The Chemical Senses

Learning Outcomes

1. What is the difference between taste and flavour?

2. What are the basic taste qualities?

3. How are chemicals transduced on the tongue?

4. What are the gustatory neural pathways?

5. What factors affect gustatory threshold?

6. How do we perceive “hot” and “cool” tastes?

7. How can olfactory stimuli be described, and categorized?

8. What are two theories of olfaction?

9. What structures are involved in olfaction? What is the transduction mechanism?

10. What are the olfactory neural pathways?

11. What factors affect olfactory threshold and identification?

12. Describe some psychological effects of odourants.

Gustation

- _______ is a combination of smell and taste, but may also include texture and other sensory aspects

- difficult to ________ foods without olfaction (Mozell et al., 1969)

- not a “___” sense; gustation is the most intimate sense

- _______ to test whether food was edible, spoiled, or poisonous

• salty maintains electrolyte balance

• sour detects vitamins, and acids that could burn tissue

• sweet helps ensure energy reserves/fuel for neurons

• bitter avoids toxins/poisons

Taste Stimuli/Qualities

• _____: in-/organic salts; cation is most important (e.g., Na⁺ in NaCl)

• ____: in/organic acids that release H⁺ ion in solution; not all acids are sour

• _____: complex organic molecules: sugar, aspartame

• ______: varied: alkaloids; some nitrogen-containing molecules; some related to but having different molecular arrangement than sweet molecules

• _____ (“brothy,” “meaty,” or “savoury”): salts of glutamic acid

- found in protein-rich foods, Parmesan cheese, tomatoes, mushrooms, and Marmite^® yeast extract

- Ikeda (1909) isolated glutamic acid, an amino acid, from seaweed

- glutamic acid + Na⁺ monosodium glutamate (MSG)

- Chaudhari, Landin, & Roper (2000): identified G protein-coupled receptors for MSG

Receptors

- tongue is covered with 4 kinds of bumps called ________

- taste ____ found on:

• fungiform: mushroom-shaped; on tip and sides

• foliate: folds along sides of tongue

• circumvallate: flat mounds surrounded by a trench; on back of tongue

• also found on soft palate

- we have up to 10,000 buds

- each bud has several taste cells (receptors)

- microvilli of receptor cells project out of taste ____

- taste buds wear out and are replaced every 7-10 days; less frequently replaced after age 45

Transduction

- ______ penetrates membrane of receptor, depolarizing it

- sour substances contain H⁺ which ______ ion channels

- other substances may form _____ with molecules in the membrane, causing chemical changes within the cell

Sensory Coding

• _______-____ (specificity coding):

- different taste stimuli activate specific fibres; firing rate represents intensity

- each nerve fibre is “labeled” as corresponding to a particular taste sensation

- activity in a few fibre types is all that’s needed to code stimuli

- however, many substances elicit more than one taste (e.g., sodium saccharine shifts from sweet to ______ with increased concentration)

• ______-_____ pattern (population coding):

- there are no specialized receptors for each taste; taste quality signaled by pattern of activity across many fibres

- each fibre type responds to a variety of substances, but seems to be tuned to respond best to one type of taste:

• likely that one single mechanism is not responsible, but by specificity and population coding working together in some way

Neural Pathways

- nerve fibres from front/sides of tongue project to brain in ______ _______ nerve

- fibres from back of tongue make up part of ________________ nerve

- fibres from mouth and throat project via _____ nerve

- fibres from the soft palate project via superficial petrosal nerve (a.k.a., greater petrosal nerve)

- these connect to nucleus of the solitary tract in medulla, which projects to:

• thalamus gustatory cortexes in frontal lobe, including frontal operculum cortex & anterior insula (conscious taste __________) orbitofrontal cortex (emotion & reward)

• hypothalamus & amygdala: aversions, cravings, _________ component of taste

Threshold

Factors affecting threshold:

• location (Bartoshuk, 1993; Collings, 1974)

- tastes not _________ on the tongue

- “tongue map” based on misleading graph published in 1942

- tastes can be perceived anywhere on tongue--except the centre, which has no taste buds

