PSYC 212 | Chapter 4 – The Chemosensory Systems: Taste and Smell
A. General Characteristics of Chemosensory Perception
What evolutionary factors led to the development of chemosensory systems?
Marine organisms needed to seek food and avoid predators
For terrestrial organisms, air is the medium: smell is the distant detector of
chemical stimuli and taste is the near detector
- Territorial marking is accomplished by by-products of excretion.
- Sexual signaling occurs by release of chemicals from specialized glands
(pheromones) usually in found in the genital areas of organisms.
Why are we sensitive to chemicals?
Humans rely much less on the chemosensory system; we rely more on vision
However it is still significant:
Smell: - Certain emitted odors have pheromonal actions in humans
- Ex: 1. Infants can detect odors of mother’s breasts and attracted to
them, while repulsed by scents of strangers 2. Menstrual synchrony
caused by emitted odors
Taste: - still provides survival value since we reject poisonous food bitter or
- Craving of salt might reflect the fact that we need to make sure we
have enough sodium every day inadequate amount leads to cardiac
- However, we are also sensitive to excessive salt intake, since high blood
pressure is also dangerous
How does human chemosensory function differ from that of other animals?
Most apparent in olfactory system
Vomeronasal organ (VNO): chemosensory organ for specific detection of
- Located in nasal cavity, but separate from main olfactory organ.
- Exists in many terrestrial mammalian species, not sure if it is in humans
Since humans have reduced reliance on odor detection, we are less sensitive
Dogs are 10,000 more sensitive to scents than humans.
How do taste and smell differ from the other sensory systems?
- Nature of stimulus that triggers our various taste and smell experiences
o For chemosensory systems, relationship between stimulus and
perception is very complex
- Proust phenomenon: odors can trigger memories with emotional
significance from the past
o Happens because smell is the only sense that has direct contact
with the limbic system (a set of brain structures responsible for neural response that trigger emotion, mood, motivation, and
sexual behavior) of the brain.
- Odors play a large role in sexual development
o Feminine heterosexual reactivity and sensitivity/aversion for male
odors are believed to trigger psychosexual development in boys
(occurs at 2-3 years old)
- Was believed that darker skin meant more sensitive olfactory system
B. The Gustatory System – Biological Mechanisms
5 categories of taste stimuli: sweet, sour, salty, bitter, and umami (MSG).
Taste receptors are clustered in taste buds on the tongue, as well as the roof
and back of mouth but in smaller amounts
B1. Taste Reception in the Tongue
The tongue is composed of many bumps and groove all over its surface
Papillae: small mounds/projections of tissue; there are many types
- Fungiform: located at front of tip of tongue, larger ones found at the
back of tongue
- Vallate: along V-shape line at very back of tongue; deeper and larger
- Foliate: at back of and along sides of tongue; create deep ridges
- Tastebuds are embedded in the grooves (walls) of papillae
o Food substance dissolved in saliva are carried into groove and
interact with taste buds to initiate primary sequence that leads to
- All gustatory sensation is through ~4600 taste buds total; ½ in vallate,
½ from other two
Taste receptor cells are located inside the taste bud
Taste bud receptor cells (not the taste buds) transduce chemical stimuli into
Tastants (dissolved chemicals) infiltrate taste buds through narrow pore that
opens into the grooves
Taste receptor cells behave as neuronssensory stimulation produces change
in membrane potential that leads to neurotransmitter release (this is the
difference between cells in the skin and the cells for taste); each taste bud has
50-150 receptors cells
2 types of receptor cells: dark and light; can be distinguished by
appearance under microscope
- Both types become narrow at the pore where cell’s membrane becomes
invaginated into microvilli (series of folds)
- When tastants enter taste bud, they interact with receptor cell
membrane at microvilli
Taste receptor cells are innervated by sensory neurons – cranial nerves carry
Gustatory signals from taste buds in the:
- Front two-thirds of the tongue are transmitted through the facial nerve - Back one-third of the tongue are carried by glossophargyngeal nerve
- Back of the throat are carried by the vagus nerve
All 3 of these cranial nerves emerge from the brain stem, not the spinal
Each nerve is associated with a ganglion (collection of neurons), which give
rise to gustatory fibers that innervate the tongue and throat.
These nerves also carry signals for touch and pain.
B2. Signal Transduction Mechanisms
Primary taste qualities each have own transduction mechanism, which fall into
But no matter which category of mechanism, the end result of chemical
stimulation upon a cell receptor is the same: membrane depolarization that
results in neurotransmitter release upon the gustatory fiber.
