TASTE & RESPONSE TO FOOD IN THE MOUTH

importance of hypotonic saliva

taste buds are able to detect salt at much lower thresholds

what are consequences of lacking the ability to taste food

• failure to recognise food with a high enough caloric content could mean a slow death from malnutrition

• failure to detect a poison could result in near-instant death

• no pleasure/ enjoyment could lead to malnourishment

what senses are involved in the acceptance or rejection of food

• taste

• smell

what are functions of taste

• aids selection of food

• detects particular chemicals

• provides the brain with information about:

  • stimulus intensity

  • duration

  • quality/ edibility of food

  • avoid unpleasant/ noxious substances

• aids digestion by saliva production

what are the types of taste categories

• sweet - energy rich

• sour - typically taste of acids

• salt - important for electrolyte balance

• bitter - sensing of diverse natural toxins

• umami - taste of amino acids e.g. in meat broth, MSG, glutamate and aspartate

  • Japanese for delicious

what factors affect our taste (6)

• genetics

• olfaction

• previous experience

• adaptation

• diseases

• saliva production

• age - saliva production changes which impacts our taste

how does the mouth respond to food taken in

• chewing »

• reflex secretion of saliva for lubrication »

• breakdown of food into smaller particles and mixing with saliva for taste »

• binding of food particles together to form a bolus for swallowing

briefly outline mastication

• chewing, biting or grinding of food

• preparation of food for swallowing

• an unconscious act - voluntary input

what is the cycle of mastication

• bolus of food initiates reflex inhibition of the muscles of mastication, causing the lower jaw to drop

• this initiates a stretch reflex of the jaw muscles that leads to rebound contraction

• the jaw rises to cause closure of the teeth

• bolus is compressed against the linings of the mouth

state the muscles of mastication

• temporalis

• masseter

• lateral pterygoid

• medial pterygoid

what is the function of the temporalis

• elevates and retracts the mandible

• pulls the jaw posteriorly

what is the function of the masseter

• elevates the mandible

• closes the mouth

what is the function of the lateral pterygoid

• has superior and inferior heads

• unilateral contraction - contralateral excursion i.e. mandible moves to the opposite side

• bilateral contraction - pulls the mandible forwards and downwards

what is the function of the medial pterygoid

• has deep and superficial heads

• elevates the mandible and therefore closes the jaw

summarise the functions of the muscles of mastication

• all move the mandible - all insert into the mandible

• secures and stabilises position of the mandible

• determines direction of movements

• involved in biting and chewing

what is the motor innervation of the muscles of mastication

motor fibres of the mandibular branch of the trigeminal nerve (CN V₃)

what structure allows us to taste

papillae (‘taste organs’)

what are papillae

raised protrusions on the tongue visible to the naked eye that contain the taste buds

where are papillae located

on the tongue, soft and hard palate, pharynx, epiglottis and larynx

what are taste buds

groups of 30-100 elongated ‘neuroepithelial’ cells

what are the types of taste buds

4 types of taste buds:

• circumvallate

• foliate

• fungiform

• filiform

outline circumvallate taste buds

• largest papillae

• contain 100s of taste buds

• located on posterior part of tongue forming a V shape

• papillae are a sunken V shaped with troughs separating from surrounding wall - taste buds are located in tiers within the trough

• 3-13 in humans

which taste category do circumvallate papillae provide

sour/ bitter sensitivity to the posterior 2/3 of the tongue

which nerve innervates the circumvallate papillae

• CN IX (glossopharyngeal)

• provides taste and general sensation

histological section of circumvallate papillae

outline foliate taste buds

• on the edge of the tongue, anterior to the circumvallate papillae

• pockets are lined with taste buds

• invaginations protect the taste buds

• 5.4 papillae per side of the tongue with 117 taste buds per papillae

which taste category do foliate papillae provide

• sensitive to sour tastes - difficult to isolate

which nerves innervate foliate papillae

• CN IX (glossopharyngeal)

• chorda tympani - branch of CN VII (facial)

histological section of foliate papillae

outline fungiform taste buds

• located on anterior 2/3 of the tongue

• mushroom shaped

• appear as red spots due to rich blood supply

• papillae at the front of the tongue have more taste buds (1-18) compared to the mid-region (1-9)

which nerve innervates fungiform papillae

• chorda tympani - branch of CN VII (facial)

histological section of fungiform papillae

outline filiform taste buds

• thin V shaped cones

• thread-like

• covers most of the anterior 2/3 of the tongue surface

• NO TASTE BUDS

• have a mechanical function instead - act like ‘velcro’ securing food to the tongue

histological section of filiform papillae

what are taste buds

functional units of (taste) papillae

how many taste buds does the tongue have

> 10,000 can occur singularly or in clusters

how many types of cells does each taste bud contain and what are they

each taste bud contains three kinds of cells:

• epithelial cells

• support cells

• sensory receptor cells

outline epithelial cells

epithelial cells

• basal

• differentiate into new receptor cells

• derived from surrounding epithelium

• renewed every 10 days

outline support cells

support cells

• contain microvilli

• secrete substances into lumen of taste bud

outline sensory receptor cells

sensory receptor cells

• peg-like extensions projecting into lumen

outline the structure of taste buds

• onion shaped

• contain 50-100 continuously maturing cells

• apical microvilli increase surface area

• receptor proteins exposed through taste pores

• ion channels/ receptors located on villi

what is the function of the taste ‘bud’

bud shields the cells from the oral cavity

what are taste buds innervated by

gustatory afferent nerves

what is the life span of a taste bud

10 days

how is a taste bud formed

• cells from the outside of the bud move inwards and develop into taste cells

what happens after a taste cell dies

• it is shed into the oral cavity

• washed away by saliva

• nerve terminals move and innervate a new taste cell

how is taste sensitivity distributed across the tongue

sensitivity to all tastes is distributed across the whole tongue and soft palate/ epiglottis - there are differences in the intensity of sensation from different regions of the tongue

outline the innervation of the gustatory system

taste transduction

• taste impulses from the anterior 2/3 of the tongue pass to the facial nerve

• impulses from the posterior regions of the tongue and throat pass through CN IX (glossopharyngeal)

