Taste and Smell L2

Question 1

Lecture 2 — Learning Objectives

Answer 1

Understand characteristics of taste and smell perception

Understand the chemical stimulus, receptors and pathways for taste

Evaluate the limits of human taste sensitivity

Understand the chemical stimulus, receptors and pathways for smell

Evaluate the limits of human smell sensitivity

Explore how taste and smell interact

Question 2

What are the characteristics of taste?

Answer 2

“gustation” is a chemical sense

Chemicals dissolve in our mouth (must be water soluble) and stimulate the taste buds in the oral cavity (tongue, soft palate, cheek, etc.)

Taste aids in the regulation of nutrients and enables the organism to ‘test’ substances prior to ingestion (important for identifying both nutrious foods and harmful substances)

Question 3

What are the characteristics of smell?

Answer 3

“olfaction” is also a chemical sense

Volatile (gaseous) chemicals are inhaled into the nasal passages (or enter via the mouth) where olfactory receptors line the membranes

Smell also conveys important non-nutritive information such as the presence of prey, predators and in some species mates (pheromones regulate sexual activity)

Question 4

How are taste and smell mediated in the first instance?

Answer 4

They are mediated in the first instance by receptors that are stimulated by chemical substances. Receptors are called ‘chemoreceptors’

Question 5

how are taste and smell linked?

Answer 5

Taste and smell are closely linked in that they are both usually involved in many activities such as food seeking and sampling (flavour involves both)

Question 6

How many primary tase qualities do we have?

Answer 6

For humans there are at least 4 basic (‘primary’) taste qualities or sensations: ‘Salty’, ‘sour’, ‘sweet’ and ‘bitter’ (Henning, 1916)

Question 7

What is a salty taste produced by?

Answer 7

Salty taste typically produced by organic salts (e.g. NaCl — table salt)

Question 8

What is a sour taste produced by?

Answer 8

Sour taste comes from acidic subtances (e.g. vinegar)

Question 9

What is a sweet taste produced by ?

Answer 9

Sweet taste produced by carbohydrates and amino acids (e.g. glucose)

Question 10

What is a bitter taste produced by?

Answer 10

Bitter taste produced by alkaloids (often poisonous) (e.g. quinine, strychnine, cocaine)

Question 11

Does taste have a survival value?

Answer 11

As nutritous substances tend to taste sweet and poisonous substances bitter this suggests that ability to sense these qualities has survival value

Question 12

What does taste quality depend on?

Answer 12

Taste quality depends on factors such as substance concentration .E.g. Lithium chloride changes from sweet to sour as concentration increases (Dzendolet & Meiselman, 1967)

Question 13

How many taste buds does human have?

Answer 13

10,000

Question 14

Where are taste buds found?

Answer 14

Found in 3 types of little bumps (papillae) on tongue

Question 15

How many taste buds do each papillae have?

Answer 15

Each papilla has anything from several hundred to just one taste bud (Bradley, 1979)

Question 16

What is the life span of each taste bud?

Answer 16

10 days (Beidler & Smallman, 1965)

Question 17

What are chemicals disolved in saliva in direct contact with?

Answer 17

Chemicals disolved in saliva are in direct contact with microvilli (finger-like structures) of receptor cells

Question 18

Refer to taste bud and tongue labelled model

Answer 18

Folliate curcumvallate
fungiform
filoform ( no taste buds; abrades food)

curcumvilliate papilla
taste buds

supporting cell
receptor cell (taste)
sensory nerve fibres
epethelial cells of tongue
put
taste pore and tips of receptor cells (microvilli)

Question 19

What are the 3 sets of afferent nerve fibers carry taste information derived from the taste buds in the tongue and oral cavity?

Answer 19

Chorda tympani: Front part of the tongue

Glossopharyngeal: Back region of the tongue

Vagus: Throat, pharynx and larynx

Question 20

what do taste stimuli interact with?

Answer 20

receptor sites and ion channels on the microvilli

Question 20

How many types of transduction mechanisms are there?

Answer 20

There are several different types of tranduction mechanisms that convert chemical stimulation into neural responses (see Smith, 1997)

Question 21

How do afferent fibres travel to the nucleii?