Ionic channel mechanism code for salty and sour tastes
Salt: taste of salt is mediate by certain free ions, most potent is Na +
All body cells have lower Na concentration inside that outside, which creates
electrochemical gradient that favors entry of Na . +
Movement of Na through a pore in receptor cell creates depolarization at
opposite end of cell where it causes neurotransmitter to be released in
Sour: produced by acidic substances that release H +
+ + +
H movement is same as that of Na , but can only occur is Na is low in
saliva. Otherwise, the two ions compete for the sodium channel; one ion can
block entry of the other.
Receptor-mediated mechanisms code for sweet and bitter tastes
Chemicals that produce sweet and bitter are more complex than ions.
Sweet: can be generated by simple sugars (sucrose, fructose, …) or amino
acids, peptides, or artificial sweeteners (aspartame, saccharine, …)
Binding of sweet tastants to receptor triggers G protein, which results in
production of cAMP in the receptor cell cAMP is a second messenger:
triggers further set of biochemical events within the cell. It works directly upon
ionic channels to produce membrane depolarization.
Bitter: produced by alkaloids (contain N) (ex: quinine) found usually in plan2+
Binding of bitter tastants trigger G protein, which results in release of Ca
ions, which trigger release of neurotransmitter release.
Umami – 5 primary taste produced by flavor enhancer monosodium
Umami receptor binds glutamate molecules and triggers series of biochemical
events in the taste cell (like sweet and bitter tastants)
Existence of umami receptor gives evidence that it is a primary taste.
More evidence: glutamate is one of the most abundant amino acids, it also
is an important excitatory neurotransmitter in the brain.
B3. Coding of Gustatory Signals
“Labelled-line” versus “cross-fiber” coding Labelled-line coding system: each nerve fiber is responsible for
transmitting information that is highly specific and restricted to a particular
perceptual aspect of the stimulus.
Problems: Individual receptor cells are actually capable of responding to
several taste stimuli through many transductional mechanisms. One nerve
fiber can receive signals from more than one taste bud, while each taste
bud can send its signals through more than one gustatory fiber.
Cross-fiber coding (discovered by Pfaffman): different taste qualities are
distinguished by the pattern of discharges across a large population of fibers
(as opposed to neural discharge within dedicated nerve fibers).
Higher centers of the brain decode this pattern to create taste perceptions.
Taste quality and intensity coding
Many gustatory fibers show a preference for a particular taste primary even
though they may also be stimulated by several different tastes.
Chemical stimuli that generate similar patterns of firing are perceived to be
closer in taste than those that generate different firing patterns.
Stimulus intensity: is reflected by the concentration of the chemical
dissolved in the saliva.
- Is encoded by the firing rate of action potentials in gustatory fibers.
Regional differences in taste across the tongue
All four primary qualities are processed by taste buds located throughout the
tongue, but there are some regional preferences for certain taste qualities:
- Tip of tongue is more sensitive to sweet and salty stimuli (carried to
- Back is more sensitive to sour and bitter stimuli (carried to
Chemical organization: organization in the layout of taste primaries, which
creates uneven distribution across the tongue.
B4. Gustatory Processing in the Brain
Subcortical relay sites for gustatory signals
Nucleus of the solitary tract (NST): collection of neurons located in bran
stem that acts as relay station for other non-gustatory signals.
- Receives gustatory fibers from neurons whose cell bodies are located in
ganglia associated with the 3 cranial nerves (VII, IX, X) emerging on
each side of brain stem.
- Signals in the incoming nerve fibers are responsive to a broad rage of
taste stimuli, but in most cases, neurons respond best to a particular
- Rough chemotopic arrangement may exist in NST, which explains
Ventral posterior medial nucleus (VPMN): located further along taste
pathway within thalamus
- Neurons are smaller and densely packed
- Responses are broadly tuned: show responses to several stimuli, but
responses to sweet and salty are more common, followed by sour and
bitter. After the signal are relayed at these sites, they are sent to the primary
Primary gustatory cortex
Projections from the thalamus arrive into the insula: area of the frontal lobe
buried within the sylvian sulcus; damage to the insula may suffer a loss of
Neurons in the primary gustatory area are responsive to all 5 taste primaries.
Majority of neurons here have a preference for particular taste that is more
selective/sharper neurons in the primary gustatory cortex are more
specifically tuned to individual primary tastes than neurons in the earlier
Hunger and satiety do not affect firing of neurons in the primary gustatory
cortex so neurons of primary gustatory cortex are involved in pure sensory
processing of taste information
Secondary gustatory cortex and beyond
Satiety and hunger do affect taste signals here.
Orbitofrontal cortex (secondary gustatory cortex): gustatory neurons here
process higher aspects of taste function.