• impulses from the base of the tongue pass through CN X (vagus)

—

• these impulses travel to the gustatory nucleus in the medulla which then sends impulses to the thalamus and cortex

what information does CN V (trigeminal) convey

• thermal sensation

• tactile sensation

• pain sensation

each taste ________ has a different ________ and a different ____________ mechanism

each taste modality has a different receptor and a different transduction mechanism

what are the 2 transduction mechanisms

• ionotropic

• metabotropic

what is the transduction mechanism dependent on

the receptor

which taste modality is the ionotropic mechanism associated with

salt

outline the ionotropic transduction mechanism

• movement of Na ions through Na channels causes depolarisation

• Ca then enters through the voltage sensitivity Ca channels

• this causes neurotransmitter release and the firing of primary afferent nerve - synaptic transmission

which taste modalities are the metabotropic mechanism associated with

• sweet

• bitter

• umami

outline the metabotropic transduction mechanism

• sweet, bitter and umami converge on a common transduction channel - TRPM5 (via phospholipase C)

• TRPM5 is a calcium activated, non-selective cation channel

• depolarisation is induced following increases in intracellular Ca²⁺

• this causes the channel to open, producing a depolarising generator potential which leads to an action potential

—

indirectly linked with ion channels on the plasma membrane of the cell through signal transduction mechanisms such as G protein-coupled receptors

what transient receptor channel does sweet taste stimuli (glucose) bind to

• T1R2

• T1R3

what transient receptor channel does umami taste stimuli bind to

• T1R1

• T1R3

what transient receptor channel does bitter taste stimuli bind to

• T2R

what is the taste of sourness essentially the taste of

sour tasting is essentially tasting acid (H+)

what is the acid-sensing channel

PKD2L1 channel (Polycystic kidney disease 2-like 1 protein)

also known as

TRPP2 channel (Transient receptor potential polycystic 2)

what type of channel is PKD2L1

a non-selective cation channel

how does sour taste protect the oral cavity

• sour tasting protects intra oral tissues from damage from acidic compounds

which taste modality is the strongest activator of saliva secretion

sourness - because saliva will buffer the protons/ H+

miracle berry or Synsepalum dulcificum - originally found in West Africa

what is the effect of ingesting miracle berry and what is it due to

• miracle berries cause sour foods to taste sweet

• this is due to a protein called miraculin

how does miraculin work at neutral and low pH

• miraculin binds to taste buds causing sour foods to taste sweet

• neutral pH: miraculin binds and blocks the receptors

• low pH (sour foods): miraculin is able to activate the sweet receptors, resulting in the perception of sweet taste

how long does the effect of miraculin last

lasts until the protein is washed away by saliva i.e. 30 mins to 1h

what are the uses of miracle berry

• sweeten palm wine

• improve the flavour of sour cornbread - miracle berry is chewed first, then food is ingested

• used as a sweetener and flavouring agent for lots of beverages and foods e.g. beer, cocktails, vinegar, pickles

process of tasting

taste modality thresholds

newborns tasting abilities

• indifferent/ averse to salt

• breast milk is sweet

at what age do babies start developing a preference for salty tastes

6-24 months

what is the range of taste cells per individual

500-10,000

who are supertasters

• more papillae - particularly fungiform on their tongues than usual

• they therefore have more taste receptors

% of population that are supertasters and number of papillae per cm

• 25% of the population - more women than men

• 165 papillae per cm

% of population that are normal tasters and number of papillae per cm

• 50% of population

• 127 papillae per cm

% of population that are non-tasters and number of papillae per cm

• 25% of population

• 117 papillae per cm

what substance is the major ingredient in tonic water

quinine - bitter taste

what does PTC stand for

phenylthiocarbamide

what does the ability to taste PTC determined by

genetics - genes encode taste receptor on tongue

what does PTC taste like to supertasters/ tasters VS non-tasters

bitter VS no taste

what is the frequency of non PTC tasters in

a) West Africa

b) China

c) India

d) Europe

a) West Africa: 3%

b) China: 6-23%

c) India: 40%

d) Europe: 30%

what kind of sensitivity does the tongue have to different taste modalities

regional sensitivity

where is sour taste usually perceived

at the sides of the tongue

where is bitter taste usually perceived

at the back of the tongue

where are salt and sweet tastes usually perceived

at the front of the tongue

how many modalities is the olfactory system sensitive to

1000s

what does a disorder of olfaction e.g. cold lead to

altered gustation i.e. loss of taste

how does adaptation affect taste

repeated exposure to the same taste modality can induce a process of adaptation - decreased perceived intensity to the same concentration of a stimulus

how can adaptation to a taste be reset

by exposure to water

how saliva contributes to taste

• taste stimuli are dissolved in saliva (solvent)

• this allows them to reach and interact with taste receptors

—

• ionic composition of saliva can affect the resting membrane potential of taste cells as the cells are bathed in saliva

• this affects how sensitive taste cells are to taste stimuli i.e. taste perception

when are sweet, salt, sour and bitter compounds tasted

only when host threshold levels are exceeded