- certain regions may be more sensitive to particular tastes

• ___________ of substances

- greatest sensitivity around 22-40 °C

• individual differences:

Fox (1931):

- synthesized phenylthiocarbamide (PTC)

- a colleague noted how bad it tasted--but Fox tasted nothing

- he called this taste blindness

Blakeslee & Fox (1932):

- gave samples of PTC to 2,500 at AAAS meeting

- found different reactions to PTC:

Bartoshuk & colleagues (1994; 1998):

- participants tasted 6-n-propylthiouracil (PROP):

- supertasters also find saccharine, caffeine, and broccoli bitter

- more sensitive to tastes, spicy foods

- due to number, size, density, and shape of papillae; has a genetic basis

Adaptation

• adaptation-produced ____________: adaptation to one taste increases sensitivity to another

e.g., adapting to a bitter substance elicits a _____ taste from water

- occurs for all basic tastes; salty is most difficult to induce, perhaps because ______ contains salt

- _________ effect: after eating cooked artichoke, 60% of people find that water subsequently tastes sweet

- _______ fruit: contains the glycoprotein miraculin, which affects sweet receptors in the presence of acids

• _____-adaptation: adaptation to one compound reduces sensitivity to another compound

- adapting to NaCl reduces sensitivity to other salts, but not to other basic tastes; also found for sour

- suggests ______ neural-receptor mechanism for salty, and for sour

- more complex for sweet and bitter tastes: cross-adaptation found for some substances, but not others

- implies sweet and bitter each have ________ coding mechanisms

e.g., there may be 12-50 subtypes of bitter receptors

- this may reflect diversity of sweet and bitter tastants

“Hot” & “Cool” Tastes

____________: sensations arising when chemical compounds in foods activate receptor mechanisms for pain, touch, or thermal perception

e.g., burning from capsaicin in chili pepper, coolness from menthol in mouthwash, stinging sensation of carbonation in the nose

_________:

- mainly found in white membrane inside chili peppers

- soluble in alcohol and vegetable oils

- ingredient in Zostrix^® (0.025%): relieves arthritis pain by lowering levels of substance P

- “heat” measured in ________ heat units (1912): how many parts of sugar water required for heat to be at absolute threshold

David Julius & colleagues (1997):

- won Nobel Prize in 2021

- capsaicin partly comprised of vanillin-shaped molecule--like endogenous endovanilloids

- capsaicin binds to transient receptor potential cation channel subfamily V member 1 (TRPV1; _________ receptor 1) protein

- TRPV1 is polymodal nociceptor of some C-fibres; also responds to heat (> 43 °C, painfully hot!)

- causes influx of Ca²⁺ and some Na⁺ into cells; triggers heat signal, inflammation

- influx correlated with Scoville units

- prolonged exposure/influx kills cells

_______:

- alcohol extracted from peppermint; potent counterirritant

- produces cool sensation, not by evaporation; acts on thermoreceptors

Ardem Patapoutian & colleagues (2002):

- won Nobel Prize in 2021

- menthol acts on transient receptor potential cation channel subfamily M member 8 (TRPM8) protein

- TRPM8 is a cold receptor (< 20 °C)

- menthol inhibits ______ of Ca²⁺ from cold receptor

- this causes depolarization/increased firing of ____ receptors

Olfaction

- smell stimuli are _________: molecules of volatile substances (give off vapours)

- we can discriminate over 1 trillion olfactory stimuli (Bushdid & colleagues, 2014)

- smell qualities:

• perfumers (called “the Nose”): ___ notes evaporate the quickest, then middle notes, lastly ____ notes; classification by comparison

• Ann C. Noble et al. (1984): developed the ____ aroma wheel; some categorization, but mostly classification by comparison

Aristotle (c. 350 BCE):

- 4 basic categories of odourants: pungent, succulent, acid, astringent

Henning (1916):

- derived _________ odours based on similarity judgments

- created odour _____:

- believed any odour was a combination of these qualities; occupies a point somewhere within prism

- problem: most people need more than 6 categories

- pine is similar to lemon & balsam, but combining the two creates odour _____

______________ Theory (Amoore, 1970)

- classification of odours based on shape of molecules

- certain shaped molecules fit into matching receptor sites

- expanded to 32 (1977)

Pros & Cons:

evidence for categories from different kinds of _______: inability to perceive a certain odour

contrary evidence: very similar-looking molecules may have very different odours; and different-looking molecules can have similar odours

What about curry powder?

