Chemosensory System

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Yogurt is a gustatory, olfactory and somatosensory

treat!

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Chemosensory • Two basic components that interact:

• Gustatory system (taste)• Olfactory system (smell)

• For each of these, we will examine: • Biological mechanisms: Stimulus, Sense organ,

receptors, & brain structures

• Perceptual characteristics: Thresholds & magnitudes, stimulus quality, identification

Chapter 4 in Chaudhuri Text

Stimuli of Taste

• There are thought to be 5 basic taste qualities: Salty, sour, sweet, bitter, & umami

• These arise as a result of chemicals (tastants) interacting with taste cells on the tongue

• The relationship between molecular structure of chemicals and their taste quality is complex and poorly understood

• The concentration of a tastant is measured in ppm or ppb (or millimoles/litre). This relates to the subjective intensity or “strength” of a taste

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Saltiness

• Arises from receptors responding to sodium (e.g., NaCl = sodium chloride = table salt)

• Sodium is necessary for many body functions

• Magnitude estimation shows response compression (b

Sourness• Arises from receptors

responding to acids (H+ ions)

• Pleasant combined with sweet, indicating “fruit”

• Unpleasant in combination with bitterness, indicating decomposition

• Magnitude estimation shows response compression (b

Sweetness

• Arises from receptors responding to carbohydrates in solution, especially saccharides

• Generally pleasant, indicating high-calorie foods

• Magnitude estimation shows response compression (b

Bitterness

• Arises from receptors responding to a variety of chemicals, esp. alkaloids.

• Generally unpleasant, indicating toxins 
(like caffeine, alcohol...)

• Magnitude estimation shows response expansion (b>1)

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Denatonium Benzoate (Bitrex™)

• Tolerability threshold for this chemical is just 30 ppm

• Used to make poisonous commercial products taste aversive to avoid accidental consumption

• Similar concept to addition of the odourant mercaptan to natural gas.

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Umami (≈ savoury)• Initially controversial

• Relatively recently discovered receptors respond to amino acid L-glutamate

• Pleasant, indicating presence of proteins

• Associated with MSG, strong tastes in seaweed, cheese, meat, broth, etc.

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The “Tongue Map”

• Suggests each part of tongue detects one of the 4 basic tastes

• Not true, but thresholds for 
sweet & salty are slightly lower in front, while those for sour & bitter are slightly lower in back

• ∴ Glossopharyngeal nerve carries (slightly) more sour & bitter and facial nerve more sweet & salty

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Piquancy• Piquancy (spicy hotness, pungency,

prickliness, etc.) is actually a cutaneous pain sense, but it does interact with taste receptors

• Capsaicinoids in peppers and other piquant foods bind with pain and heat-detecting FNEs in the mouth

• Piquancy is measured in Scoville heat units (SHU). 
 Jalapeno ≈ 4000 
 Habanero ≈ 100 000 
 Naga Jolokia ≈ 1 000 000 


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Scoville Scale

The ratio of water to a substance such that it is diluted to the point that it cannot be tasted.

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Questions

• What are the five basic tastes? • Which ones express response expansion?

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The Taste System• Tongue contains four types of papillae:• Filiform - shaped like cones and located

over entire surface (not mentioned in text)

• Fungiform - shaped like mushrooms and found on sides and tip

• Foliate - series of folds on back and sides• Circumvallate - shaped like flat mounds in a

trench located at back

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The Taste System

• Taste buds are located in all types papillae except for filiform

• Tongue contains ≈ 6,000 taste buds• Each taste bud has several taste cells with

microvilli that extend into the taste pore

• Transduction occurs when chemicals contact the receptor sites on the tips

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The Papillae

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Three Types of Papillae

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Taste Bud

• Taste bud contains (L)ight and (D)ark cells

• Each cell has microvilli that extend into the taste pore

• The microvilli have receptors at the tips

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Taste Receptors

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Transduction

• There are different transduction mechanisms for different basic tastes

• Ionic Channel mechanisms for salty (Na+) and sour (H+) tastes

• G-protein coupled receptor (GPCR) mechanisms for sweet and bitter (& umami)

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Salty & Sour: Ionic Channels

Positive ions from salts (Na+) or acids (H+) travel through ionic channels in the taste cells, depolarizing them and leading to neurotransmitter release

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Sweet: GPCR & cAMP

Wide variety of tastants trigger GPCR to realease cAMP messenger in sweetness-detecting taste cells.

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Bitter: GPCR & Calcium

Wide variety of tastants 
(e.g. alkaloids such as caffeine, nicotine, & quinine) trigger GPCR to release calcium 
(Ca++) messenger in bitterness-detecting taste cells.

