Sensory aspects of eating and drinking

Question 1

What sensory systems are involved when we eat and drink? (5)

Answer 1

– Smell – many qualities (but it is the ‘hidden sense’)
– Taste – few qualities, but motivationally significant
– Skin senses (touch)
• Common chemical sense – very few qualities (whole body, especially mucosa)
• Somatosensation/Proprioception – few qualities (static & dynamic)
• What we perceive is an integrated sensation - flavour

Question 2

What is the sense of taste and its 6 different sensations?

Answer 2

• Located primarily on the surface of the tongue
• We appear able to perceive several qualitatively different sensations (note hedonics-function)
• Sweet (e.g. sucrose, saccharine) – Energy - Pleasant
• Sour (e.g. acids) – Ripeness/Vitamin C, fermentation (bacteria) – Un/Pleasant
• Bitter (e.g. plant alkaloids) – Toxicity (LD50 correlation) - Unpleasant
• Salty (e.g. mineral salts) – Depletion & Preference (Miners) – Un/Pleasant
• Umami (e.g. MSG) – Allergy quackery (in toms, cheese, breast milk) - Pleasant
• Fat (e.g. fatty acids) – Energy – BUT may have no conscious correlate

Question 3

What are the 3 receptors on the human tongue?

Answer 3

– Receptors are located in structures called taste buds
– Taste buds are grouped into structures called papillae
• Vallate papillae (fried eggs) - 9 in adults, 250 buds/papillae (function: swallow reflex – last chance to check?)
• Foliate papillae (ridges) - 10 in adults, 120 buds/papillae
• Fungiform papillae (dots) - 30/cm2 [tip] - 8/cm2 [mid], 3 buds/pap (function: most sensitive – immediate detection of tastants?)

Question 4

What is a taste bud? (5)

Answer 4

• Each bud contains cells with microvilli
• These cells last 2 days
• The bud is filled with mucus
• Each bud may have more than one type of receptor located on the microvilli
• Tongue is equally sensitive to all tastes in any area and most sensitive to all of them at the tip of the tongue

Question 5

What are the 2 types of taste receptors?

Answer 5

• There are two basic types of receptor present upon the taste bud’s microvilli
– Ion gated channels
• Salt detectors (Na+)
• Acid detectors (H+)
– Protein gated channels
• Sweet, bitter, umami, fat
• It appears that each of these may occur in several forms
– For bitter – may be 14 different receptors perhaps driven by selection pressure to avoid poison?
– For sweet – just one receptor

Question 6

Where do taste receptors project to in the brain? (6)

Answer 6

• After the cell depolarises an action potential passes along onto the chorda tympani nerve
• The first major processing point is the Nucleus of the solitary tract in the brain stem
• Information is then routed along two discrete pathways
– To the brain stem (ingestive/protective reflexes)
– To the insula and orbitofrontal cortices (perception of taste quality, intensity & hedonics)
• The insula is primary taste cortex, and the orbitofrontal cortex, can be thought of as secondary taste cortex
• Patients with discrete insula lesions are able to tell a taste is present, but have some trouble with its quality
• The insula also supports taste-related functions – notably the emotion of disgust

Question 7

What is the emotion of disgust? (12)

Answer 7

• Animals including humans respond with disgust to bitter tastes
• In humans disgust seems to occur to a much broader range of stimuli than just bitter tastes (in contrast to animals)
• Disease cues (body products, body envelope violations, death, spoiled food, signs of ill-health etc)
• Incest
• Perhaps even to some moral violations
• Disgust responding involves
• A characteristic facial expression
• A particular qualia - revulsion
• Nausea
• An intense desire to withdraw
• If the elicitor is touched - contamination
• A preparatory immune response
• Disgust responding/perception is impaired in people with damage to their insular cortex (e.g., in Huntington’s chorea)

Question 8

How does the brain form a representation of what is stimulating the taste receptors?

