Taste is detected by taste buds located in the mouth and throat. Taste buds contain receptor cells that are stimulated by chemicals in foods and trigger nerve impulses. There are 5 primary tastes: sweet, sour, salty, bitter, and umami. Receptor cells for different tastes use different mechanisms like ion channels or second messengers. Nerve impulses travel to the brainstem and thalamus which process taste information. Disorders can impair one's ability to taste. Maintaining oral hygiene and adding zinc can help correct some taste disorders.

1. Physiology of Taste
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com

2. The student will be able to
 Learn the structural organization and functions of taste buds
 Draw a schematic diagram of taste pathways
 Understand the mechanism of gustatory transduction
 Comprehend the physiological basis of abnormalities of taste sensation

3. Introduction
The sense of taste is known as gustation
Gustatory receptors are chemoreceptors
Sense of taste and smell allow us to separate undesirable or
even lethal foods from those that are pleasant to eat.
Both senses are strongly tied to primitive emotional and
behavioral functions

4. Introduction
Taste is mainly a function of taste buds in the mouth
13 probable chemoreceptors in the taste cells are identified
2 sodium receptors, 2 potassium receptors, 1 chloride
receptor, 1 inosine receptor, 2 sweet receptors, 2 bitter
receptors, 1 glutamate receptor, 1 hydrogen receptor

5. Primary taste sensations
1. Sour
2. Salty
3. Sweet
4. Bitter
5. umami

6. Primary taste sensations
A person can perceive hundreds of different tastes
They all are thought to be combinations of elementary taste
sensations

7. Sour taste
Caused by acids
By the hydrogen ion concentration
Intensity of the taste is approximately proportional to the
logarithm of the hydrogen ion concentration
More acidic the food, the stronger the sour sensation
becomes

8. Sweet taste
Some of the chemicals that cause this taste include sugars,
glycols, alcohols, aldehydes, ketones, amides, esters, some
amino acids, small proteins
Specifically most of the substances that cause sweet taste
are organic chemicals

9. Salt taste
Ionized salts elicit this taste
Mainly by sodium ion concentration
Quality varies from one salt to other
Because some salt elicit other taste sensations in addition to
saltiness

10. Bitter taste
Substances that give bitter taste are almost organic
substances
Two important class of substances
Long chain organic substances that contain nitrogen
Alkaloids – includes quinine, caffeine, nicotine, etc.

11. Bitter taste
Some substances initially taste sweet but have bitter after taste
Saccharin
When bitter taste occur in high intensity, the animal or human
rejects the food
Many deadly toxins found in the poisonous plants are alkaloids

12. Umami taste
Japanese word – delicious
Pleasant taste sensation
Different from sour, salt, sweet and bitter
Food containing l-glutamate gives this taste
Meat extracts and aging cheese

13. Umami taste
Precise molecular mechanisms responsible for Umami taste
are still unclear

14. Threshold for taste
Threshold for stimulation of sour taste by hydrochloric acid
averages 0.0009 N
For stimulation of salty taste by sodium chloride, 0.01 M
For the sweet taste by sucrose, 0.01 M
For the bitter taste by quinine, 0.000008 M
Bitter taste sensitivity is more than others (Protective function)

15. Taste blindness
Some people are taste blind for certain substances
Especially for different types of thiourea compounds
Substances used by psychologists for demonstrating taste
blindness is phenylthiocarbamide
15-30% of people exhibit taste blindness to
phenylthiocarbamide

16. Taste Bud and its function
Diameter 1/20 mm
Length 1/16 mm
Taste bud is composed of about 50 modified epithelial cells
Taste cells and sustentacular cells (supporting cells)
Taste cells are continuously replaced by the mitotic division of
surrounding epithelial cells

17. Taste Bud and its function
Some taste cells are young cells
Others are mature cells lies towards centre of the bud
Life span of each taste cell is 10 days for lower mammals
In humans it is not known
The outer tips of the taste cells are arranged around a minute
taste pore