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Coding of Gustatory Signals

• How are different taste sensations coded by the gustatory system?

• There are two broad ways that signals can be carried from receptor to brain

• “Labelled line”• “Cross-fibre coding”

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Labelled Line Coding

• Each receptor picks up just one modality and transmits it via a single dedicated fibre

• Therefore, one fibre can provide information about modality and intensity

• Thermal touch sensations work like this.

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Labelled Line CodingStrongStimuli

Fibre Response

WeakStimuli

Fibre Response

WarmthReceptors

ColdReceptors

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Cross-fibre Coding• Each receptor responds to multiple modalities and transmits

signals about both along the same fibre.

• Response strengths differ based on modality and intensity• Therefore, one fibre cannot provide disentangled

information about modality and intensity.

• To discern what modalities are being experienced, we must compare patterns of activity across fibres that are tuned more (though not exclusively) to one modality or another.

• Taste works more like this.

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Cross-fibre CodingStrongStimuli

Fibre Response

WeakStimuli

Fibre Response

“Sweet” receptor
(but also responds weakly to bitter)

“Bitter” receptor
(but also responds weakly to sweet)

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From Tongue to Brainstem

• Three cranial nerves carry taste signals

• Facial nerve (VII)• Glossopharyngeal (IX)• Vagus nerve (X)

• Note ganglia, similar to DRGs (but aren’t)

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Subcortical Relays in Gustatory System

• The 3 nerves synapse in the nucleus of solitary tract (NST) in the brain stem

• Then they travel to the Ventral Posterior Medial Nucleus (VPMN) of the thalamus

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Cortical Areas in Gustation

• VPMN neurones project first to Primary gustatory cortex (PGC)

• Neurones in PGC are more sharply tuned to the 5 basic tastes than are earlier ones

• Their responses are NOT modulated by hunger and satiety states, although neurones at later stages are

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(brainstem)

(thalamus)

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Cortical Areas in Gustation

• From PGC, signals travel to Orbitofrontal Cortex (OFC), part of which is the Secondary Gustatory Cortex (SGC)

• From SGC, signals travel to hypothalamus and amygdala, areas important for memory, motivation and emotion

• OFC neurones integrate information from many senses, including olfactory, somatosensory and visual.

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Responses of neurones in OFC are affected by hunger/satiety signals

Critchley & Rolls, 1996

TOP: Consuming dairy cream reduces firing rate of neurones in monkey OFC.

BOTTOM: Monkey’s response to the cream (bottom panel).

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Pathways of Gustatory System

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Questions

• Taste buds are located on what structures?• What does a taste bud consist of?• Where do the nerves that carry taste signals

converge in the brain first?

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Perceptual Aspects of Gustation

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Intensity & Quality

• Intensity (aka “Strength) of a taste is related to the concentration of a tastant, usually measured in ppm.

• Assessed via thresholds and magnitude estimations• Quality (aka “Flavour”) has many dimensions and is

difficult to quantify

• Assessed via various methods, including multi-dimensional scaling

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Absolute Thresholds

• Absolute thresholds vary widely from one tastant to another.

• In general, bitter substances have lower thresholds, followed by sour, salty & sweet

• The presence of one tastant affects threshold of another (e.g., sweet blocks salty)

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Absolute Thresholds

• Temperature affects threshold as well (cold beer tastes less bitter). Threshold lowest at about 27 C°.

• Sensitivity drops with age, esp. for bitter and salty tastes.

• Thresholds vary slightly depending on area of tongue

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Suprathreshold

• Weber fraction for detection concentration differences is .15 - .25, depending on tastant.

• Magnitude estimation shows response compression for most tastants, except bitter ones.

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Taste Quality

• No simple relationship between molecular structure of tastants and taste quality

• E.g., number of hydrogen atoms in molecule does not determine its taste

• Quite different from touch (and vision and hearing)

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Taste Quality

• Henning’s taste tetrahedron• An early attempt at

geometrically representing taste

• Any taste, according to this, is a simple mix of the four primaries

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Taste Quality

• Multidimensional scaling (MDS) shows that it is not that simple

• In MDS, Ss are asked to taste pairs of tastants and rate their similarity.

• Tastants are then placed in a multidimensional space such that similar ones are close and different ones are distant.

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Multidimensional Scaling

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Taste Quality

• One method for assessing taste quality is to have tasters rate tastants across a number of arbitrarily-selected descriptor terms.

• Data is represented as “star diagrams”• The shapes of the diagrams can be used to

compare different tastants.

