PHYSIOLOGY OF TASTE AND SMELL CLINICAL AND APPLIED ASPECTS

1. THE CHEMICAL SENSES-TASTE AND
SMELL
PRESENTED BY- Dr JONCY V ABRAHAM
1ST YR PG DEPT OF PROSTHODONTIA

2. SENSE OF TASTE
• 4 PRIMARY SENSATION OF TASTE
• SOUR
• SALTY
• SWEET
• BITTER

3. • SOUR TASTE-
1.IT IS CAUSED BY ACIDS
2.PROPORTIONAL TO THE LOGARITHM OF THE HYDROGEN ION CONCENTRATION
SALTY TASTE-
1.ELICITED BY IONIZED SALTS
2.CATIONS OF SALT ARE MAINLY RESPONSIBLE FOR THE SALTY TASTE.
• SWEET TASTE-LIST OF CHEMICALS CAUSING SWEET TASTE ARE:
1)SUGARS
2)GLYCOLS
3)ALCOHOLS
4)ALDEHYDES
5)KETONES
6)AMIDES
7)ESTERS

4. • BITTER TASTE-SUBSTANCE GIVING BITTER TASTE ARE ALMOST
ENTIRELY ORGANIC SUBSTANCES.
1)LONG CHAIN ORGANIC SUBSTANCES THAT CONTAIN NITROGEN
2)ALKALOIDS. The alkaloids include drugs such as QUININE, CAFFEINE,
STRYCHININE, AND NICOTINE.

5. THRESHOLD FOR TASTE
• SOUR TASTE BY HYDROCHLORIC ACID -0.0009N
• SALTY TASTE BY SODIUM CHLORIDE-0.01M
• SWEET TASTE BY SUCROSE -0.01M
• BITTER TASTE BY QUININE-0.000008M

6. TASTE BLINDNESS
• MANY PEOPLE ARE TASTE BLIND FOR CERTAIN SUBSTANCES
ESPECIALLY FOR DIFFERENT TYPES OF THIOUREA COMPOUNDS.
• SUBSTANCE USED FREQUENTLY FOR DEMONSTRATING TASTE
BLINDNESS IS PHENYLTHIOCARBAMIDE BY PSCYHOLOGISTS.

7. TASTE BUD AND ITS FUNCTION
• COMPOSED OF 50 MODIFIED EPITHELIAL
CELLS.
• SOME SUPPORTING CELLS CALLED
SUSTENTACULAR CELLS.
• OTHER ARE TASTE CELLS.
• TASTE CELLS ARE CONTINUALLY BEING
REPLACED BY MITOTIC DIVISION FROM THE
SURROUNDING EPITHELIAL CELLS.
• OUTER TIP OF TASTE CELLS ARE ARRANGED
AROUND A MINUTE TASTE PORE.
• MICROVILLI OR TASTE HAIR PROTRUDE
OUTWARD INTO THE TASTE PORE TO
APPROACH CAVITY OF MOUTH.

8. • THESE MICROVILLI PRODUCE RECEPTOR SURFACE
FOR TASTE.
• AMONG THE BODIES OF TASTE CELLS IS A
BRANCHING TERMINAL NETWORK OF SEVERAL
TASTE NERVE FIBERS.
• VESICLES FORM BENEATH THE CELL MEMBRANE
NEAR THE FIBERS.
• CONTAINS NEUROTRANSMITTER WHICH IS
RELEASED THROUGH THE CELL MEMBRANE TO
EXCITE THE NERVE FIBER ENDINGS IN RESPONSE
TO TASTE STIMULATION.

9. LOCATION OF TASTE BUDS
• CIRCUMVALLATE PAPILLAE WHICH FORM
A V LINE ON POSTERIOR SURFACE OF THE
TONGUE.
• FUNGIFORM PAPILLAE(moderate
numbers) OVER THE FLAT ANTERIOR
SURFACE OF THE TONGUE.
• FOLIATE PAPILLAE LOCATED IN FOLDS
ALONG THE LATERAL SURFACES OF THE
TONGUE.
• ADULTS HAVE 3000-10,000 TASTE BUDS.
• AT AGE OF 45 YRS MANY TASTE BUDS
DEGENERATE.

