expresses only one of 500 olfactory genes
 Receptor cells are interspersed by much smaller number of
sustentacular cells
 Basal cells along the basement membrane of the olfactory
epithelium regularly divide & yeild differentiated cells that replace
lost neurons.
Dr. Misbah-ul-Qamar

10. Olfactory Receptor Cell
 It is a bipolar neuron
 Apical surface of receptor cell exhibits a knob that
emits 4-25 olfactory hair or cilia.
 Cilia are nonmyelinated with a length of 2μ & a
diameter of 0.1μ.
 Cilia contain the receptors which provide binding sites
and project into the mucus.
 Axons of olfactory receptor cells collectively form the
olfactory nerve.
Dr. Misbah-ul-Qamar

11. Glands of Bowman
 Spaced among the receptor cells.
 Secrete mucous onto the epithelial surface of olfactory
membrane.
 Mucus contains some proteins which increase the
actions of odoriferous substances on receptor cells
Dr. Misbah-ul-Qamar

12. Olfactory Bulb
 It is a neural structure of forebrain involved in olfaction.
 It lies over the cribriform plate of the ethmoid bone that
separates the cranial and nasal cavities.
Dr. Misbah-ul-Qamar

13. Olfactory Bulb
Dr. Misbah-ul-Qamar

14. Olfactory receptor protein
 This protein is located in the membrane of each
olfactory cilium
 Each receptor protein is a long molecule which threads
its way through the membrane about 7 times, folding
inward & outward
 Outside fold binds with odorant
 Inside fold is coupled to the G-protein
 G-protein itself is combination of 3 subunits: α,βand ϒ
Dr. Misbah-ul-Qamar

15. Stimulation of Olfactory Cells
 Odorant molecules diffuse into the mucus
 Binding to the receptor protein that is linked to a
cytoplasmic G-protein
 Α-subunit of the G-protein separates away
Dr. Misbah-ul-Qamar

16. Stimulation of olfactory cells
Separated unit activates adenyl cyclase
Formation of Cyclic AMP
sodium channels are activated
sodium ions enter the receptor cell & depolarize it
Dr. Misbah-ul-Qamar

17. Dr. Misbah-ul-Qamar

18. Upon stimulation of the
olfactory cells
 The depolarization of receptor cell leads to production
of action potential in the olfactory sensory fibers.
 Membrane potential of un-stimulated olfactory cell is -
55mV with baseline activity of of AP(once every 20 sec
to 2-3 per sec)
 Depolarization brings membrane potential to -30mV
with 20-30AP/sec
Dr. Misbah-ul-Qamar

19. Cascading Effect
 A pattern to enhance the effect of a weak odorant
molecule.
How is it achieved?
 A single dissolved molecule can activate many receptor
proteins
 Activated G-protein complex activates multiple molecules of
adenylyl cyclase
 Each of these molecules causes formation of many times
more molecules of cAMP
 Each cAMP opens still many times more sodium ion
channels
Dr. Misbah-ul-Qamar

20. Importance of cascading effect
 This process multiplies the excitatory effect of even the
weakest odorant and greatly enhances the sensitivity of
the system to the slightest stimulus.
Dr. Misbah-ul-Qamar

21. Initial olfactory sensation
 The intensity of initial olfactory stimulation is
proportional to the logarithm of the stimulus strength.
 Although the determination of differences in intensity of
any given odor is poor in olfactory system
 Concentration of an odor must be changed by 30%
before a difference can be detected
Dr. Misbah-ul-Qamar

22. Rapid Adaptation of olfactory
sensations
 Olfactory receptors adapt about 50% during the first
second and thereafter adapt very little and very slowly.
 Olfactory adaptation is mainly a central mechanism
achieved through
 Adaptation of receptors: Olfactory receptors are phasic
receptors
 Psychological adaptation: far greater than receptors’
adaptation
 Additional adaptation occurs within CNS
Dr. Misbah-ul-Qamar

