MECHANISMS
OF
TASTE & OLFACTION
By
Dr. Khaled Khalil
By the end of this lecture, you should be able to:
A. Describe the primary taste sensations.
B. Explain the structure and mechanism of stimulation of taste
buds.
C. Discuss the taste pathway.
D. Describe the mechanism of stimulation of olfactory cells.
E. Discuss the olfactory pathway.
F. Explain the role of central nervous system in olfaction.
G. Explain the clinical correlation of taste sensations and
olfaction.
Taste sensation
Primary taste modalities
All tastes are varying combinations of four primary
tastes: salty, sour , sweet, and bitter
and umami, a
meaty or savory. These primary taste sensations are
elicited by the following stimuli:
1. Salt taste is stimulated by chemical salts, especially
NaCl (table salt).
2. Sour taste is caused by acids. The citric acid content
of lemons.
Sweet taste is evoked by glucose and other organic
molecules with similar structures but no calories, such
as saccharin, aspartame, and other artifcial sweeteners,
can also interact with “sweet” receptor binding sites.
Bitter taste is elicited by caffeine, nicotine, strychnine and
morphine .
Umami taste is triggered by amino acids, especially
glutamate (in monosodium glutamate) . The presence of
amino acids, as found in meat for example, serves as a
marker for a desirable, nutritionally protein-rich food.
Taste receptors
• Taste receptor cells are present in in taste buds located in
taste papillae on the dorsum of the tongue, pharynx and
upper esophagus.
• A taste bud consists of a group of 50 to 150 receptor cells,
as well as supporting cells and basal cells. The
chemoreceptor cells synapse at their bases with primary
afferent nerve fibers.
• The apices of taste receptor cells have microvilli to
increase surface area exposed to oral contents.
 Each receptor cell responds in varying degrees to all five
primary tastes but is generally preferentially responsive to
one of the taste modalities. The richness of fine taste
discrimination beyond the primary tastes depends on subtle
differences in the stimulation patterns of all the taste buds in
response to various substances,
 Taste perception is also influenced by information derived
from
other
receptors,
especially
odor
as
well
as
psychological factors associated with past experiences with
the food.
Mechanism of stimulation of taste buds:
a) For a chemical substance (tastant) to bind to the taste receptor,
it must be in a solution or dissolved in saliva.
b) Taste binding protein (TBP) (produced by Ebner's glands that
secrete mucus into the cleft around circumvallate papillae)
transports the tastant and concentrates it at taste buds.
c) Then the tastant binds to its specific receptors in the microvilli of
gutstatory cells.
d) The binding of the tastant to a taste receptor opens or closes ion
channels, causing potential changes in the taste receptor cell
and release of the chemical transmitter e.g. glutamate which
excite the nerve fibers to generate nerve impulses.
For each tastant, there is a different mechanism :
Central Gustatory Pathway
First order neurons:
It carries taste sensation from the taste buds to the
gustatory nucleus (nucleus solitarius) as follow;
a) From anterior 2/3 of the tongue are carried by the chorda
tympani nerve (branch of the facial nerve).
b) From the posterior 1/3 of the tongue are carried by the
glossopharyneal nerve.
c) From the pharynx, hard and soft palate and epiglottis are
carried by the vagus nerve.
Second order neurons:
From the gustatory nucleus (nucleus solitarius) axon of 2nd
order neuron ascend in the ipsilateral (same side) medial
lemniscus and pass directly to the ventral posteromedial
nucleus of the thalamus (VPMNT).
Third order neurons:
 From the thalamus (VPMNT), the axons of the 3rd order
neurons pass in the sensory radiation to the face area of
the somatosensory cortex in the ipsilateral postcentral
gyrus.
 They also pass to the anterior part of the insula which is
anterior to the face area of the postcentral gyrus.
 This area that mediates conscious perception of taste and
taste discrimination.
Disorders of taste sensation
• Hypogeusia : Decreased ability to taste
due to nerve lesion, aging, cigarette
smoking.