- blend of up to 30 spices, hundreds of volatile chemicals and aromatics, but has a certain smell

The Olfactory System & Transduction

- olfactory mucosa (at the roof of the nasal cavity) contains mucous and the olfactory __________, which is about 5 cm²

- stimulus molecules may be picked up by olfactory _______ proteins (OBP) in mucous layer

- molecules bind to olfactory _________ (ORs) on cilia of olfactory ________ neurons (ORNs, also called olfactory sensory neurons)

- this opens ion channels in membrane, leading to depolarization

- odour chemicals may also ________ affect ORNs

- 10,000,000 ORNs; up to 350 different types of ORs

- ORNs replaced every 5-7 weeks

Linda Buck and colleagues (1999):

- Buck & Richard Axel shared Nobel Prize in 2004

- used fluorescent imaging of binding sites

- odours are encoded _______________

- different odourants encoded by a particular combination of ORs

- but each OR is a component of the code for many odourants

- ORs are like letters of the alphabet, used to make up different words

- about 1,000 _____ code for odour receptors in mammals

- only about 250-400 are active in humans, and vary from person to person

e.g., “_________ _____” phenomenon:

• certain compounds in asparagus are metabolized and excreted in urine, giving it a distinctive smell

• although everyone produces these metabolites, only about 40% of people have the _____ required to smell them

Neural Pathways

- olfactory _____ (axons of ORNs) goes to olfactory ____ (bottom of front of the brain)

• axons of ORNs and dendrites of olfactory bulb neurons form clusters called _________

• evidence shows that glomeruli are grouped according to type of receptor or stimulus molecule (chemotopic map)

- one pathway goes to primary olfactory cortex (piriform cortex) in temporal lobe and then secondary olfactory cortex (orbitofrontal cortex)

- another pathway goes to ______ system (involved in memory, emotions, feeding behaviours)

- olfaction--our most primitive sense--is the only sense that does not travel through the thalamus before reaching the cortex

Threshold

- factors affecting threshold:

• age

• _______

• _________ cycle

• individual differences may be as great as __×

de Vries & Stuiver (1961):

- dogs can detect substances in concentrations 300-10,000× lower than humans

- but: human olfactory receptors triggered by ___ molecule

- dogs have 1 _______ ORNs; humans: 10 _______

- dogs have 100-150 _____ per ORN; humans: 6-8

Identification

Desor & Beauchamp (1974):

- exp’t. 1 task: identify 64 common smells (e.g., fried liver, motor oil, popcorn, _____)

• results: ~__% correct

• good accuracy: coffee, paint, banana, chocolate

• poor: cigar, cat feces, ham, crayon, sawdust (less ________)

- exp’t. 2: observers trained to name 32 smells correctly

• 5 days later, recall smell name: accuracy = 98%

• problem is _________ of smell names from memory

Effects of SARS-CoV-2:

- SARS-CoV-2 attacks sustentacular cells that support ORNs; cilia on ORNs also show damage

- around half of COVID-19 patients experience disorders of taste or smell; most recover their sense of smell

- can be helped by olfactory training: regularly sniffing odourants from several categories; mechanism believed to be plasticity/regeneration of neurons in the olfactory system

de Araujo et al. (2005): top-down effects

- participants presented with isovaleric acid

- ratings were significantly worse when it was labeled “body odour” (-0.86) than when it was labeled “_______ ______” (-0.10)

Psychological Effects of Odourants

____________: study of the influence of smells on behaviour, mood, and emotion

“Aromatherapy”

- the term “aromathérapie” was coined by chemist René Maurice Gattefossé (1928)

- largely based on unsupported claims about the effects of essential oils and/or aroma compounds

e.g., different scents believed to affect mood, alertness, “energy”, “purity”, “harmony”, etc.