23.  Neuronal mechanism for adaptation: centrifugal fibers
from brain backward to granule cells (inhibitory cells in
olfactory bulb).
 This feedback inhibition suppress relay of smell signals
providing adaptation
 This is not a physiological process which takes place at
the level of receptors but rather a mechanism altering
perception.
Dr. Misbah-ul-Qamar

24. Primary Olfactory Sensations
 As many as 100
 Narrowed down to 7 which are:
 Camphoraceous
 Musky
 Floral
 Peppermint
 Ethereal
 Pungent
 putrid
Dr. Misbah-ul-Qamar

25. Threshold for smell
 Even a minute quantity of stimulating agent in the air
can elicit a smell sensation
Example:
 Methylmercaptan can be smelled when one 25 trillionth
of a gram is present in each ml of air
Dr. Misbah-ul-Qamar

26. Threshold for different
olfactory sensation
These are the lowest concentrations of a chemical that
can be detected
 Ethyl ether: 5.8mg/L of air
 Chloroform: 3.3mg
 Peppermint oil: 0.02mg
 Butyric acid: 0.009mg
 Artificial musk: 0.00004mg
 Methyl mercaptan: 0.0000004mg
Dr. Misbah-ul-Qamar

27.  Some other examples of substances which may be
detected at very low concentrations include
 Hydrogen sulfide: 0.0005 parts per million (ppm)
 Acetic acid: 0.016ppm
 Kerosene: 0.1ppm
 Gasoline: 0.3ppm
Dr. Misbah-ul-Qamar

28.  Some toxic substances are odorless i.e., they have
odor detection threshold higher than lethal
concentrations
 Example
 CO2; detected at 74000ppm but lethal at 50000ppm
Dr. Misbah-ul-Qamar

29. Affective Qualities
 Smell sensation either could be pleasant or unpleasant.
 Threshold of some odorant molecules is extremely low(
1/25 billionth of a mg)
 Range of sensitivity is only 10-50 times.
Dr. Misbah-ul-Qamar

30. Odorant binding proteins
(OBPs)
 Olfactory epithelium contains one or more OBPs
 These proteins are produced by supporting cells &
released in extracellular space
Functions of OBPs
 These proteins may concentrate the odorants & transfer
them to receptors
 They may partition hydrophobic ligands from air to an
aqueous phase
 They sequester odorants away from site of odor
recognition to allow for odor clearance
Dr. Misbah-ul-Qamar

31. Olfactory pathway
Dr. Misbah-ul-Qamar

32. Transmission of Signals into
CNS
 The olfactory fibers( axons of receptor cells) collect into
bundles of 20 or more pass through perforations in
the cribriform plate of ethmoid  enter the olfactory
bulb.
 Olfactory bulb is a complex neural structure containing
several different layers of cells
 Each olfactory bulb is lined by small ball like neural
junctions(glomeruli)
 Fibres terminate in relation to glomeruli.
Dr. Misbah-ul-Qamar

33. Olfactory Glomerulus–
1st relay station
 This is a tangled knot of mitral and tufted cell dendrites
and olfactory nerve fibres.
 Each of the glomeruli receives synaptic input from only
one type of olfactory receptor (which in turn responds
to only one discrete component of an odorant)
 Glomeruli sort & file various components of odoriferous
molecule before relaying signal to higher levels.
 Mitral cells in glomeruli refine the smell signals.
Dr. Misbah-ul-Qamar

34. Other functions of olfactory
glomerulus
1. Olfactory glomeruli demonstrate lateral inhibition which
sharpens & focuses olfactory signals
 This mechanism is mediated by
 Periglomerular cells
 Granule cells
2. Extracellular field potential in each glomerulus
oscillates & helps to focus the signals reaching the cortex
Granule cells regulate the frequency of oscillation
Dr. Misbah-ul-Qamar

35. Olfactory Tract
 It is formed by the axons of mitral and tufted cells.
 It leaves the olfactory bulb after receiving signals and
enter specialized regions of the cortex.
 Both olfactory tract & bulb are an anterior outgrowth of
brain tissue from the base of the brain
Dr. Misbah-ul-Qamar

