2. Professor A.M.A Abdel Gader MD, PhD, FRCP (Lond., Edin), FRSH (London)
Sensation of Taste
Professor A.M.A Abdel Gader
MD, PhD, FRCP (Lond., Edin), FRSH (London)

3. 3

6. Chemical Senses TASTE SMELL Both determine the flavour of food
Taste and smell are closely linked even though they involve different receptors
and receptive processes.
This suggests an overlap in central processing.

7. Anatomy of Taste Sensation

8. Taste Buds

9. Sensation of Taste – Anatomy of Taste - cont.
Receptors:
Located in taste buds in:
Tongue
Epiglottis
Soft Palate
Pharynx

10. Anatomy of Taste Buds – cont.
10,000 taste buds found on tongue, soft palate & larynx
Taste buds consist of:
~50 receptor cells (type 3) surrounded by supporting cells
Basal cells (type 1 &2) develop into supporting cells then receptor cells
Gustatory hairs project through the taste pore
Life span of 10 days

12. Anatomy of Taste Buds – cont.

13. Anatomy of Taste Buds - cont
Papillae are found on the front, sides and back of the tongue.
(The response is not as specific as indicated on the right).

14. Anatomy of Taste Buds - cont

15. Anatomy of Taste Buds - cont
Each taste bud is innervated by 50 nerve fibers
Each nerve fiber receives input from about 5 taste buds
There are about 4500 taste buds per average tongue

16. Nerve supply of tongue Fibres from:
Epiglottis
Palate
Pharynx
VAGUS

17. Taste Pathway Tongue ML Frontal Operculum Vagus Nerve Mouth & Larynx
Insular Cortex
Mouth & Larynx
Tongue
Chorda Tympani Nerve
Vagus Nerve
Glossopharyngeal Nerve
Thalamus
Nucleus Solitary Tract (NST) ML

18. Taste Pathway

19. Taste Pathway
Taste information is send to the CNS by the crainial nerves # 7, 9 and the taste nucleus (n. tractus solitarius) thalamus insular cortex

20. Physiology of Taste

21. Attachment to Receptors
Physiology of Taste - cont
Dissolution in Saliva
Attachment to Receptors
Generator Potential
Action Potential

22. Physiology of Taste – cont: Primary modalities of taste:
Sensitivity differs in different areas, but all tastes can be perceived at most areas of the tongue
Its not
this
simple

23. Responses of Taste buds:
Physiology of Taste – cont: Primary modalities of taste:
Responses of Taste buds:
Each taste bud responds strongly to one type of taste
But they also respond to other tastes as well

24. Primary modalities of taste:
Physiology of Taste - cont
Primary modalities of taste:
Sour
Salt
Sweet
Bitter
umami (deliciousness), a taste associated with glutamate & other nucleotides has receptors located at the back of the pharynx.

25. Physiology of Taste – cont: Primary modalities of taste:
Sour
Salt
Sweet
Bitter
Evidence for 4 modalities???

26. Physiology of Taste – cont: Primary modalities of taste:
Sour, Salt, Sweet, Bitter
Evidence for 4 modalities:
Cocaine on the tongue:
Sensations disappear in the following order
Pain
sweet
sour
bitter
salt
touch

27. Physiology of Taste – cont: Primary modalities of taste:
Sour, Salt, Sweet, Bitter
Evidence for 4 modalities:
Gymnemic acid on tongue:
Bitter & sweet ……..disappear
Sour & salt ……………remain

28. Artificial sweeteners:
Sensation of Taste – Physiology of Taste – cont: Chemical structure and taste thresholds:
Threshold concentration.
µmol/L
Sour ... Acidity by {H+} – HCL
Salt … Sodium chloride ……………..… 2000
Sweet ..Sucrose ……………………………….. 10,000
glucose ……………………………….. 80,000
Saccharin ……………………………. 23
Bitter ..Strychnine hydrochloride … 1.6
Quinine sulphate …………………….. 8
Artificial sweeteners:
Aspartame, Cyclamate

29. Physiology of Taste – cont:
Mechanism of stimulation of taste sensation:
Sour:
Acids (H+)
Blocks K+ channels
Salt taste
Na+
Depolarization

30. Physiology of Taste – cont:
Mechanism of stimulation of taste sensation:
Sweet
G protein activation of adenyl cyclase c-AMP K conductance
Bitter
G protein Activatn. of Phospholipase C IC-insitol (PO4)3 Ca2 release

31. Deep inside a ‘salty taste’ taste bud
Sour is similar
Electrical signal
A bucket brigade transmits the signal through the cell

32. Ion channels Saltiness or sodium receptors allow
sodium ions to cross the membrane,
thereby causing depolarization.
Ion channel
Sourness receptors operate by closing
potassium channels, which allows a
positive charge to build up, thereby
causing depolarization of the cell.

33. Sweet taste
We have determined that the G protein subunits a-gustducin, a-transducin, g13, b1 and b3 are expressed at high levels in TRCs. a-Gustducin is expressed selectively in TRCs and other chemosensory cells. a-Gustducin's biochemical properties closely resemble those of the a-transducins, including the ability to activate retinal phosphodiesterase (PDE6). a-Gustducin and a-transducin activate TRC-expressed PDE1A; gustducin's bg component, b3g13, activates the b2 form of phospholipase C (PLCb2) (see below).

34. Different ions, different receptors for different tastes
THE 4 “BASIC” TASTES ARE SALTY, SOUR, SWEET AND BITTER,
Also: UMAMI (MSG)?, METALLIC? FAT? AMINO ACID?
Different ions, different receptors for different tastes

35. Mechanism of stimulation of taste sensation - cont:
Saltiness and sourness are transduced directly by sodium and hydrogen ions respectively.
The transduction process for sweetness and bitterness involve second messengers.

36. Sensation of Taste – cont.
Discrimination of intensity of taste:
Poor (like smell)
Requires 30% change to allow discrimination of intensity

37. Sensation of Taste – cont.
Adaptation to taste:
Decreased sensation from repeated stimulus
Entirely peripheral at
the receptors

38. Sensation of Taste – cont.
Ethnic variation in taste sensation:
(Genetics)
Phenylthiocarbamide (PTC):
In dilute solution
Taste sour in 70% of Caucasians
Tasteless in 30% of Caucasians (Inherited defect- autosomal recessive trait)

39. Sensation of Taste – cont.
After-effects in taste sensation:
(Taste tricks):
Taste modifier Meraculin (a glycoprotein extracxted from miracle fruit):
When applied to tongue makes acids taste sweet
The Miracle fruit-origin
of miraculin

40. Sensation of Taste – cont.
Adaptation to taste:
Decreased sensation from repeated stimulus
Entirely peripheral at
the receptors

41. Clinical considerations
Ageusia: Absence of sense of taste
Dysgeusia: Disturbed sense of taste
Hypogeusia: Diminshed sense of taste
Hypergeusia: increased sense of taste

42. 42

43. 43

44. 44

45. 45

46. Thank you
Shukran Gazeelan