- Is a gateway where taste signals reach other brain areas (hypothalamus
o Neurons in these areas have encoding behavioral features (ex:
motivation, emotion, desire, memory)
- Combine sight, smell, texture, and taste of food into comprehensible
- Is affected by hunger: which causes enhanced sensation of taste
o Reduced firing in cases of satiety (full), increased firing in hunger
C. The Gustatory System – Perceptual Characteristics
Taste signals generated by a certain substance can produce an immediate
reflexive response as to ingest or reject (as little as 50 msec)
Two perceptual aspects of taste:
- Intensity: related to concentration of the tastant, lingering effects of
the tastant over time, differences in detection sensitivity over the
tongue, and context in which its effects are tested for.
- Quality: relies on subjective description of the sensory experience
C1. Psychophysical techniques for taste measurement
- General: - Lack of agreement on the nature of taste qualities
- Finding that different methods can yield different results
- Biggest: stimulus cannot be effectively controlled because chemical
substances are difficult to apply and remove in a precise manner on the
Electrogustometry – stimulate sour and salt ionic channel
Delivers small electrical current through an electrode or metal disk to a
specific point/region of the tongue and oral cavity.
Advantage: electrical stimulus can be applied in a highly discrete manner
with respect to time and space Disadvantage: taste sensations evoked by electrical stimulation are limited
to only sour and salty.
Regional and whole-mouth chemogustometry – use of chemical solution to
asses taste function
- Application of chemical solution to restricted part of tongue by filter
o Used to access different thresholds, or to obtain estimate of
sensory intensity, or identity taste quality
- OR solution flows over limited extent of tongue
o Advantage: stimulation area is constant and interference effects
of saliva are minimalized.
- Explores basic aspects of gustatory function and clinical aspects of
- Use entire mouth and tongue to make assessment of gustatory function
- Three-drop test: subject has to distinguish which of 3 drops contains
o Detection threshold is defined as the concentration at which the
subject can identify the correct drop 3 trials in a row
Which are usually high values since volume is low
- Eight-cup test: subjects have to identify which of 4 cups among 8
contain the stimulus
o Detection threshold is defined as the concentration at which the
subject can make the perfect separation between stimulus and
Assessment of taste abnormalities
Ageusia: total loss of taste; condition that can arise from injury to any
gustatory nerves or from side effects of some medications
Hypogeusia: reduction in taste sensitivity; arises from smoking, symptoms of
diseases (influenza, diabetes, hypertension)
Dysgeusia: taste distortions; food perceived to have taste of different
C2. Perception of Intensity
Lowest to highest threshold: bitter (urea is unusually high), sour, salty, &
sweet (saccharin is low)
Masking: taste thresholds are affected by other primary tastants in a mixture
The presence of one tastant might alter the threshold of the other; (sweet
Temperature: relationship between threshold and temperature is a U-shaped
function for the taste primaries.
- Temperature for lowest threshold (highest sensitivity) is between body
and room temperature o Believe to reflect that binding of tastant molecule to gustatory
receptor cell needs an optimal thermal condition.
Suprathreshold intensity perception
Weber fraction: smallest detectable concentration change 15-25%
- Fraction for sweet taste is ~unaffected by age, but bitter taste
- Power law: exponent < 1 = negatively accelerating function where
nervous system compresses physical information into a sensory
o Taste functions for most primary functions show this compressive
Temporal and spatial factors
Detection thresholds: - susceptible to various temporal conditions (ex: how
rapidly tastant is present on the tongue, what was presented before, how long
taste may linger after detection) +
- Made in the presence of saliva, which contains Na and other ions that
can have an interfering effect
o Residual background due to saliva/lingering effects of prior
tastant can affect detection and discrimination thresholds.
o To reduce background factors: increase flow of solution on the
Reduces detection threshold since greater number of
tongue molecules are present on the tongue per unit of
Spatial extent of stimulation also affects taste intensity perception:
- Greater area of stimulation = lower threshold
o Due to recruitment of additional taste receptors that add to the
neural signal for that particular taste
- Sweet and salty thresholds are lower at front end of the tongue
- Sour threshold is lowest at the back of the tongue
- Bitter threshold is lowest in the soft palate
o Even though bitter threshold lower at front of tongue, the
suprathreshold relationship between perceived intensity and
stimulus concentration is a steeper function at the back of the
Adaptation and cross-adaptation
Adaptation: when perceived intensity or sensitivity of a tastant decreases
while the taste is continuously applied to the tongue. Time required depends
on concentration of tastant.
Example: saliva (tasteless but contains many ions); cannot be food.
Cross-adaptation: when perceived intensity of a compound decreases due
to the adaptation to a different one.
- Occurs when two different molecules have chemical affinity for same
- Compounds from the taste primaries do not cross-adapt
o Each primary has independent/separate transduction mechanism PTC/PROP thresholds – non-tasters, tasters, and supertasters
While making PTC, chemist accidentally released some into the air, colleagues
complained about its intense bitterness, while he could not detect it.
Non-tasters: ~25% of people who have normal taste sensitivity but cannot
detect PTC/PROP bitterness (have high bitterness thresholds)