Sensation of Taste

2. Sensation of Taste Professor A.M.A Abdel Gader MD, PhD, FRCP (Lond., Edin), FRSH (London) Professor of Physiology, College of Medicine & King Khalid University Hospital Riyadh, Saudi Arabia

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

18. TastePathway

19. TastePathway Taste information is send to the CNS by the crainial nerves # 7, 9 and 10 the taste nucleus (n. tractus solitarius)thalamus insular cortex

20. Physiology of Taste

21. 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. 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. 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. Sensation of Taste –Physiology of Taste – cont:Chemical structure and taste thresholds: Threshold concentration. µmol/L • Sour ... Acidity by {H+} – HCL .......... 100 • 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. Ionchannel Sourness receptors operate by closing potassium channels, which allows a positive charge to build up, thereby causing depolarization of the cell.

33. Sweet taste

34. 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 secondmessengers.

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. 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) Sensation of Taste – cont.

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

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

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

46. Thank you Shukran Gazeelan