Answer 21

Afferent fibers travel to nucleii in the brainstem and then via the thalamus to the primary taste area in the parietal lobe of the cortex (near the somatosensory cortex). Brain-damage impairs taste (Pritchard, 1991)

Question 22

What do the fibres that project to the orbitofrontal cortex do?

Answer 22

Involved in the behavioural significance/reward value of food and perhaps the degree of ‘pleasantness’ of sensory stimuli in general (e.g. Francis et al., 1999)

Question 23

How is taste quality encoded?

Answer 23

Most receptor cells respond to some extent to all 4 basic kinds of taste stimuli, although with different sensitivity (e.g. Arvidson & Friberg, 1980)

Many taste responsive cells in the thalamus also respond to all tastes (Doetsch et al., 1969; Scott & Erickson, 1971)

Question 24

So how does the brain differentiate between different substances?

Answer 24

One solution is Cross-fiber theory (Pfaffman, 1955; Erickson, 1968, 1984)

Although most neurones in the taste system respond to several taste stimuli, each responds best (is ‘tuned’) to a particular substance (e.g. salt)

Question 25

How has cross fibre theory been supported?

Answer 25

Supported by electrophysiological recordings from individual taste sensitive cells in hamster (Frank, 1973), rat (Scott & Chang, 1984) and primate (Pfaffman et al., 1976)

The pattern of firing across different neurones (to a particular stimulus) is thus different

Question 26

Where is info about taste quality/identity coded?

Answer 26

information about taste quality/identity can be coded in the pattern of activity within an ensemble or group of neurones

Question 27

What are the Limits of Human Taste Sensitivity?

Answer 27

Detection thresholds (min. substance concentration)

Question 28

What do detection thresholds for taste depend on?

Answer 28

Substance tested, temperature (McBurney et al., 1973)

mouth region tested (Collings, 1974)

viscosity (Paulus & Haas, 1980)

presence of other substances (Stevens, 1995)

Question 28

When is taste sensitivity greatest?

Answer 28

Taste sensitivity greatest (detection thresholds lowest) between 22 to 32 °C regardless of taste quality

Question 29

Does sensitivity vary across the tongue?

Answer 29

Not all papillae are equally responsive and sensitivity to specific substances varies over the tongue surface

Question 30

How do each papillae effect each part of the tongue?

Answer 30

Front (‘sweet’ & ‘bitter’ tasting substances), back sides (‘sour’), front sides (‘salt’)

Question 31

What is the soft pallate sensitive to?

Answer 31

in addition the soft palate maximally sensitive to bitter tasting substances

Question 32

How have Human Neural and Psychophysical Responses to Taste been studied?

Answer 32

Electrophysiological recording from taste fibers innervating the front of the tongue (chorda tympani) were made during surgery (Borg et al., 1967)

Patient made magnitude (intensity) estimates of taste substances (e.g. sucrose, citric acid, salt) applied in various concentrations on tongue

Question 33

What were the results of Human Neural and Psychophysical Responses to Taste studies?

Answer 33

• Very close correspondence between the 2 sets of data
• Responses of taste nerve and subject both increased with the concentration of the substances
• Neural code for taste intensity appears to be the overall activity evoked by a stimulus
• Subjects can discriminate intensity differences (min. molar concentrations) of ~ 15 to 25 % for sucrose (McBurney, 1978)

Question 34

Which two substances are intensely bitter?

Answer 34

synthesised compounds phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP)

Question 35

What does the ability to taste PTC and PROP depend on?

Answer 35

depends on inheritance of single pair of genes. ‘Tasters’ have one or both dominant genes and ‘nontasters’ recessive genes (Bartoshuk, 1988)

Question 36

How do tasters, nontasters and supertasters react to PTC and PROP?

Answer 36

They taste quite bitter to ~ 50 % of population (‘tasters’), tasteless or undetectable to ~ 25 % (‘nontasters’) and extremely bitter, causing choking or gagging, to ~ 25 % (‘supertasters’)

Question 37

What is different about supertasters?

Answer 37

‘Supertasters’ have twice as many papillae (and taste buds) on tongue than ‘nontasters’

Question 38

How to the different types of tasters react to common substances?