Answer 8

– Labeled lines (stimulus A – receptor for A activates only A sensitive neurons – thus the presence of ‘A’ is determined)
– Patterns (stimulus A – generates a unique pattern of activity across many neurons – presence of ‘A’ is determined by recognizing ‘A’s’ unique neural pattern)
• A pattern based explanation assumes that the brain recognises a pattern of activity across many nerve fibres and that different patterns produce different taste qualities

Question 9

What is the evidence for labeled lines in taste?

Answer 9

– Certain fibers in the chorda tympani are selectively responsive to different tastes (i.e. fibre X is only active when salt is tasted)
– On this basis we might assume that when fibre X is active, this results in a ‘salty taste’ qualia
– Such selective fibres have been observed for sweet, salty, sour and bitter
As increase [NaCl], increase activty of fiber but others subs dont increase activity of that fiber

Question 10

How do we taste?

Answer 10

• Basic qualities may be defined by activity within specific nerve fibres (labeled line)
– For example, ‘salty’
• But whether it is one sort of saltiness or another may depend upon the pattern of activity
– For example, ‘metallic salty’ vs ‘mineral
salty’

Question 11

What are individual differences in taste? (10)

Answer 11

• Some people could taste a very bitter substance called PTC and others could not
• This difference was genetically determined
• There are large and significant individual differences in sensitivity to PROP
• Recent research suggests three groups of people
– Non-tasters (30%), tasters (40%) and supertasters (20%)
• Supertasters find PROP disgustingly bitter
• Supertasters appears to have more taste buds than, tasters and non-tasters. This has the following effects
• Greater sensitivity to sweet & bitter tastes
• Dislike for bitter tasting vegetables (especially sprouts and other members of the Brassicae family including cabbage, broccoli and cauliflower)
• Greater sensitivity to irritants such as chilli and carbonic acid (responsible for ‘fizz’ in carbonated drinks)
• Supertasters are often leaner as well, as they may be more sensitive to fats in food (and so need less fat to get equal ‘pleasure’)

Question 12

What does the stimulation of the taste system do?

Answer 12

Stimulation of the taste system also stimulates the production of saliva which assists digestion and makes food more palatable

Question 13

How does our taste change with age?

Answer 13

• Our ability to taste declines with age, but not until we are into our late 60’s
– Reductions in taste sensitivity are associated with lower body weight in the elderly and with reduced appetite

Question 14

What is the common chemical sense? (5)

Answer 14

• The primary function of the common chemical sense is to allow for the speedy identification and removal of harmful chemical irritants from the skin
• Humans (unlike most animals) actively seek to add irritants to their diet
• It is called the common chemical sense (CCS) as it is located over the whole area of the body but receptors are more densely grouped on the mucosa
• In the mouth, many CCS receptors are located around the base of taste buds, so if you have more taste buds, you have more of these receptors too
• When these receptors are stimulated in sufficient number the body has a reflex response
– Tears, salivation, running nose, sweating

Question 15

What do we perceive with the CCS? (6)

Answer 15

• The receptors responsible for the CCS are called ‘free nerve endings’ and appear to
– Detect temperature (hot/cold) - confusion studies with capsaicin and menthol
– Damage from excessive temperature
– Chemical stimulation
• Many researchers believe that we can only experience the following sensory dimensions
– Intensity (weak to strong)
– Hot/Cold (quality; could be more - Anosmic studies)
– Hedonics (pleasure to pain)

Question 16

Why (3) and how (4) do we like the burn of hot peppers?