11. MECHANISM OF STIMULATION OF
TASTE BUDS
• RECEPTOR POTENTIAL-
MEMBRANE OF TASTE CELL IS NEGATIVELY CHARGED w.r.t OUTSIDE.
TASTE SUBSTANCE ON TASTE HAIRS CAUSES PARTIAL LOSS OF THIS NEGATIVE
POTENTIAL.
TASTE CELL IS DEPOLARIZED.
DECREASE IN POTENTIAL IS APPROXIMATELY PROPORTIONAL TO THE LOGARITHM
OF THE CONCENTRATION OF THE STIMULATING SUBSTANCE.
THIS CHANGE IN POTENTIAL IN TASTE CELL IS THE RECEPTOR POTENTIAL FOR
TASTE.

12. • MECHANISM:
• BINDING OF THE TASTE CHEMICALS TO THE PROTIEN RECEPTOR MOLECULES
THAT PROTRUDE THROUGH THE VILLUS MEMBRANE.
• OPENS ION CHANNELS AND ALLOWS SODIUM IONS TO ENTER AND DEPOLARIZE
THE CELL.
• TASTE CHEMICAL IS GRADUALLY WASHED AWAY FROM THE TASTE VILLUS BY
SALIVA.HENCE REMOVES THE STIMULUS.
• TYPE OF RECEPTOR PROTIEN IN EACH TASTE VILLUS DETERMINES THE TYPE OF
TASTE IT WILL ELICIT.

13. TRANMISSION OF TASTE SIGNALS INTO THE CENTRAL
NERVOUS SYSTEMS
• TASTE IMPULSES FROM ANTERIOR
2/3RD OF TONGUE PASSES FIRST INTO
VTH NERVE.
• THEN THROUGH CHORDA TYMPANI
INTO FACIAL NERVE.
• INTO THE TRACTUS SOLITARIUS IN THE
BRAIN STEM.
• TASTE SENSATION FROM
POSTERIOR1/3RD OF THE TONGUE AND
OTHER POSTERIOR REGION IS
TRANSMITTED BY
GLOSSOPHARYNGEAL NERVE INTO THE
TRACTUS SOLITARIUS BUT AT SLIGHTLY
LOWER LEVEL.

14. • FROM BASE OF THE TONGUE AND
OTHER PARTS OF PHARYNGEAL
REGION TO TRACTUS SOLITARIUS BY
THE VAGUS NERVE.
• ALL TASTE FIBERS SYNAPSE IN THE
NUCLEI OF TRACTUS SOLITARIUS.
• SENDS 2ND ORDER NEURONS TO A
SMALL AREA OF VENTRAL POSTERIOR
MEDIAL NUCLEUS OF THE
THALAMUS.

15. • 3RD ORDER NEURONS FROM
THALAMUS TO THE LOWER TIP OF THE
POSTCENTRAL GYRUS IN THE PARIETAL
CORTEX.
• IT CURLS DEEP INTO SYLVIAN FISSURE
AND INTO ADJACENT
OPERCULARINSULAR AREA.

16. TASTE REFLEXES INTEGRATED IN BRAIN STEM
• FROM TRACTUS SOLITARIUS A LARGE NUMBER OF IMPULSES ARE TRANSMITTED
WITHIN BRAIN STEM DIRECTLY TO INFERIOR AND SUPERIOIR SALIVATORY
NUCLEI.
• THESE INTURN TRANSMIT IMPULSES TO THE SUBMANDIBULAR,
SUBLINGUAL,AND PAROTID GLANDS TO HELP CONTROL THE SECRETION OF
SALIVA DURING THE INGESTION OF THE FOOD.

