Physiology of Gustation by Dr Bisrat Getachew.(Beba)

1. Prepared by- Bisrat G. / ORL-HNS (R1)
Moderator - Dr. Alene / ORL - HNS Surgeon
April 2018 GC

2. Outline
 Introduction
 Development of Taste System
 Anatomy of Taste system
 Basic taste Parameters
 Central and Peripheral Gustatory Pathways
 Taste measurement
 Clinical correlates
 Bibliography

3. Taste
 Taste is the sensation produced when a substance in the mouth reacts
chemically with taste receptor cells located on taste buds in the oral cavity,
mostly on the tongue.
 Taste is a form of chemoreception which occurs in the specialised taste
receptors in the mouth.
 Taste, along with smell (olfaction) and trigeminal nerve stimulation
determines flavors of food or other substances.
 This system is present in both vertebrates and invertebrates and well
developed even in a newborn infant.

4. DEVELOPMENT OF THE TASTE SYSTEM
 Evidence shows that there is a functional taste system
present in utero.
 Even premature infants respond pleasurably to sucrose
and glucose.
Tongue Development

5. Anatomy of the Taste system
 Taste buds are located throughout the oral cavity,
In the pharynx,
The laryngeal epiglottis, and
At the entrance of the esophagus.
 Taste buds on the dorsal lingual epithelium are the
most numerous 4000-5000 per tongue.
Each taste bud contains 50 to 100 taste receptor
cells.
 Each with a taste pore opens into bud's centre, termed
the taste pit.

6. Cont…
 The tongue is covered with thousands of small bumps
called Papillae which are visible to the naked eye.
 Here, taste buds are contained within three major classes of
papillae, the fungiform, foliate, and circumvallate, while the
filiform papillae are nongustatory.
 Microvillae, on which the taste receptors that interact with
the tastants are located, project from these epithelial cells
into the taste pit.

7. Cont…
Taste buds are aggregations of 30 to 100
individual elongated neuroepithelial cells (50 μm in
height, 30 to 70 μm in width).
Taste cells are bipolar cells, which connect to
the epithelial surface of the oral cavity via
dendritic processes and to a nerve axon at the
base which
invades the bud and ramifies extensively.
Oral cavity
Bipolar cells
Nerve axon

8. Fungiform papillae
 Most densely distributed on the tip and sides of the tongue.
 These papillae generally contain one to several taste buds per
papilla.
 They are innervated by the chorda tympani branch of the facial (VII
cranial) nerve and appear as red spots on the tongue because they
are richly supplied with blood vessels.
 The total number of fungiform papillae per human tongue is
around 200.

9. Foliate papillae
 Situated on the edge of the tongue slightly anterior of the
circumvallate line.
 They are predominantly sensitive to sour tastes and are innervated
by the glossopharyngeal (IX cranial) nerve.
 Average five foliate papillae per side of the tongue with
approximately 120 taste buds per foliate papillae.

10. Circumvallate papillae
 are sunken papillae, with a trough separating them from the surrounding wall.
 The taste buds are in tiers within the trough of the papillae.
 They are situated on the surface of the circumvallate line and form an inverted V on
the posterior of the tongue.
 They confer a sour/bitter sensitivity to the posterior one-third of the tongue.
 They are innervated by the glossopharyngeal (IX cranial) nerve.
 There are 3 to 13 circumvallate papillae per tongue with about 250 taste buds per
papillae.

11. Cont…
 The filiform papillae are mechanical and nongustatory.
 In addition, there are 2,500 taste buds on the epiglottis, soft palate, and
laryngeal and oral pharynx.

12. Cont…
 The lifespan of a mammalian taste cell is approximately 10- 14
days.
 Cells in each taste bud contain the
 sensory receptor cells,
 supporting cells, and
 basal cells that differentiate into new
receptor cells.
NB. As a cell ages => a nerve terminal detaches, :finds a
developing cell, and forms a new synapse.
The new cell has to be of the same taste specificity as the old
dying cell

13. Basic Taste Parameters
 Taste in the gustatory system
allows humans to distinguish between safe and harmful food,
and
To gauge foods’ nutritional value.
 The five specific tastes received by taste receptors are
saltiness, sweetness, bitterness, sourness, and savoriness,
often known by its Japanese term "umami" which translates to
‘delicious’.