Answer 16

• Why?
– Bland diets, Rice (Asia), Corn (Mexico) – salivation
– Medicine effect - Vitamin C
– Release of endogenous opioids – Naloxone study
• How?
– In Mexico exposure starts around 7 years
– Chilli sauce is always available, but children are never forced into using it
– Concentration is gradually increased
– It appears that people learn to love it (i.e. they come to know that it does not harm them and this then allows them to enjoy the ‘burn’)

Question 17

How do we smell? (7)

Answer 17

• Our smell receptors are located behind the bridge of the nose and can be accessed by two separate pathways
– Sniffing (orthonasal)
– Via the back of the throat (retronasal)
• Each of these pathways is associated with its own perceived location
• Sniffing makes us feel that an odour is located in the environment, while when the odour is in our mouth, it is perceived as part of that food – how does this happen?
– This type of question is called a ‘binding problem’ and is a major issue for cognitive neuroscience
– Odour location binding may be caused by nasal airflow direction and by inhibition of olfactory attention by the presence of a taste in the mouth
• Much of the sensation that we term ‘taste’ or ‘flavour’ when we eat and drink is in fact smell
• The sensations that we can experience in this modality appear to exceed the other flavour senses by many orders of magnitude

Question 18

What happens with the nasopharynx during the different phases of eating and drinking?

Answer 18

• During certain phases of eating and drinking volatiles ascend via the nasopharynx and bind to the same receptors that are stimulated during sniffing
– Volatiles in food are pumped into the nasopharynx during chewing and on exhalation, when the soft palate (velopharyngeal flap) opens
– This flap is normally shut during eating and drinking to stop food and drink getting into the nose

Question 19

What is the olfactory receptor surface? (3)

Answer 19

• We have about 4-6cm2 of receptor tissue - the olfactory mucosa
• The tissue is bathed in mucus and the ORN’s extend microvilli into this medium
• The mucosa has a variety of functions
– Clearing ‘old’ smells away
– Transport
– Protection

Question 20

What are the olfactory receptors? (5)

Answer 20

• There are between 300-500 different olfactory receptors in humans and maybe 800+ in rodents
• Each olfactory receptor neuron on the epithelium expresses just one type of receptor
• All belong to a group called GProteins
• Chemicals bind to the G-Protein and result in depolarisation of the cell and an action potential

• There may be other classes of receptor that are sensitive to reproductive related chemicals
- Scent of symmetry
- Faces vary in symmetry
- More symmetrical faces are liked more
- The smell from people with more symmetrical faces is liked more too
- MHC (Major histocompatability complex) type – immune genes
- Needs to be different between sexual partners to maximise off-spring fitness
- Partners with dissimilar MHC have more kids
- Partners with similar MHC have more miscarriages
- Even female perfume choice seems to be selected to complement MHC type
- Smell seems to be our main mode for detecting MHC type

Question 21

What are olfactory glomeruli?

Answer 21

• As you know each ORN expresses one type of receptor
• The olfactory receptor types are randomly distributed across the olfactory epithelium
• Each receptor type is sensitive to different chemicals but there is considerable overlap in sensitivity
• Information from each receptor type converges on a structure called a glomeruli in the olfactory bulb
• There are about the same number of glomeruli (300-500) as there are receptor types (300-500)
When we sniff something there is a spatial (and temporal) pattern of activation across all of the 300-500 glomeruli – as we will see this is crucial to how we manage to perceive odours

Question 22

How does olfactory information flow to the brain?

Answer 22

• Information from the glomeruli in the olfactory bulb (OB) travels then to the olfactory cortex (PC - paleocortex), orbitofrontal cortex (OFC; neocortex), amygdala (AC;
fear), mediodorsal thalamus (MD; attention role) and the hypothalamus (Hy)
• The neural architecture of olfaction is unique amongst the senses
– Direct access to neocortex without obligatory thalamic processing
– Initial paleocortical processing
– Direct access to hippocampus & amygdala

Question 23

How do we smell?

Answer 23

• In essence our sense of smell is a pattern recognition system
• Most odours are complex mixtures of chemicals - coffee contains 600 or so volatile (i.e. smelly) chemicals, but we just perceive ‘coffee
Each olfactory receptor type is sensitive to many different chemicals
• This effectively rules out ‘labeled lines’ just as the complex nature of the stimulus does too (i.e. we don’t have a ‘coffee’ receptor)
• Rather the brain uses the pattern of activity across the 300-500 glomeruli to recognise the odour
• It appears to do this by matching the glomerular pattern to
patterns that have already been experienced before (and encoded in to odour memory)
• Crucially, it is this pattern matching process that generates our conscious perception of odour quality (that’s coffee!)
• We all have different smell worlds that are dependent upon our history of smelling
• Children are poorer at telling odours apart than adults, even though they have normal acuity (they can detect whether an odour is present or absent)
• Different cultures perceive different culturally specific odours in different ways

Question 24

Are wine tasters really experts?