19. Taste Bud and its function
From the tip of each taste cell several microvilli or taste hairs
protrude outward into the taste pore to approach the cavity of
the mouth
These microvilli provide receptor surface for taste
Taste cells ARE INNERVATED BY taste nerve fibers that are
stimulated by taste receptor cells

20. Location of taste buds
Found on the three types of papillae of the tongue
Large number – circumvallate papillae
Moderate number –foliate papillae
Moderate number – fungiform papillae
Additional taste buds located on the palate, tonsillar pillars,
epiglottis and proximal esophagus

21. Location of taste buds
Adults have 3000 – 10,000 taste buds
Beyond the age of 45 years many taste buds degenerate
Decrease in taste sensitivity in old age
Each taste bud is specific for a particular taste sensation
However, they can be stimulated by other taste sensations at
high concentration

22. Mechanism of stimulation of taste buds
The membrane of the taste cell is negatively charged on the
inside with respect to outside
Application of taste substance
Partial loss of negativity
Depolarization of taste cell
Receptor potential for taste

23. Mechanism of stimulation of taste buds
 Taste substance binds with the protein receptor molecule present on the
outer surface of taste receptor cell
 Opening of ion channels
 Entry of sodium or hydrogen ions
 Depolarization
 Taste stimuli is gradually washed away by saliva
 Removal of stimulus

24. Mechanism of stimulation of taste buds
Salty and sour taste – opening of ion channels
Sweet and bitter taste – activation of second messenger
transmitter substance – intra-cellular chemical changes –
elicit taste signals

25. Mechanism of stimulation of taste buds
Salty taste
Sodium selective channel (ENac)
Entry of sodium
Depolarize the membrane
Receptor potential

26. Mechanism of stimulation of taste buds
 Sour taste
 Triggered by protons (H+ ions)
 ENacs permit entry of H+ ions
 H+ ions binds and block K+ channels
 Fall in K+ permeability
 Depolarization
 Receptor potential

27. Mechanism of stimulation of taste buds
Bitter taste
Some substances bind and block K+ sensitive channel
Some acts through G protein receptors
Some are membrane permeable and do not need G proteins
Example - quinine

28. Mechanism of stimulation of taste buds
Umami taste
Activation of metabotropic receptors
How they cause depolarization is yet to know

29. Generation of nerve impulse by taste bud
On first application of taste stimulus
Rate of discharge of nerve fibres from taste buds rises to
peak in fraction of second
Then adapts within next few seconds
Back to a lower, steady level as long as taste stimulus
remains

30. Taste pathway
Transmission of taste signals from the tongue and pharyngeal
region into the central nervous system
Taste impulses from anterior two-thirds of the tongue pass
first into the lingual nerve
Then through the chorda tympani into the facial nerve
Finally into the tractus solitarius in the brain stem

31. Taste pathway
Taste sensations from circumvallate papillae on the back of
the tongue
And from other posterior regions of the mouth and throat
Transmitted through glossopharyngeal nerve
Into tractus solitarius

33. Taste pathway
 From base of the tongue
 And other parts of pharyngeal region
 Transmitted through vagus nerve
 All these taste fibers synapse in the posterior brain stem in the nucleus
of tractus solitarius
 Second order neurons from this nuclei to VPMT (Ventero Postero Medial
nucleus of Thalamus)

34. Taste pathway
From thalamus, third order neurons are transmitted to the
lower tip of post central gyrus of cerebral cortex
Gustatory cortex

35. Rapid adaptation of taste
Taste sensation adapt rapidly
Completely with in a minute or so of continuous stimulation
Mechanisms are not known

36. Disorders of taste
 Loss of ability to taste – Ageusia
 Reduced ability to taste – Hypogeusia
 Distorted sense of taste – dysgeusia
 These conditions can occur due to advanced age or medications or poor
oral hygiene
 Improved oral hygiene and adding zinc supplements to diet can correct
the taste disorders in some individuals

37. THANK YOU