36. Dr. Misbah-ul-Qamar

37. Unique Feature of Olfactory
Tract
 Main olfactory tract does not first pass through the
thalamus before reaching cortex.
Dr. Misbah-ul-Qamar

38. Dr. Misbah-ul-Qamar

39. Cortical Areas of Olfaction
 Medial olfactory area
 Lateral olfactory area
Dr. Misbah-ul-Qamar

40. Medial Olfactory Area
 It exerts primitive behavioral aspects of olfactory
signals e.g: licking, salivation & other feeding
responses caused by smell of food or by emotional
drive associated with smell.
 It is represented by septal nuclei
 Signals from this area project to hypothalamus and
other regions for controlling same aspects of olfaction
Dr. Misbah-ul-Qamar

41. Lateral Olfactory Area
 This area is concerned with specific behavioral responses related
to odors i.e: learned control of food intake
 Example: aversion to food that have caused nausea & vomiting
 The area is composed of following regions:
Prepiriform area
Piriform area
Cortical amygdaloid region
 From here, the signals are directed to less primitive limbic
structures e.g: Hippocampus
Dr. Misbah-ul-Qamar

42. Newer Olfactory Pathway
 Signals from primary cortical olfactory area are
projected to dorsomedial thalamic nucleus and then to
orbitofrontal cortex.
 It is a phylogenetically newer pathway
 Involved in conscious perception+ analysis of odor and
also odor discrimination
Dr. Misbah-ul-Qamar

43. Olfactory pathway
Dr. Misbah-ul-Qamar

44. Main olfactory destinations
 Primary olfactory cortex piriform cortex
 Amygdala
 Entorhinal cortex
Dr. Misbah-ul-Qamar

45. Odor discrimination
 How different odors are discriminated from one another
is exactly not resolved
 There is theory that receptors are selectively sensitive
 If 2 odors are mixed, the resulting intensity is always
less than the sum & perceived intensity is dominated by
stronger component
 The direction from which a smell comes may be
indicated by slight difference in the time of arrival of
odorant molecules in the two nostrils
Dr. Misbah-ul-Qamar

46. Detection of pheromones by
VNO
 Pheromones/ vomeropherins
non-volatile, odorless chemical signals passed
subconsciously from one individual to another.
 Vomeronasal organ (VNO)
it is an accessory olfactory organ found in many animals
including mammals, located half an inch inside nose next
to the vomer bone.
Dr. Misbah-ul-Qamar

47. Vomeronasal organ (VNO) in
human
 In humans VNO was considered as vestigial or
nonfunctional
 Recently it is found that this organ is present in the
form of vomeronasal pits on anterior part of nasal
septum
 Receptors of the pit detect odorless human
pheromones at a very low concentration in air
 This organ is also called Jacobson’s organ as
discovered by Ludvig Jacobson in 1813
Dr. Misbah-ul-Qamar

48. Sixth sense?
 Binding of a pheromone to its receptor on surface of
neuron in VNO triggers AP that travels through non-
olfactory pathways to the limbic system, governing
emotional response.
 Messages conveyed by VNO bypass cortical
consciousness
 This subconscious detection of odorless chemical
messengers in air is considered an extra sense of
humans
Dr. Misbah-ul-Qamar

49. Abnormalities of Olfactory
Sensation
 Anosmia total loss for all odors
 Temporary permanent
 Temporary anosmia is due to obstruction of nose which
occurs during
 Common cold
 Nasal sinus
 Allergic conditions
 Permanent anosmia occurs during lesion in olfactory
tract, meningitis & degenerative conditions such as PD
& Alzheimer’s.
Dr. Misbah-ul-Qamar

50. Disadvantages of anosmia
 Person is unable to experience enjoyment of pleasant
aromas & a full spectrum of tastes
 The individual is at greater risk because they are not
able to detect odor from dangers (gas leak, fire, spoiled
food)
Dr. Misbah-ul-Qamar

51.  Hyposmia 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. It may be temporary or permanent. It occurs due
to same causes of anosmia.
Dr. Misbah-ul-Qamar