Answer 24

• Wine tasters are somewhat better, but not much
– They are no better at discrimination than regular wine drinkers, but both are better than non-wine drinkers
– They can match a description they gave to a particular wine about 48% of the time, compared to 28% regular wine drinkers
– Their expertise lies in applying language to sensation and knowing feature clusters associated with particular varieties (and some tricks)
– The phenomenon of verbal overshadowing

Question 25

How does somatosensation and proprioception work in the mouth?

Answer 25

• We have, in the mouth, a range of receptors located in and on the tissue surface, and deep in the muscles and joints of the mouth (as with elsewhere in the body)
– Somatosensation is our perception of objects (and their properties) contacting the body – it is both an active and passive sense
– Proprioception is our perception of the location of our muscles and joints in ‘space’
– They are intimately related and I’ll deal with them as one‘system’
• These are important in feeling
– Pressure (i.e. chewing food)
– Texture (i.e. crispness & fattiness)
– Astringency (i.e. pinched-up & shriveled, such as tannins in wine)
• This is probably the most poorly explored sensory system
in the context of flavour, but it is clearly important in the
perception of texture and fat

Question 26

How do we perceive fat?

Answer 26

• A significant component of fat perception in food involves texture
• Descriptive terms for fat are textural
– Greasy, oily, creamy, thin, watery
• Fat content can be accurately gauged by the fingers alone
• However, this is not the whole story
• Smell (rat studies - Yes vs human studies – No?)
• Taste (as noted earlier) - currently contentious…
– In rat ‘yes’, they have receptors that detect fat
– Humans do appear able to discriminate fats in the absence of textural and olfactory cues, which just leaves taste (sort of)

Question 27

What is flavour?

Answer 27

• No language surveyed has a term that distinguishes the
olfactory from the taste component of eating & drinking
– Taste and smell in the mouth seem to be treated linguistically as a single entity (contrast with ‘red’ and ‘green’ for example)
• When people lose their sense of smell, they typically report also having lost their sense of taste, as food now tastes bland
• People are poor at discriminating the components of flavours, even if trained to do so, and especially odours
• While children know you need eyes to see and ears to hear, most adults do not know that you need a‘nose to taste’
• So we can conclude – broadly – that at least for the two major components of flavour, taste and smell, these seem to be treated as a single entity in the mouth

Question 28

What is Odour-taste synesthesia?

Answer 28

• Synesthesia refers to the experience of a sensation normally associated with one sensory system, when another sensory system is stimulated
• Most sysnesthesias are rare (<1 in 10,000) but odour-taste
synesthesia is universal
• Synesthesias, even these rare ones appear to be learnt
• For odour-taste synesthesia it arises from the frequent co-occurrence of certain smells and tastes
• Apart from rarity the key difference between these rare synesthesias and odour-taste synesthesia is awareness
• People become aware that they are synesthetic - that is they recognise that the experience of the colour RED whenever they see the letter A is unusual
• People are not aware that when they describe an odour as smelling‘sweet’ that this is a form of synesthesia, because they do not appreciate that ‘sweet’ is a quality associated with the sense of taste
• Not only can smells acquire ‘taste-like’ properties they can also:
– Acquire fat-like properties (smell fatty)
– Acquire irritant-like properties (smell acidic)

Question 29

How does the brain make “flavour”?

Answer 29

• Information from taste, smell, irritation and proprioception first all converge (along with visual and auditory information too) in the orbitofrontal cortex
• In this structure there are cells that respond to combinations of information from different senses and it may be here that our unitary sensation of flavour arises