Reading:
BCP Chapter 8
www.knowabouthealth.com
The Chemical Senses
 Most ancestral and common of
the senses.
 Present even in the simplest
single-cell organisms (e.g.,
bacteria).
 Functions:
 Finding food sources
 Judging nutritional value and
safety of foods.
 Avoiding predators and
hazardous environments.
 Social communication,
mating (“pheromones”).
 Monitoring internal
physiological state.
www.georgeretseck.com
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“Taste” vs. “Flavor”
 Taste refers to the sensations
relayed by taste receptor cells in
the oral cavity.
 Foods activate different, unique
combinations of only 5 basic
tastes.
 Flavor depends on both taste
(gustation) and smell (olfaction),
i.e. flavor is a multisensory
percept.
 Visual, auditory (crunch) and
somatosensory (texture, pain and
temperature) factors also
influence the percept of flavor.
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Tastes
 Five basic tastes:





Sweet (non-ionic)
Sour (ionic)
Salty (ionic)
Bitter (non-ionic)
Umami (MSG, glutamate;
non-ionic).
 Basic tastes are innate
 Other complex tastes are
acquired and modified by
experience.
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Taste and Survival
 A sensitive and versatile taste system has
great survival value for organisms
exploiting wide variety of food sources.
 Gustation divides the world into
 Nutrients: generally attractive, e.g., sweet
and salty.
 “Anti-nutrients”: repulsive, e.g., sour and
bitter.
 Sensitivity to bitter tastants (which
number in the thousands) can be extreme
(nanomolar thresholds), because such
tastants in the natural world are often
poisonous.
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Taste Chemistry
 Some taste perceptions are directly related to
tastant chemistry:
 Salts (nutrients) taste “salty”.
 Acids (anti-nutrients) taste “sour”.
 Other common taste perceptions can be elicited
by a wide variety of unrelated chemicals, e.g.,
“sweet”:
 Sugars
 Certain proteins (e.g. thaumatin)
 Sugar substitutes: saccharin (benzoic sulfinide),
aspartame (aspartic acid/phenylalanine di-peptide),
sucralose (“Splenda”, a chlorinated sucrose).
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The Tongue
Taste is primarily a function of
the tongue.
Taste buds are grouped in three
of the four accessory structures
called papillae: vallate, foliate
and fungiform
There are subtle regional
differences in sensitivity to
different tastes over the lingual
surface, but most of the tongue
is sensitive to all tastes.
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Taste Buds and Taste Cells
TB’s are morphologically
specialized accessory
epithelial structures
containing taste cells.
 2000 – 5000 TB’s on human
tongue.
There are roughly 50 – 100
taste cells per taste bud, along
with basal stem cells.
 Modified epithelial
(“short”) sensory receptors.
 Differentiate from basal
stem cells in the taste bud.
 Turnover about every 10
days.
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Functional Morphology of Taste Cells
 Taste cells are “short” receptors
 Apical pole: microvilli
 Protrude through taste pore into mucus
of oral cavity.
 Provide large surface area to maximize
contact with dissolved tastants.
 Basolateral pole:
 Contains typical organelles of epithelial
cells.
 Synapses onto primary gustatory
afferents (first-order taste neurons),
which project to brain via the central
gustatory pathway.
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Taste Transduction
 Direct transduction: Some
tastants are ions that carry
currents through ion channels.
 Indirect transduction: Other
(non-ionic) tastants bind
selectively to specific G proteincoupled membrane receptors.
 Individual taste cells may employ
both types of transduction.
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Direct Transduction: Salty and Sour
 Salty:
 Na+ ions permeate amiloride-
Salt
Sour
sensitive Na+ channels, directly
depolarizing membrane.
 Sour:
 H+ ions (protons) permeate
amiloride-sensitive Na+
channels…
 … AND block K+ channels,
directly depolarizing membrane.
 Depolarization opens Nav and
Cav channels, leading to AP’s
(sometimes) and transmitter
release, respectively.
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Indirect Transduction: Bitter/Sweet/Umami
 Bitter, sweet, or umami tastants
transduced by G protein-coupled
pathways.
 Binding of tastant to receptor activates G
protein/PLC/IP3 cascade.
 IP3 elevates internal [Ca2+]


IP3 causes release of Ca2+ from internal stores.
Opens unique Ca2+-activated Na+ channel,
depolarizing membrane, opening voltagedependent Cav channels.
 Ca2+ influx further raises [Ca2+]]i, triggers
transmitter release, synaptic stimulation of
primary gustatory afferents.
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Early Processing of Tastes
 Receptor potential magnitude
proportional to both type AND
concentration of tastant.
Selectivity in Taste
Cells
 Taste cell selectivity for basic tastes
varies: 90% respond to two or more
tastes.
 Primary gustatory afferents branch
many times, innervating numerous
taste buds and, within each taste bud,
several taste cells. Thus, the electrical
activity recorded from a single sensory
fiber represents the input of many taste
cells. Afferents exhibit taste
preferences, suggesting they receive
input from taste cells with common
tastant selectivity.
Taste Selectivity in
Gustatory Afferents
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Central Pathways for Taste
Gustatory afferent neurons leave
the mouth as part of the 7th, 9th,
and 10th cranial nerves to the
solitary nucleus of the medulla.
Medulla  ventral posterior
nucleus of the thalamus 
primary and secondary
gustatory cortex (ipsilateral).
10th
9th
7th
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Central Representation of Taste
Many central gustatory neurons
exhibit a strong response to a
specific tastant but are also able
to respond to other tastants.
Imaging studies of the gustatory
cortex show that different tastes
(salty, sour, sweet and bitter) are
represented by specific spatial
patterns containing both distinct
and overlapping regions. These
data support both a labeled line
and a population (distributed)
code of taste.
www.cell.com
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Pathways from Primary Gustatory Cortex
Forebrain:
Taste/Flavor
Perception
Primary
Gustatory
Cortex
• Orbitofrontal cortex
(location where
chemical signals
first merge to form
percept of flavor).
Medullary
motor nuclei:
Feeding
Behavior
Hypothalamus,
Amygdala:
Motivational &
Hedonic Value
of Food
•
•
•
•
•
Swallowing
Chewing
Gagging, vomiting
Salivation
Respiration
• Hunger
• Palatability
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