52. Abnormalities (cont’d)
 Hyperosmia exaggerated sensation. Also called
olfactory hyperesthesia. Perceptual disorder. May
occur in brain injury, epilepsy & neurotic conditions.
 Phantosmia olfactory hallucination smelling
something that is not there
 could be central or peripheral
Dr. Misbah-ul-Qamar

53. Phantosmia
 Phantom smells (imaginary odours) are not uncommon
 Brief episodes of phantosmia can be triggered by
 Temporal lobe seisures
 Epilepsy
 Head trauma
 Onset of a migrain
Dr. Misbah-ul-Qamar

54. Why do we sniff to smell
something better
 It increases our ability smell enhancing the detection of
odorous molecules in the air
 Sniffing causes a peripheral drive in brain to
synchronize rythmic activity, which is the concurrent
firing of neurons in olfactory bulb with breathing.
Dr. Misbah-ul-Qamar

55. Gustation
Sense of
Taste
Dr. Misbah-ul-Qamar

56. Main function of sensation of
taste???
 Taste is a relative crude sense that serves primarily as
gatekeeper to GIT.
 Used to separate undesirable foods from the pleasant
ones
 To avoid lethal foods
Dr. Misbah-ul-Qamar

57. Taste is cumulative sense
 It is mainly a function of taste buds
 One’s sense of smell also contributes strongly to taste
perception
 The texture of food is detected by tectual senses of
mouth
 Presence of substances in food that stimulate pain
endings (pepper) greatly alter taste experience
Dr. Misbah-ul-Qamar

58. Taste bud
The taste buds are ovoid bodies
with a diameter of 50-70μ
In adults about 10,000 taste buds
are present------the number is more
in children
In old age, many taste buds
degenerate & the sensitivity of taste
becomes weak
Insects have taste organs in their
feet, antennae & mouthparts
Dr. Misbah-ul-Qamar

59. Functional unit of taste:
A Taste Bud(3000-10,000 in
number)
Composition of the taste bud
Receptor cells Sustentacular cells
(modified epitelial (supporting cells)
cells) about 50 in number few only
Sensory nerve fibres are intertwined among the cell bodies
Dr. Misbah-ul-Qamar

60. Physiologic structure of taste
bud
 Taste bud is a bundle of taste receptor cells
 Its supporting cells are embedded in the covering of
papillae
 These cells are divided into 4 groups: type I, type II,
type III &type IV(basal cells)
Dr. Misbah-ul-Qamar

61. Cells of taste bud
 Type I & IV are supporting cells
 Type I, II &III have projections called microvilli
 Microvilli project into an opening in the epithelium
covering the tongue
 Neck of each cell is attached to the neck of others
 There are tight junctions between epithelial cells & the
neck portion of type I, II & III cells so that only the tip of
these cells are exposed to fluid in oral cavity
Dr. Misbah-ul-Qamar

62. Regeneration in taste cells
 Cells of taste buds undergo constant cycle of growth ,
apoptosis & regeneration.
 Why regeneration required: most receptors are carefully
sheltered from direct exposure to the environment
 Taste receptor cells, by virtue of their task, frequently come
into contact with potent chemicals so they have to be
replaced continuously
 Epithelial cells surrounding the taste bud differentiate first
into supporting cells & then into receptor cells
Dr. Misbah-ul-Qamar

63. Taste pore
 Formed by apical surfaces of taste cells
 Taste hair protrude from the pore
 Surface for taste molecules provided by taste hair/
microvillus
Dr. Misbah-ul-Qamar

64. Tongue papillae
 Taste bud are smaller closer to the tip of tngue & larger
toward the back
 found in relation to tongue papillae
Location of papillae
 Fungiform : on ant. 2/3 of tongue
 Circumvallate: forming a V-shape, on post. 1/3 of tongue
 Foliate: along lateral margins of tongue
 Filiform: have no taste buds
Dr. Misbah-ul-Qamar

65. Tongue Papillae
Dr. Misbah-ul-Qamar

66. Dr. Misbah-ul-Qamar

67. Filiform papillae
 These are small & conical shaped papillae
 Situated over the dorsum of tongue
Dr. Misbah-ul-Qamar

68. Fungiform papillae
 These are round in shape
 number of taste buds in each is moderate (up to 10)
Dr. Misbah-ul-Qamar