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Star Chart AKA web chart, spider chart, star plot, cobweb chart, irregular polygon, polar chart, or kiviat diagram

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Individual Differences

• Individuals vary widely in their taste thresholds and, of course, preferences

• The study of the pleasurable (or not) aspects of taste is called “hedonics” (same root as hedonism)

• An extreme example of individual differences is in the tastant PTC/PROP

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• PTC is a chemical that has a bitter quality to 75% of people (“PTC tasters”), but no taste to 25% of people (“PTC non-tasters”).

• Among PTC tasters, about 25% (so ≈ 6% of population) are “supertasters” who find PTC horribly bitter.

• Supertasters are hedonically different, generally disliking things such as coffee, alcohol, & dark chocolate

PTC/Prop

(basically everything good!)

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The Perception of Flavour

• Flavour ≠ Taste• Flavour combines smell, taste,

somatic sensations (texture, temperature, pain/piquancy) and even visual inputs.

• But food odour is especially important

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The Perception of Flavour

• Odor stimuli from food in the mouth reaches the olfactory mucosa through the retronasal route

• The taste of most compounds is influenced by olfaction, but a few, such as MSG are not

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Questions

• What proportion of the population are PTC tasters? What is the reason for the difference?

• What does the term “hedonics” refer to?

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Chemosensory System, Part II

Olfaction

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Overview of Topics

• A controversy: Pheromones in humans• Biological basis of olfaction: Stimulus, olfactory

mucosa, olfactory bulb, brain areas.

• The puzzle of olfactory coding• Perceptual aspects: Thresholds & Magnitudes,

higher order phenomena

Chapter 4 in Chaudhuri text (2nd half)

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Pheromones

• Olfaction is linked very closely to the “Four Fs of survival”: Fighting, Fleeing, Feeding and...

• Messenger chemicals called pheromones can control these basic behaviours, among many other things

• There is (weak) evidence that these messenger chemicals are unconsciously at work in humans.

Fornication

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Pheromones in Humans

• Many other species (snakes, new world monkeys) have a Vomeronasal Organ (VNO) and an Accessory Olfactory Bulb (AOB).

• These structures are critical to pheromone detection

• Humans do not seem to have either of these (though there is some controversy), so how could we respond to pheromones?

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Pheromones in Humans• Some work supports the existence of “cranial nerve

0” (CN0) in humans (it definitely exists in many other species)

• CN0 projects from nasal cavity to septal nuclei and other brain areas known to regulate sexual behaviour

• CN0 may be involved in human pheromonal control of sexual arousal, but this is highly controversial

R. Douglas Fields, Sex and the Secret Nerve, February/March 2007; Scientific American Mind

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Pheromones in Humans

• Experiment by Stern and McClintock suggests that pheromones are responsible for phenomenon of menstrual synchrony in humans

• Method: Secretions from underarms of donor women were wiped on upper lips of recipient women

• Secretions from the donors taken at the beginning of their cycles led to a shortened length of the recipients’ cycles

• Secretions from the ovulatory phase lengthened recipients’ cycles

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Pheromones in Humans• Miller et al. (2007) studied influence

of lap dancers’ menstrual cycles on the tips they received

• Results: Dancers get more tips when ovulating, less when menstruating. Dancers on the pill show no variation in tipping

• Miller speculates that this pattern of findings may be a result of pheromone messengers, but other explanations are possible

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Stimulus of Olfaction

• As with gustation, olfaction involves detecting chemicals

• Olfactory stimuli are (generally) airborne chemicals known as odorants

• The relationship between chemical structure and odor quality is complex and poorly understood

• The concentration of odorants in air relates to the subjective intensity or strength of a smell

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The Puzzle of Olfactory Quality

Two chemically similar molecules yield very different odors

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The Puzzle of Olfactory Quality

Two chemically dissimilar molecules yield very similar odors

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Structure of the Olfactory System

• Odorant molecules enter the nose

• They are carried to the olfactory mucosa, located at the top of the nasal cavity

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Figure 4.18

Odorants are carried along the mucosa, coming in contact with the sensory neurones and activating them. Their signals are sent to the glomeruli in the olfactory bulb and then on to the brain.