Ilmberger et al. (2001):

- presented essential oils including peppermint, jasmine, ylang-ylang, & menthol, vs. control group

- measured motor and RT

- no differences between groups found

- but: some correlation between subjective evaluations and objective performance (effects are _____________, i.e., _______)

Nakamura et al. (2009):

- rats were placed in a restraint, which causes stress

- then inhaled linalool (found in mint plants, citrus fruits, and cinnamon)

- results:

• white blood cell levels returned to normal

• causes changes in expression of 115 genes

- odourants can reverse the effects of stress on the ______ system

Memory

- ______ effect: certain odourants trigger associated autobiographical memories

• named after Marcel Proust (1913), who wrote À la Recherche du Temps Perdu (In Search of Lost Time, or Remembrance of Things Past)

• narrator described how the smell of petites madeleines dipped in tea triggered memories

• this unleashed 3,000 pages of recalled childhood memories

Schab (1990):

- presented 40 adjectives (e.g., large, beautiful)

- task: generate antonyms, while exposed to the smell of _________

- next day: free recall of the antonyms (surprise task)

gp 1) presented smell of _________: 21% recall

gp 2) no smell: 13% recall

- also obtained for odour of mothballs

Herz et al. (2004):

- possible mechanism for enhanced memory is the greater emotionality of odourants

- familiar scent (e.g., perfume associated with a person, place or event) induced greater activity in the amygdala than a control odour (e.g., generic perfume) or visual image (e.g., photo of the perfume bottle)

Pheromones

- definition: chemicals secreted by animals that transmit information to others of the same species

- may affect (but are not limited to) sexual behaviours

e.g., female silkworm moth releases bombykol; can attract a male from over 3 km away

e.g., ants release a trail pheromone to mark their path from a food source

- pheromones detected by ___________ organ (VNO) in base of nasal cavity

• found in many mammals and other animals

• may exist in humans, but is vestigial

- are there human sexual pheromones?

• ____: secreted from abdominal gland of male musk deer

▸ detected in quantities as little as 9.1 × 10^-13 g

▸ close to human ____________

▸ no evidence of effectiveness

• ___________: secreted in human sweat, mostly by women

▸ sex attractant for ____

▸ males smelling it while viewing photos of females rated them more sexually __________

• androstadienone: derivative of male sex hormone testosterone present in _____

▸ improved women’s mood, but only when men were present

- no true human ______ pheromone yet identified--likely only needed by simpler species

Sexual Arousal

Alan R. Hirsch & Gruss (1999):

- measured increase in ______ blood flow due to scents:

Hirsch & colleagues (1998):

- measured increase in _______ blood flow due to scents:

- may be due to:

• ___________ response to sexual partner/favourite food

• nostalgic recall

• __________ of odour

• neurophysiological response

It is unlikely that any of these scents are acting as pheromones; these studies have not been replicated.

For Further Reading

Diane Ackerman. (1990). A natural history of the senses. Vintage.

Christopher Chabris & Daniel Simons. (2010). The invisible gorilla: And other ways our intuitions deceive us. Crown Archetype.

Roger N. Shepard. (1990). Mind sights: Original visual illusions, ambiguities, and other anomalies. W. H. Freeman.

Daniel J. Levitin. (2006). This is your brain on music: The science of a human obsession. Dutton.

Jamie Ward. (2008). The frog who croaked blue: Synesthesia and the mixing of the senses. Routledge.

David J. Linden. (2015). Touch: The science of hand, heart, and mind. Viking Penguin.

Sandra Blakeslee, & Matthew Blakeslee. (2007). The body has a mind of its own: How body maps in your brain help you do (almost) everything better. Random House.

Avery Gilbert. (2008). What the nose knows: The science of scent in everyday life. Crown Publishers.

Bob Holmes. (2017). Flavor: The science of our most neglected sense. W. W. Norton.

John McQuaid. (2015). Tasty: The art and science of what we eat. Scribner.

This document copyright © 1995-2024 Karsten A. Loepelmann. All rights reserved. Viewing this page is taken as acceptance of the copyright agreement.