This document provides information on smell, taste, and their physiology. It discusses the key features and functions of smell, including olfactory receptors and pathways. It also examines taste sensation in depth, covering the receptors, primary modalities, stimulation methods, pathways, importance in digestion and homeostasis, abnormalities of the tongue, and taste abnormalities. Diagrams are included to illustrate tongue papillae, taste buds, modalities, and pathways.
30. Importance of taste sensation
Defensive
function
1st line of defense
against toxic or
undesired food
Affects GIT
function
Increases GIT
secretion and
motility
(unconditioned
reflex)
Psychic
effect
Causes psychic
happiness with
tasty food
Homeostasis
1.Pregnant women seek
for Ca++ rich diet.
2.Diabetics seek sugary
food.
3.Addisonian patients
seek salty food.
31. Taste receptors (Taste buds)
Characters
i.e. are stimulated
by a substance dissolved in saliva.
:
1.Dorsum and sides of tongue.
2.Soft and hard palate, epiglottis
and pharynx.
they are barrel-
shaped structures in the walls of
circumvallate, fungiform and
villiform papillae (giving the tongue
its characteristic rough appearance).
Structure
cells receptor cells
cells
Type 1 & 2
cells
Type 3 cells
which synapse
with sensory
nerve fibers.
Arise from tongue
epithelial cells.
Differentiate into
receptor cells.
If the dorsum of the tongue becomes smooth,
glazy red and clean, it will indicate malnutrition,
lack of vitamins or even toxic state.
32. Tongue Papillae
Circumvallate Fungiform Filliform
Most prominent.
Arranged in V-
shaped manner on
tongue dorsum (its
apex toward the
throat).
Concentrated at
of the tongue.
On
of the tongue.
Figure: Tongue Papillae
34. There are many which
cover the apical surface of type 1,
2 and 3 cells and those microvilli
project to the surface of the
tongue through tinny opening
known as .
The only parts of taste buds
which are in
oral cavity are the
through the taste pores as the
neck of all cells are connected to
each other by .
35. Primary Taste Modalities
Sweat Salt Sour
(Acidic)
Bitter Umami
Meaty
Felt at tip of tongue
(mainly).
Stimuli:
1.Organic sugar as
glucose and sucrose.
2.Saccharine: 700
times more potent
than sucrose (non
caloric).
3.Thaumatin: 100000
more potent than
sucrose.
Felt on anterior part of
dorsum of the tongue.
Stimulus: ionized
substances as NaCl.
Felt on edges of the tongue.
Stimulus: H+. Organic acids
penetrate more readily
giving more sour taste.
Felt at back of dorsum
of the tongue.
Stimulus: quinine,
strychnine and caffeine
Felt at dorsum of the
tongue.
Stimulus: amino acid
and monosodium
glutamate.
Saccharine is useful to obese
and diabetic persons. Why?
36. Figure: Primary Taste
Modalities.
Any other taste is a
mixture by different
degrees of 1ry taste
modalities.
All 5 primary taste
modalities are sensed
on the pharynx and
epiglottis.
37. Mode of stimulation of Taste Receptors
Sweat Salt Sour
(Acidic)
Bitter Umami
Binds with G-
protein increasing
cAMP decreasing
K+ conductance in
membrane of taste
cells resulting in
depolarization.
Increases Na+ influx
through opening of
passive Na+ channels.
Activation of H+-gated
cation channels causing
K+ channels blockage
causing depolarization.
Binding to G-protein a
gustducin decreasing cAMP.
Phospholipase C activation
increasing intracellular
Ca+2.
Water stimulates taste receptors by
washing certain ions from them
causing their discharge.
Metabotropic and/or
ionotropic glutamate
receptors’ activation
38. Figure: Mode of stimulation of sweat taste
receptors
The substance to be tasted must have a certain
threshold of concentration e.g. glucose 80mmol/L
while NaCl is 2mmol/L.
In old age, the substance concentration must be
increased due to atrophy of many taste buds.
Taste sensation is increased by:
1. Smell.
2. Heat (that is why hot food and drinks appears
more tasty).
39. Figure: Mode of stimulation of
sweat taste receptors
A protein that binds, concentrates
and transmits taste-producing
substances to the receptor cells is
secreted by Ebner’s glands on the
dorsum of the tongue.
Gustducin is a G-protein associated
with taste found in some taste
receptor cells.
Each taste bud contains receptors
for more than one taste modality
(even the 5) with predominance of
one.
40. Pathway of taste
1st order neuron 2nd order neuron 3rd order neuron
From anterior 2/3 of tongue: sensation
carried by chorda tympani branch of facial
(VII) to geniculate ganglion to upper part
of nucleus solitarius of medulla oblongata.
From posterior 1/3 of tongue: sensation
carried by glossopharyngeal (IX) nerve to
petrosal ganglion to middle part of nucleus
solitarius of medulla.
From palate, pharynx and epiglottis:
sensation carried by vagus (X) nerve to
nodosa ganglion to lower part of nucleus
solitarius of medulla.
In: the nucleus
solitarius of
medulla oblongata
of the same side.
Its fibers ascend
in the ipsilateral
medial lemniscus.
In: posteroventral
nucleus of the
thalamus (PVNT)
the same side.
Its fibers pass in the
sensory radiation to
reach anterior
insula and the
frontal operculum
in ipsilateral cortex.
41. Figure: Pathway of taste
Local application of an
electric current on tongue
can evoke taste sensation.
Used to locate facial nerve
lesion.