17. NOTE-
• RECENTLY UMAMI (MEANS DELICIOUS IN JAPANESE) A FIFTH TASTE SENSE HAS
BEEN ADDED TO FOUR CLASSICAL TASTE MODALITIES.
• THE TASTE IS PLEASANT AND SWEET BUT DIFFERS FROM STANDARD SWEET
TASTE.
• MEDIATED BY GLUTAMIC ACID ACTING ON METABOTROPIC GLUTAMATE
RECEPTORS(mGluR4) LOCATED ON TONGUE AND STOMACH.
• GLUTAMIC ACID IS ABUNDANT IN BREAST MILK AND IS PRESENT IN MOST OF THE
NATURAL FOODS eg – CHEESE, MILK, MEAT AND SEAFOOD.
• GLUTAMIC ACID IS CLOSELY INVOLVED IN SMOOTH DIGESTION AND ABSORPTION
OF PROTEINS.

18. APPLIED ASPECTS
1. AGUESIA- IS ABSENCE OF SENSE OF TASTE
SULPHYDRYL GROUP OF DRUGS LIKE CAPTOPRIL AND
PENICILLAMINE OR VITAMIN B12 OR ZINC DEFICIENCIES CAN CAUSE
TEMPORARY LOSS OF TASTE OF SENSATION.
CAN BE DUE TO DAMAGE TO THE LINGUAL OR GLOSSOPHARYNGEAL
NERVE.
2. HYPOGEUSIA- DIMINISHED TASTE SENSITIVITY SEEN WITH AGING
AND TOBACCO USE.
3. DYSGUESIA OR PARAGUESIA- UNPLEASANT DISTURBED
SENSE OF TASTE CAUSE A METTALIC , SALTY, FOUL OR RANACID TASTE.

19. SENSE OF SMELL
• OLFACTORY MEMBRANE-
1. MEDIALLY THE MEMBRANE FOLDS
SLIGHTLY DOWNWARD OVER THE
SURFACE OF THE SUPERIOR
SEPTUM.
2. LATERALLY IT FOLDS OVER THE
SUPERIOR TURBINATE.
3. IN EACH NOSTRIL THE OLFACTORY
MEMBRANE HAS A SURFACE AREA
OF ABOUT 2.4 SQUARE
CENTIMETERS.

20. OLFACTORY CELLS
• RECEPTOR CELLS ARE
OLFACTORY CELLS.
• BIPOLAR CELLS DERIVED
ORIGINALLY FROM CNS.
• 100 MILLION OF THESE CELLS
INTERSPERSED AMONG
SUSTENTACULAR CELLS.

21. • MUCOSAL END OF OLFACTORY
CELLS FORMS KNOB FROM
WHICH 6-12 OLFACTORY HAIRS
OR CILIA PROJECT INTO THE
MUCOUS.
• 0.3 MICROMETER IN DIAMETER
AND 200 MICROMETER IN
LENGTH.
• SPACED AMONG THE
OLFACTORY CELLS ARE MANY
SMALL GLANDS OF BOWMAN
THAT SECRETE MUCOUS ONTO
THE SURFACE OF OLFACTORY
MEMBRANE.

22. STIMULATION OF THE OLFACTORY CELLS
• MECHANISM OF EXCITATION OF
OLFACTORY CELLS
1.THE ODORANT SUBSTANCE
COMING IN CONTACT WITH THE
OLFACTORY SURFACE FIRST
DIFFUSES IN THE MUCOUS THAT
COVERS THE CILIA.
2.IT BINDS WITH THE RECEPTOR
PROTEIN THAT PROTRUDES
THROUGH THE CILIARY
MEMBRANE.
3.THIS RECEPTOR IS A LONG
MOLECULE THAT THREADS ITS WAY
THROUGH THE MEMBRANE SEVEN
TIMES FOLDING INWARD AND
OUTWARDS

23. • THE INSIDE OF FOLDING RECEPTOR IS
COUPLED TO A SO CALLED G-
PROTEIN.CONSIST OF 3 SUBUNITS.
• ON EXCITATION THE ALPHA SUBUNIT
BREAKS FROM G PROTEIN AND
IMMEDIATELY ACTIVATES THE ADENYL
CYCLASE.
• ACTIVATED CYCLASE CONVERTS
INTRACELLULAR ATP TO cAMP.
• cAMP ACTIVATES MEMBRANE
PROTEIN A GATED SODIUM ION
CHANNEL .LARGE NO OF SODIUM ION
POURS INTO THE
CYTOPLASM.INCREASES POSITIVITY
OF THE CELL MEMBRANE AND THUS
EXCITING THE OLFACTORY NEURON
AND TRANSMITTING ACTION
POTENTIAL TO CNS THROUGH
OLFACTORY NERVE.