69. Circumvallate papillae
 These are large structures
 Each papilla contains many taste buds (up to 100)
Dr. Misbah-ul-Qamar

70. Other locations for few taste
buds
 Palate
 pharynx
 Tonsils
 Epiglottis
 Proximal esophagus
Dr. Misbah-ul-Qamar

71. 5 Primary taste sensations
 Sour
 Salty
 Sweet
 Umami
 bitter
Dr. Misbah-ul-Qamar

72. Sour taste
 Caused by acidic substances
 Recepter involved is called epithelial Na channel
(ENaC)
 Although the proton which enters the receptor is H+
 Another channel involved is nucleotide gated cation
channel
 Intensity of this taste is approximately proportional to the
logarithm of hydrogen ion concentration
 The more acidic the food the stronger the sour
sensation
Dr. Misbah-ul-Qamar

73. Salty taste
 Receptor involved is ENaC
 Cations of ionized salts (mainly by Na+ ion
concentration)
 Anions also contribute to a lesser extent
 Quality of taste varies from one salt to another
 Some salts elicit other taste sensations in addition to
saltiness
Dr. Misbah-ul-Qamar

74. Sweet taste
 Not caused by any single
class of chemicals
 Some of the types of
chemicals that cause this
taste include:
Sugars, glycols, alcohols,
aldehydes, ketones, amides,
esters, some amino acids,
some small proteins, sulfonic
acids, halogenated acids
 most of the substances that
cause a sweet taste are
organic chemicals.
 The inorganic substances
which produce sweet taste
are lead & beryllium.
 Slight changes in chemical
structure (addition of a
simple radical) can often
change the substance from
sweet to bitter
Dr. Misbah-ul-Qamar

75. Umami taste
 Designates a pleasant taste sesation
 Umami is japanese word meaning ‘delicious’
 it is qualitatively different sensation from sour, salty, sweet or bitter
 It serves as a marker for a desirable, nutritionally protein rich food
 It is triggered by the presence of amino acids especially L-
glutamate(e.g: meat extract, aging cheese)
 Receptor for this taste is metabotropic whose activation is intensified
by
 Guanosine monophosphate (GMP)
 Inosine monophosphate(IMP)
Dr. Misbah-ul-Qamar

76. Bitter taste
 Not caused by single type of chemical agent
 Substances that give bitter taste are almost entirely organic
substances
 Two particular classes of substances cause bitter taste
 alkaloids,
 long chain nitrogen containing items
 Examples: quinine, caffein, strychnine, nicotine
 Many plants, fungi, & some animals produce toxins as a
natural defense mechanism.
Dr. Misbah-ul-Qamar

77.  Most bitter tastants are detected by GPCRs.
 Taste cells that detect bitter possess 50-100 bitter receptors
 Each of the receptors respond to a different flavor of bitter
 Because each cell has diverse family of receptors, a wide
variety of unrelated chemicals all taste bitter despite their
diverse structures
 This mechanism expands the ability of receptor to detect a
wide range of potentially harmful chemicals
Dr. Misbah-ul-Qamar

78. Important examples of bitter
substances
 Quinine is bitter tasting toxin with antimalarial
properties extracted from a tree bark. It blocks most
classes of K channels & causes nonspecific memb.
depolarization
 There are some substances which initially taste sweet
but have a bitter aftertaste
 This characteristic makes the substance objectionable
to some people
 Example: saccharine
Dr. Misbah-ul-Qamar

79. Other taste like sensations
 Taste of fat constitute a sixth basic taste but the
transduction mechanisms are not fully delineated.
 Chemical sensations that mimic hot (e.g: the burning
sensation associated with chilli pepper) & cold (e.g:
menthol) are not tastes but rather are mediated by
somatosensory pathways located in oral cavity or nasal
passage.
Dr. Misbah-ul-Qamar