14. Sweetness
 Sweetness, usually regarded as a pleasurable sensation, is produced by the
presence of sugars and a few other substances.
 Sweetness is often connected to aldehydes and ketones, which contain
a carbonyl group.
 Sweetness is detected by a variety of G protein coupled receptors coupled
to the G protein gustducin found on the taste buds.
 Taste detection thresholds for sweet substances are rated relative
to sucrose, which has an index of 1.
 The average human detection threshold for sucrose is 10 mmoles per
liter.

15. Sourness
 Sourness is the taste that detects acidity.
 Sour taste is detected by a small subset of cells that are
distributed across all taste buds in the tongue.
 The sourness of substances is rated relative to dilute hydrochloric
acid, which has a sourness index of 1.
 The most common food group that contains naturally sour foods
is fruit, such as lemon, grape, orange, tamarind, and
sometimes melon.

16. Sourness cont…
 At low concentrations, it is pleasant but once it reaches high
concentrations it becomes an unpleasant taste and leads to avoidance.
 It guides acid-base regulation and is sensitive to extracellular pH changes

17. Saltiness
 The simplest receptor found in the mouth is the sodium chloride (salt) receptor.
 Saltiness is a taste produced primarily by the presence of sodium ions.
 The saltiness of substances is rated relative to sodium chloride (NaCl), which has an
index of 1.
 Salt-water homeostasis.
NB : The size of lithium and potassium ions most closely resemble those of sodium, and thus the
saltiness is most similar.

18. Bitterness
 Bitterness is the most sensitive of the tastes, and many perceive it as unpleasant,
sharp, or disagreeable, but it is sometimes desirable.
 Common bitter foods and beverages include coffee, unsweetened cocoa, South
American mate, bitter gourd, olives,
 Quinine is also known for its bitter taste and is found in tonic water.
 The taste thresholds of other bitter substances are rated relative to quinine, which
is thus given a reference index of 1.
 Warning system designed by nature to protect against the ingestion of harmful
compounds

19. Savoriness /Umami/ (旨味)
 Savory, or savoriness is an appetitive taste and is occasionally described by its
Japanese name, umami or meaty.
 Umami, or “scrumptiousness”, was first studied with the scientific method and
identified by Kikunae Ikeda.
 He isolated a substance he called ajinomoto, Later identified as the
chemical monosodium glutamate(MSG), and increasingly used independently as a
food additive.
 This taste helps guide the intake of peptides and proteins.

20. Pungency / spiciness / 6th taste
 Substances such as ethanol and capsaicin cause a burning sensation by inducing
a trigeminal nerve reaction together with normal taste reception.
 This particular sensation, called chemesthesis, is not a taste in the technical sense,
because the sensation does not arise from taste buds, and a different set of
nerve fibers carry it to the brain.
The piquant ("hot" or "spicy") sensation provided by such foods and spices plays an important role in a
diverse range of cuisines across the world—especially in equatorial and sub-tropical climates, such
as Ethiopian, Peruvian, Hungarian, Indian, Korean.

21. Coolness
 Some substances activate cold trigeminal receptors even when not at low
temperatures. This "fresh" or "minty" sensation can be tasted
in peppermint, spearmint, menthol, ethanol, and camphor.
 Unlike the actual change in temperature described for sugar substitutes,
this coolness is only a perceived phenomenon.

22. Taste Transduction
 Stimuli entering the mouth First interact with sites on the microvilli within the taste
pore.
 2nd - Taste receptors trigger transduction cascades, which then activate synapses
and cause the excitation of nerve fibers.
 3rd - A signal is produced, which is then carried to the brain, relaying information
on the identity and intensity of the gustatory stimulus.