24. MEMBRANE POTENTIAL AND ACTION POTENTIAL IN OLFACTORY CELLS.
• THE MEMBRANE POTENTIAL OF UNSTIMULATED OLFACTORY CELLS AS
MEASURED BY MICROELECTRODES AVERAGES ABOUT -55 MILLIVOLTS.
• CELLS GENERATE CONTINUOUS ACTION POTENTIAL AT A VERY SLOW RATE
VARYING FROM ONCE EVERY 20 SECS UPTO 2 TO 3 PER SECOND.
• ODORANTS CAUSES DEPOLARIZATION DECREASING THE NEGATIVE POTENTIAL
FROM -55 DOWN TO AS LOW AS -30 MILLIVOLTS OR EVEN LESS.
• NUMBER OF ACTION POTENTIAL INCREASESTO ABOUT 20 PER SECOND WHICH IS
A HIGH RATE FOR THE MINUTE FRACTION OF A MICROMETER OLFACTORY NERVE
FIBERS.
• A FEW ODORANTS HYPERPOLARIZE THE OLFACTORY CELL MEMBRANE WHICH
DECREASES THE NERVE FIRING RATE.
• OLFACTORY RECEPTORS TENDS TO OBEY THE PRINCIPLES OF TRANSDUCTION.

25. TRANSMISSION OF SMELL SIGNALS INTO THE
CENTRAL NERVOUS SYSTEM
• THE OLFACTORY BULB ALSO
KNOWN AS CRANIAL NERVE I IS AN
ANTERIOR OUTGROWTH OF THE
BRAIN TISSUE FROM THE BASE OF
THE BRAIN HAVING BULBOUS
ENLARGEMENT.
• IT LIES ABOVE THE CRIBRIFORM
PLATE SEPERATING THE BRAIN
CAVITY FROM THE UPPER REACHES
OF THE NASAL CAVITY.
• CRIBRIFORM PLATES HAVE
MULTIPLE SMALL PERFORATIONS
THROUGH WHICH AN EQUAL
NUMBER OF SMALL NERVES PASS
UPWARD FROM OLFACTORY
MEMBRANE IN NASAL CAVITY TO
ENTER OLFACTORY BULB IN
CRANIAL CAVITY.

26. • SHORT AXONS TERMINATING IN
MULTIPLE GLOBULAR STRUCTURES
WITHIN THE OLFACTORY BULB CALLED
GLOMERULI .
• EACH OLFACTORY BULB HAS
THOUSANDS OF GLOMERULI AND IT’S
THE TERMINUS FOR ABOUT 25000
AXONS FROM THE OLFACTORY CELLS.
• EACH GLOMERULUS IS THE TERMINUS
FOR DENDRITES FROM 25 LARGE
MITRAL CELLS AND ABOUT 60
SMALLER TUFTED CELLS.
• THESE CELLS IN TURN SENDS AXONS
THROUGH THE OLFACTORY TRACT TO
TRANSMIT OLFACTORY SENSATION TO
THE CNS.

29. APPLED ASPECTS
1. ANOSMIA- COMPLETE ABSENCE OF SENSE OF SMELL.
2. PARAOSMIA- ALTERATION IN CHARACTER OF SMELL.
3. HYPOSMIA- REDUCTION OF THE SENSE OF SMELL.
 PATIENTS WITH ADRENAL INSUFFICIENCY HAVE GREATLY INCREASED
SENSITIVITY FOR SMELL.

30. REFERENCES
• GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY 12TH
EDITION.
• A.K JAIN TEXTBOOK OF PHYSIOLOGY VOL II 6TH EDITION.