Olfactory nerve

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Structure of the Olfactory System

• Each olfactory sensory neurone has many receptors on its cilia

• There are ≈ 1000 types of receptor

• Each olfactory sensory neurone has only one type of receptor on it

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OSNs are GPCRs

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OSNs & Glomeruli

• Glomeruli are bundles of OSN axons in olfactory bulb

• There are about 2000 of them per bulb• Each glomerulus consists of the axons of

one type of OSN

• IOW, each type of OSN projects to two glomeruli in each bulb

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Glomeruli

• Information about different odourants is mapped onto different glomeruli

• Each glomerulus serves as an independent coding unit

• Another example of parallel processing

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Cortical Areas in Olfaction

• From olfactory bulbs, signals travel via mitral/tufted cells (M/T cells) to O1, primary olfactory cortex

• Note that we do not stop off in thalamus! Smell is the one sense that goes straight to cortex before going back to subcortex

• O1 is composed of many areas, the most important of which is piriform cortex

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The piriform cortex may be the site of re-integration in olfactory processing

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Subcortical Areas in Olfaction

• From O1 signals travel to thalamus & hippocampus, where they are thought to modulate emotional reactions to scents

• Finally, signals arrive in OFC, where integration with other senses occurs

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Coding of Olfactory Signals: Labelled Lines or Cross-fibre coding?

• Each given type of OSN projects to only one type of glomerulus, so anatomy suggests labelled lines. However:

• Multiple odourants can stimulate the same type of OSN, and its associated glomeruli

• Multiple OSN types, and their associated glomeruli, respond to any given odourant

• So functionally, we have cross-fibre coding

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Malnic’s Combinatorial Code for Odour

• Malnic et al. suggests that each OSN responds to a particular type of segment of an odourant molecule.

• This means that: • A given molecule can activate many receptors• A given receptor can respond to many molecules

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Malnic’s Combinatorial Code for Odour

• Malnic et al. proposed that each discriminable odourant is coded for by a complex cross-fibre pattern of activation across the different OSN types (and associated glomeruli)

• Each pattern is called the odour image for that odourant• Specific receptor types are part of the profile for multiple

odourants

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These two odorants have similar molecular shape and produce a similar scent as well as a similar recognition profile

These two odorants have similar molecular shape but produce a different scent as well as a different recognition profile

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Malnic’s Combinatorial Code for Odour

• Each odourant has a number of molecular features, shown as coloured shapes

• Each feature activates a given type of receptor

• Each specific pattern of activated receptors signals a specific scent

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Enantiomers

• Molecules that are identical except that they are mirror images of one another

• A challenge for Malnic’s theory because they have the same molecular features but can smell different or the same

• Carvone enantiomers can smell like caraway or spearmint.

• Limonene enantiomers smell the same

Caraway Spearmint

Carvone Enantiomers(aka steroisomers)

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Questions

• What is a glomerulus?• What is the first site of integration in smell?

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Olfactory System: Perceptual Characteristics

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Challenges in Olfactory Psychophysics

• Which of the infinite variety of odourants to test? No clear organization of odourants to guide decisions.

• Adaptation happens rapidly and lasts. Thus, time between trials must be lengthy.

• Methods for odourant delivery are either poorly controlled or technically challenging

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Methods in Olfactory Psychophysics

• 2AFC vial sniffing: Simple, but noisy due to differences in sniff volume and rate

• Olfactometer: Delivers well-controlled puffs of air to the nose. But, very complex and intricate machine.

Professor Hugo T. Farnsworth’s Smell-o-scope

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Olfactometer

http://socrates.berkeley.edu/~borp/

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Absolute Thresholds

• Detection thresholds vary greatly across odourants

• Also, there is greater individual variability here than expected, up to 1000x differences in threshold!

• Age, as with other sense, raises thresholds• Smoking, contrary to popular belief, does

not. Nor does blindness lower them.

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Difference Thresholds & Magnitude Estimation

• Weber fraction for odour concentration has traditionally been measured (using 2AFC vial tests) at around 25%

• But more careful measurements, using olfactometers, suggest a value closer to 7%

• When relating magnitude to concentration, power law exponent is betwen .2 and .7, depending on the odourant.

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Describing Odour Quality

• Difficult because there is no consensus on odour primaries (unlike with taste).

• Henning attempted to describe all odours as combinations of 6 primaries

• However, many odours fall outside the prism and people find it difficult to describe odours in terms of Henning’s “primaries”

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Henning’s Odour Prism

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Odour Primaries

• Some have suggested that odour primaries might be based on aspects of molecular structure

• But, as we’ve seen, there are 1000 different types of OSN’s and odour seems to be coded as a pattern across them. This argues against the idea of odour primaries.

• Multi-dimensional scaling research supports the idea that there may be no primaries, showing no relationship between odour similarity and molecular structure

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Odor Identification

• We can identify many odours, but naming them is often difficult

• The threshold concentration for identifying an odourant is often 10x higher than for detecting it

• Females show greater odour identification accuracy

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Odour & Emotion

• Olfaction is sometimes said to be the sense most closely attached to emotion

• Olfactory signals have a strong link to the limbic system, which processes emotion

• Some scents (trees, beach) consistently elicit positive emotions.

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Questions

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