23. INNERVATION OF THE TONGUE
 Taste receptor cells do not have an axon. Information is relayed onto
terminals of sensory fibers by transmitters.
 These fibers arise from the ganglion cells of three cranial nerves.
 CN VII supplies
 the buds within the fungiform and filiform papillae on the anterior two-thirds
of the tongue (via the chorda tympani nerve),
 as well as those of the soft palate (via the lesser palatine nerve branch of the
greater petrosal nerve).
 A branch of the trigeminal nerve serves general sensation.

24. Cont…
 CN IX via its lingual tonsillar branch, supplies the taste buds of
the circumvallate papillae and most of those of the foliate
papillae within the posterior third of the tongue.
 The superior laryngeal nerve, a branch of the vagus, innervates
taste buds on the laryngeal surface of the epiglottis.
 Taste buds within the nasopharynx are innervated by its
pharyngeal branch.

25. Central Pathways and neural communications
 Taste fibers from cranial nerves VII, IX, and X form part of the fasciculus solitarius
and synapse centrally in the medulla (in a thin line of cells called the nucleus of
the solitary tract)
 From there, the information is relayed to the somatosensory cortex for the
conscious perception of taste and to the hypothalamus, amygdala, and insula,
giving the so-called affective component of taste.
 NST receives input from the amygdala (regulates oculomotor nuclei output), bed
nuclei of stria terminalis, hypothalamus, and prefrontal cortex. NST is the
topographical map that processes gustatory and sensory (temp, texture, etc.)
information

26. Cont…
 Reticular formation (includes Raphe nuclei responsible for serotonin production)
is signaled to release serotonin during and after a meal to suppress appetite.
 Hypoglossal and thalamic connections aid in oral-related movements.
 Hypothalamus connections hormonally regulate hunger and the digestive system.
 Edinger-Westphal nucleus reacts to taste stimuli by dilating and constricting the
pupils.
 The frontal operculum is speculated to be the memory and association hub for
taste.
 The insula cortex aids in swallowing and gastric motility.

27. Cont…
 Taste nerves normally inhibit one another across the midline.
 Therefore. when a nerve is damaged there is a decrease in
inhibitory impulses from that side.
 This in turn leads to an increase in responses from the opposite
side. this disinhibition of the damaged nerve leads to an overall
presentation of taste and total ageusia or loss of taste is very rare.

28. Taste testing and background
 Taste studies looked at two primary aspects of taste: acuity and intensity.
 Taste acuity measures two different parameters:
(a) taste detection or threshold testing, which is a measure of the
concentration at which a distinction can be made between
a taste substance (tastant) and water;. and
(b) taste recognition, that is, which is sweet, sour;. salty, or bitter.
 Taste intensity is a suprathreshold measure of taste qualities.

29. Cont…
Whole mouth testing
 In the whole--mouth taste testing, five concentrations of sucrose. sodium chloride,
citric acid, and caffeine are presented in a solution form. the
subject sips, swishes, and expectorates it.
 The subject then indicates as sweet,. salty, sour, or bitter and rates its intensity on a
segmented visual analog scale (VAS) with the extremes labeled as very strong and
very weak.

30. Cont…
Regional taste testing
 Well validated taste test used by the several chemosensory centers
to assess gustatory dysfunction
 Employs four suprathreshold stimulants:
 NaCl for salty,
 Sucrose for sweet
 Acetic or citric acid for sour, and
 Quinine sulfate or caffeine for bitter.
 A drop of the tastant is placed on the tongue with a micropipette and the subject is
asked to identify the solution

31. Cont…
 A total of 96 trials are employed ( 4 tastants x 6 trials x 4 tongue regions) and is
generally administered over 45 minutes
 This is a forced choice test in which patients are asked to identify solutions, with
their tongue extended from their mouths, by pointing to a card labeled, ‘sweet,'
sour; "bitter" and "salty.“
 Patients are asked to rinse their mouths with distilled water between presentations.
 When a patient is unable to identify a solution. he or she is encouraged to guess.