80. Just to regain your attention!
Dr. Misbah-ul-Qamar

81. Taste discrimination
 The type of receptor protein & its specific action in each
taste villus determines the type of taste that will be
perceived
Example:
 For Na & H ions: receptor proteins open specific ion
channels in apical membranes of taste cells
 For sweet & bitter taste: portion of receptor protein that
protrude through apical membrane activates 2nd
messenger transmitter substances intracellular
chemical changes eliciting taste signal
Dr. Misbah-ul-Qamar

82. How difference in taste is
appreciated
 Each taste bud typically responds to only one of the
five primary taste substances
except
When an item is present in very high
concentration
Dr. Misbah-ul-Qamar

83. Taste discrimination
 This discrimination is coded by patterns of activity in
various taste bud receptors
 Each receptor cell responds in varying degrees to all
primary tastes but is generally preferentially responsive
to one of the taste modalities
 So the discrimination depends on subtle differences in
the stimulation patterns of all taste buds
Dr. Misbah-ul-Qamar

84. 13 taste receptors
 Sodium receptors(2)
 Potassium receptors(2)
 Chloride receptors(1)
 Adenosine receptors(1)
 Hydrogen ion receptors(1)
 Inosine receptors(1)
 Sweet receptors(2)
 Bitter receptors(2)
 Glutamate receptor(1)
Dr. Misbah-ul-Qamar

85. Taste transduction
 The process in which taste chemoreceptors convert
chemical energy into action potentials in taste nerve
fiber
 Dissolved substances act on exposed microvilli (taste
hair/cilia) development of receptor potential
generation of action potential
Dr. Misbah-ul-Qamar

86. Initiation of receptor potential
 Like most sensory receptor cells, membrane of taste
cell is negatively charged on inside
 Application of taste substance causes partial loss of
negative potential
 Decrease in potential is approximately proportional to
the logarithm of concentration of stimulating substance
 The change in electrical potential is called receptor
potential
Dr. Misbah-ul-Qamar

87. Receptor potential
application of substance to be tasted
depolarization of receptor cell
(by opening ion-specific channels)
response in associated nerve fibres
Dr. Misbah-ul-Qamar

88. Mechanism of stimulation of
taste sensation
 Presence of Free H+ in
acid H+ blocks K+
channel decrease in
passive movement of K+ out
of cell reduction in internal
negativity
 Presence of salt entry of
positively charged Na+
through specialized
channels receptor
depolarization
Dr. Misbah-ul-Qamar

89. Mechanism of stimulation of
taste sensation
 Presence of glucose
activation of cAMP second
messenger pathway
phosphorylation & blockage
of K+ channels
 Bitter tastant activation of
G-protein & phospholipase C
messenger system Ca
release
Dr. Misbah-ul-Qamar

90. Threshold for taste
 For sour by HCl: 0.0009M
 For salty by NaCl: 0.01M
 For sweet by sucrose: 0.01M
 For bitter by quinine: 0.000008M
 What is taste index: reciprocals of taste thresholds
Dr. Misbah-ul-Qamar

91. How taste nerve is excited
 Taste nerve fibers form a branching terminal network
 This network is interwoven around the bodies of taste
cells
 Some of these fibers invaginate into folds of taste cell
membrane
 Many neurotransmitter vesicles form beneath cell
membrane near fibers release of NT substance
excite the nerve fiber endings
Dr. Misbah-ul-Qamar

92. Dr. Misbah-ul-Qamar

93. adaptation
 A strong immediate signal by taste nerve weaker
continuous signal
 On first application of taste stimulus rate of discharge
of nerve fibers rises to a peak in small fraction of a
second
 Adaptation occurs within few seconds rate of
discharge of impulses falls back to a lower steady level
 Taste adaptation occurs both at receptors & at CNS
level
Dr. Misbah-ul-Qamar

94. Transmission of signals into
CNS
 1st order neurons of taste pathway are in the nuclei of 3
different cranial nerves
 Dendrites of these neurons are distributed to the taste
buds
Dr. Misbah-ul-Qamar