• Ageusia : Inability to taste due to Vit. B12
or zinc deficiency.
• Dysgeusia : unpleasant perception of taste
giving metalic or rancid taste.
Smell (Olfactory) Sensation
The olfactory (smell) sensation is a chemical sense involving
receptors which are sensitive to volatile chemical substances.
Olfactory Mucosa:
 It is located on the superior nasal concha and the upper
third of the nasal septum.
 The total area of the olfactory mucosa in both sides is about
5 cm2.
Stimulation of the olfactory receptor:
1) The odorant substance must be;
 Volatile in order to be sniffed into the nose to reach the
olfactory receptors.
 Sufficiently water soluble in order to pass through mucous
layer coating the mucosa to reach the olfactory cells
 Lipid soluble in order to penetrate the cells membrane of
the olfactory cells.
2) The olfactory cells are stimulated when air blasts upwards
into the superior region of the nose. The amount of air
reaching this region is greatly increased by sniffing, an action
that includes contraction of the lower part of the nares on the
septum, deflecting the airstream upward.
3) The odorant substance, on coming in contact with the
olfactory mucosa, first diffuses into the mucus that covers the
cilia, then binds with odorant-binding proteins (OBP) that trap
and concentrate the substances and transfer them to the
olfactory receptors.
3) The odorant receptors are G-protein coupled receptors.
The specific G-protein in olfactory receptors is called Golf (Gprotein in olfactory receptor). Binding the odour to the
receptor activates Golf, which stimulates adenylate cyclase
to produce cAMP. cAMP opens Na, Ca channels resulting in
depolarization and generation of receptor potential which at
threshold levels will fire an action potential
The Olfactory Pathways
1st order neuron:
Olfactory
receptors
are the first order
neurons. Each has a
single nonmyelinated
axon
which
pass
through
cribriform
must
plate
the
to
end in the glomeruli
in the olfactory bulb.
Olfactory bulb and 2nd order neurons:
 The second order neurons are the mitral and tufted cells
present in the olfactory bulbs.
 Their dendrites synapse with the axons of the olfactory
receptors in globular structures called the olfactory
glomeruli.
 Each glomerulus contains the synapses of about 25,000
olfactory receptors and 100 of second order neurons.
 There are about 60 such glomeruli in each bulb. Axons of
the mitral and tufted cells form the olfactory tract that
leaves the bulb to the olfactory cortex.
Olfactory Cortex:
The axons of the mitral and tufted cells form the olfactory tract
which enters the brain between the cerebellum and
mesencephalon, where they divide into 2 tracts that terminate on
medial and lateral olfactory areas
A) The medial olfactory area:
 The medial olfactory area consists of a group of nuclei located
immediately anterior to the hypothalamus e.g. septal nuclei, and
primitive portions of limbic system.
 This olfactory area is concerned with:
a)
Primitive responses to olfaction, such as licking the lips,
salivation caused by the smell of food.
b)
Primitive emotional drives associated with smell.
B) The Lateral Olfactory Area:
In the lateral olfactory area, fibers may take one of 2 pathways:
1) To Prepyriform and pyriform cortex plus the cortical portion of the
amygdaloid nuclei ----> then pass into almost all portions of the
limbic system, especially the hippocampus. It is concerned with:
 Learning to like or dislike certain foods.
 Behavioral responses to food e.g. development of absolute
aversion to foods that have caused nausea and vomiting
 Olfactory conditioned reflexes.
2) To the dorsomedial thalamic nucleus and then project to the
orbitofrontal cortex.This system is responsible for:
 The
conscious
perception
of
olfaction
interpretation and significance of the odour.
i.e.
analysis,
Disturbance of olfaction:
1) Anosmia: means complete absence of the sense of smell
2) Hyposmia: means diminished olfactory sensitivity. More than
75% of humans over the age of 80 have an impaired ability to
identify smells.
3) Dysosmia: means distorted sense of smell e.g.
 Olfactory hallucinations due to lesion in the prepyriform
cortex.
 Hypersensitivity for both smell and taste as in Addison
disease