32. Intensity Testing
 Threshold or intensity testing is much more difficult.
 Intensity testing involves a VAS, which is a 9-level scale of increasing shades of gray
with the minimum at one end and maximum at the other.
 The problem with such testing is that the strength of a tastant is likely to vary
across individuals, for example, the most intense sweet experienced by a
supertaster is much more intense than the same concentration experienced by a
nontaster.

33. Electrogustometry
 Electrogustometry utilizes a weak electric stimulus to produce a sour
taste when applied to taste receptors.
 It is useful to measure taste in specific loci of the anterior tongue.
 Its main disadvantage is the inability to measure specific tastes
 Electrogustometry is more practical in clinical settings than
chemogustometry.

34. Videomicroscopy
 Described by Miller and Reedy in 1990
 It is a noninvasive procedure designed to provide a 50 to 100-fold magnification
to resolve the number and location of taste pores on the lingual papillae without
destructive tissue preparation.
 Videomicroscopy (VM) allows the study of taste perception in terms of quantified
taste receptor organs.
 VM has the capability to successfully view, capture, and store high-resolution
digital images.

35. Terms
Supertasters
 A supertaster is a person whose sense of taste is significantly more sensitive than
average.
 The cause of this heightened response is likely, at least in part, due to an increased
number of fungiform papillae.
Aftertaste
 Aftertastes arise after food has been swallowed. An aftertaste can differ from the food
it follows.
 Medicines and tablets may also have a lingering aftertaste, as they can contain certain
artificial flavor compounds, such as aspartame (artificial sweetener.

36. Disorders of Taste
 Ageusia: Absence of sense of taste
 Dysgeusia: Disturbed sense of taste
 Hypogeusia: Diminished sense of taste
 Phantom Taste >>>>>>
Gustatory agnosia - inability to recognize a taste sensation, even though the taste
processing, language and general intellectual functions are intact.

37. TASTE ALTERING CONDITIONS
Aging
 While age causes a decrement in all taste sensitivities, sweet taste in almost all
studies reviewed seemed the most robust and survived the longest
 Studies M>F
 More papillae => more sensitivity => lower threshold
 Found degenerative changes in taste receptor with a regression of gustatory
papillae.
 Olfaction is more affected than taste and leads to a decrease in flavor overall.

38. Cont…
Systemic Diseases
 Systemic diseases like diabetes, gastric disease, hypothyroidism, liver disease,
chronic renal failure, Alzheimer and Parkinson disease affect taste.
 Premenopausal women often complain of the “burning mouth syndrome," which is
believed to be related to a lowering of the bitter taste threshold and a release of
oral pain inhibition
 Smoking affects taste especially the bitter sense.

39. Cont…
Nerve Damage
 This can occur from surgery involving pressure on the gustatory nerve, for example,
during a tonsillectomy, middle ear surgery, or from a resection of the nerve
involved with a tumor.
 This symptom may be reversible within 2 years after tonsillectomy.
 Patients should be informed of the risk of postoperative taste disturbance after
tonsillectomy as being one of the rare complications of this surgery.

40. Cont…
Radiation and Chemotherapy
 A metallic taste, is a well-known complication of radiation and chemotherapy for
head and neck cancer.
 longitudinal study on newly diagnosed head and neck cancer patients and found
that sour taste was most affected after radiation and chemotherapy.
The number of taste buds was also significantly decreased after radiation. While taste generally
recovered after 6 months of therapeutic radiation, the number of taste buds did not return to the
preradiation levels.

41. Cont…
THERAPY FOR TASTE LOSS
 No specific therapy or pharmacologic agents are available to reverse the loss of
taste with aging and in general the prognosis is poor.
 Suggested that zinc supplementation be tried to enhance taste along with vitamins
A, B12, and folic acid for the maintenance of taste bud integrity.
 Epidermal Growth factor ???

42. Bibliography
 Scott brown ORL – HNS 7thed. Vol 2, section 15, Chap 143, Abnormalities of taste,
P. 1840- 1849
 Bailey’s HNS – ORL 5thed. Vol I, Section III, Part 50, Taste P. 729 – 735
 Online references