Answer 38

Caffeine in cup of coffee perceived as bitter to ‘tasters’ but not ‘nontasters’ (Hall et al., 1975)

• ‘Tasters’ also perceive KCl (a table salt subsitute) as bitter and that the taste of sodium benzoate (preservative found in many foods) is readily noticeable (Bartoshuk et al., 1988)
• ‘Supertasters’ find many taste stimuli to be much more bitter and ‘hot’ stimuli to be hotter than those who are less sensitive (Bartoshuk, 1993)

Question 39

How have smells been attempted to be categorised?

Answer 39

Henning (1916) proposed 6 primary odour sensations of ‘fragrant’, ‘putrid’, ‘ethereal’ (fruity), ‘burned’, ‘resinous’ and ‘spicy’

• In practice difficult or impossible to classify odours reliably using 6 categories (verbal labels)

Question 40

How does smell differ from taste?

Answer 40

Unlike taste, there is no agreed set of primary odour qualities or sensations and the number of distinct odours is very large (~ 10 000)

The relationship between a substance’s smell and its chemical properties is not straightforward.

Question 41

What are the characteristics of smell?

Answer 41

Typical chemical stimuli for olfaction are organic, rather than inorganic, volatile substances

• Usually composed of complex mixtures of chemical compounds emitted by vegetation, decaying matter and scent-producing glands of animals

Question 42

Does smell have a survival value?

Answer 42

Ability to sense these natural odours has survival value (identification and location of foods, toxic substances, predators and potential mates)

Question 43

refer to smell diagram

Answer 43

olfactory epethelium
baffles
tongue
axon to olfactory bulb
olfactory cell
olfactory binding protien
mucus
odorant
air
cillia

Question 44

What are the Neural Pathways for Smell Perception?

Answer 44

Oderant molecules interact with specific sites on membranes of cilia

Olfactory receptor cells send axons through holes in a bone at the top of nasal cavity, to form the olfactory nerve, to the olfactory bulb of the brain

Axons from the olfactory bulb project to several other regions of the brain including the olfactory cortex (below anterior portion of the frontal lobe), thalamus and lower brain centres of the limbic system (involved emotion)

Question 45

What evidence supports the ‘lock-and-key’ hypothesis (Amoore, 1970)

Answer 45

Specific proteins in cilia membrane (locks) have a unique 3-dimensional structure to which particular oderant molecules (keys) may bind depending on their shape and size

• Initiates a number of biochemical processes that result in action potentials in cell axon

Question 46

How is Odour Quality Encoded?

Answer 46

Olfactory nerve fibers (the axons of olfactory receptor cells) respond to some extent to a wide variety of different odours, but with different sensitivity (Kauer, 1991)

Question 47

What do individual olfctory fibres indicate?

Answer 47

the presence of odorous substances, but alone provide ambiguous information about what it is

Question 48

So how does the brain differentiate between different odours?

Answer 48

One solution is the cross-fiber patterns of activity like those proposed for taste (Kauer, 1987, 1991)

Consequently information about odour quality/identity could be coded in the specific pattern of activity within an ensemble or group of neurones
There is some evidence in support of this idea in the patterns of neural activation across the olfactory bulb (e.g. Skarda & Freeman, 1987)

Question 49

Detection thresholds for smell:

Answer 49

Substance tested, odorant purity, the way it is delivered to the olfactory epithelium and can vary greatly from moment to moment in the same individual (e.g. Stevens et al., 1988)

Question 50

The human olfactory system is remarkably sensitive

Answer 50

For example subjects can detect mercaptan (foul-smelling compound added to natural gas) at a concentration of 1 part per 50 billion parts of air! (Geldard, 1972)

Question 51

Odour sensitivity also depends on other factors such as gender and age

Answer 51

• Females are generally more sensitive, on average, to odours than males (e.g. Koelga & Koster, 1974) and the elderly are less sensitive than young adults (e.g. Cain & Gent, 1991)

Question 52

Many individuals have anosmia (‘odour blindness’)

Answer 52

• Amoore et al. (e.g. 1977) have reported 76 different anosmias. Some common (1 in 3 cannot smell the camphorous odor of 1, 8 cineole) and others rare (1 in 1000 cannot detect putrid n-butyl mercapton). Consistent with the idea of a very large number of receptor types

Question 53

Does Smell Interact with Taste?

Answer 53

Without smell, the ability to identify foods by taste alone is poor (Mozel et al., 1960). Smell also greatly affects food flavour (Murphy & Cain, 1980)