95. Afferent taste signals
 From ant.2/3 of tongue:
signals travel in branches of trigeminal nerve
joins the chorda tympani( branch of facial nerve)
 From post.1/3 of tongue:
signals carried by fibres in glossopharyngeal nerve
 From epiglottis + other areas
signals carried within branches of vagus
Dr. Misbah-ul-Qamar

96. Dr. Misbah-ul-Qamar

97. Transmission of signals into
CNS (cont’d)
 Afferent signals through axons of 1st order neurons
enter into the nucleus solitarius located in brain stem(
precisely medulla oblongata) through solitary tract.
 Neurons of tractus solitarius are 2nd order neurons.
 Axons of these run through medial leminiscus.
Dr. Misbah-ul-Qamar

98. Transmission of signals into
CNS (cont’d)
 Next, the third order neurons are in the posteroventral
nucleus of thalamus so axons pass rostrally to
ventromedial nucleus of thalamus.
 Then, axons from 3rd order neurons project into parietal
lobe & signals reach the cerebral cortex.
Dr. Misbah-ul-Qamar

99. Final taste perception
 In ventral region of postcentral gyrus which curls into
lateral fissure of cerebral cortex.
 Taste center: center for taste sensation is in the
opercular insular cortex i.e: in lower part of postcentral
gyrus which receives cutaneous sensations from face.
Dr. Misbah-ul-Qamar

100. Dr. Misbah-ul-Qamar

101. Unique feature of gustatory
pathway
 Unlike most sensory input, gustatory pathways are
primarily uncrossed
 The taste fibers do not have an independent cortical
projection.
Dr. Misbah-ul-Qamar

102. Pathway for taste reflex/
salivation
 Fibres course from the solitary tract directly to the
superior and inferior salivatory nuclei.
Dr. Misbah-ul-Qamar

103. Taste Pathway
Dr. Misbah-ul-Qamar

104. Activation of saliva secretion
 It is a taste reflex.
 Mediated by preganglionic parasympathetic fibres to
sup., inf. salivatory nuclei and then postganglionic
fibres to submandibular, sublingual and parotid glands.
 30 ounces of saliva every 24 hours
 Sympathetic activation causes dry mouth
Dr. Misbah-ul-Qamar

105. Taste preference
 It simply means that an animal will choose certain types
of foods over others
 Taste preference often change in accord with body’s
need for certain specific substances
 The phenomenon of taste preference & aversion
results from a mechanism located in central nervous
system, not in taste receptors
Dr. Misbah-ul-Qamar

106. Abnormalities of Taste
Sensations
 Ageusia
 Hypogeusia
 Dysgeusia
 Metallic dysgeusia
 Taste blindness
Dr. Misbah-ul-Qamar

107. Ageusia
 Total loss of taste.
 It may be permanent or temporary
 Lesion in facial nerve, chorda tympani or mandibular
division of trigeminal nerve causes loss of taste in anterior
2/3 of tongue
 Lesion in glossopharyngeal nerve leads to loss of taste in
post. 1/3 of tongue
 Temporary ageusia occcurs due to certain drugs
 Captopril
 Penicillamine
 Substances containing sulfhydryl group
Dr. Misbah-ul-Qamar

108. Prevalence of ageusia
 Ageusia is uncommon except in patients with Sjogren
syndrome.
 Sjogren patients suffer from an autoimmune disease that
impairs exocrine gland function, including salivary glands.
Saliva is required to carry tastants in dissolved form
through taste bud pore.
Dr. Misbah-ul-Qamar

109. Taste abnormalities
 Hypogeusia: decrease in taste sensation. It is due to
increase in threshold for different taste sensations.
 Metallic dysgeusia ( a persistent metallic taste) is a
common side effect of many antibiotics (e.g:
tetracycline & metronidazole) & antifungals.
 Taste blindness rare genetic disorder, inability to
recognize substances by taste
Dr. Misbah-ul-Qamar

110.  Dysgeusia it is a disturbance in taste sensation
 Paroxysmal unpleasant hallucinations of taste & smell
occur
 Condition in which dysgeusia present
 temporal lobe syndrome specially when anterior region
of temporal lobe is affected
Dr. Misbah-ul-Qamar

111. Dr. Misbah-ul-Qamar