1. Sense of Taste (Gustatory Sense) Preratred by Ihsan Husain B. V. M & S
Sense of Taste (Gustatory Sense) Preratred by Ihsan Husain B.V.M & S., M.Sc. in Medical Physiology Last Update

2. Taste is mainly a function of the taste buds in the mouth.
Taste buds experience the food through:
Texture of food, as detected by tactual senses of the mouth.
Presence of substances in the food that stimulate pain endings, such as pepper.

3. Taste sense allows a person to select food:
In accord with desires.
In accord with the body tissues’ metabolic need for specific substances.

4. Chemoreception perceived in the cerebral cortex
Receptors are only in the head region
Dorsum of the tongue
Soft palate
Epiglottis
Pharyngeal wall
Significance of taste
Taste is the main stimulant to saliva flow
Governs food choice.
Perceived = ادراك

5. Primary Sensations of Taste
5 categories of taste
Salty.
Sweet.
Sour.
Bitter.
Umami (Meaty).

6. The sense of taste, is evoked by receptors that consist of barrel-shaped taste buds. Located primarily on the dorsal surface of the tongue, each taste bud consists of 50 to 100 specialized epithelial cells with long microvilli that extend through a pore in the taste bud to the external environment, where they are bathed in saliva.

7. There are at least 13 chemical receptors on the taste cells, among these are: 2 sodium receptors, 2 potassium receptors, 1 chloride receptor, 1 adenosine receptor, 1 inosine receptor, 2 sweet receptors, 2 bitter receptors, 1 glutamate receptor, and 1 hydrogen ion receptor.

8. Although scientists long believed that different regions of the tongue were specialized for different tastes, this is no longer believed to be true. All areas of the tongue are able to respond to all five categories of taste. Indeed, each taste bud contains cells sensitive to each taste category.

9. Each taste cell in the taste bud activates a sensory neuron that conveys information specific to only one taste modality.
For example, the sweet taste evoked by sugar is carried into the brain only on the sensory neurons devoted to sweet.
Each sensory nerve is recruited to convey only one taste modality

10. Acid taste (Sour)
Acid taste is due to the presence of hydrogen ions (H+), and the intensity of this taste sensation is directly proportional to the hydrogen ion concentration. That is, the more acidic the food, the stronger the sour sensation becomes.
H receptor depolarization Ca+ influx neurotransmitter release.

11. Salt taste
The salty taste of food is due to the presence of sodium ions (Na +).
Gustant (tastant) type is NaCl
NaF tastes less salty.
Possible theory: opening of Na channels which lead to passing of Na+ into the cell that cause depolarization & release their transmitter.
There is evidence to
suggest that the anions can pass through the tight junctions
between the receptor cells, and that the Cl − anion passes
through this barrier more readily than the other anions. This
is presumably related to the ability of Cl − to impart a saltier
taste to the Na + than the other anions do.

12. Sweet taste
The sweet taste is not caused by any single class of chemicals. Some of the types of chemicals (gustant) that cause this taste include sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids.
gustant bind to receptors on the plasma membrane that are linked to G proteins. The G proteins then activate second messengers inside the gustatory receptor cell. Different second messengers cause depolarization in different ways, but the result is the same (release of neurotransmitter).

13. Most of the substances that cause a sweet taste are organic chemicals and even slight changes in the chemical structure, such as addition of a simple radical, can often change the substance from sweet to bitter.

14. Bitter taste
Like sweet taste, most of the substances that cause a bitter taste are organic chemicals.
Two particular substances are especially likely to cause bitter taste sensations:
long-chain organic substances that contain nitrogen.
Alkaloids; including many drugs such as quinine, caffeine, strychnine, and nicotine.

15. Neurotransmitter is released by the same mechanism as in sweet taste.
Bitter taste is the most acute sensation and is generally associated with toxic molecules (although not all toxins taste bitter).
Neurotransmitter is released by the same mechanism as in sweet taste.
The bitter taste, when it occurs in high intensity,
usually causes the person or animal to reject the food.
This is undoubtedly an important function of the bitter
taste sensation, because many deadly toxins found in
poisonous plants are alkaloids, and virtually all of
these cause intensely bitter taste, usually followed by
rejection of the food.

16. Umami taste
is a Japanese word (meaning “delicious”) designating a pleasant taste sensation that is qualitatively different from sour, salty, sweet, or bitter.
Umami is the dominant taste of food containing L-glutamate, such as meat extracts and aging cheese, and some physiologists consider it to be a separate, fifth category of primary taste stimuli. A taste receptor for L-glutamate may be related to one of the glutamate receptors expressed in neuronal synapses of the brain.
However, the precise molecular mechanisms responsible for umami taste are still unclear.

17. Neural Pathway
Taste impulses from the anterior two thirds of the tongue pass through the facial nerve into the tractus solitarius in the brain stem.
Taste sensations from the back of the tongue and from other posterior regions of the mouth and throat are transmitted through the glossopharyngeal nerve also into the tractus solitarius.

18. Finally, a few taste signals are transmitted into the tractus solitarius from the base of the tongue and other parts of the pharyngeal region by way of the vagus nerve.
All taste fibers synapse in the posterior brain stem in the nuclei of the tractus solitarius.
These nuclei send second-order neurons to the thalamus.
From the thalamus, third-order neurons are transmitted to the parietal cerebral cortex, where it curls deep into the sylvian fissure in which these signals are analyzed.

19. Flavor
The total sensation induced when a particular foodstuff is introduced into the mouth
Taste is a major component of flavor
Flavor depends on simultaneous stimulation of
Olfaction
If olfaction is blocked, tasting some flavors can be difficult
Touch receptors in the palate
e.g. flavor of alginate due to texture
Reactions to irritants on the lingual surface
Chilly pepper & ginger stimulate nociceptors of trigeminal nerve - hot flavor

20. Factors affecting taste perception
Oral temperature
Cold – reduces the receptors ability
e.g. Ice-cream need to be more sweet than hot desserts to be acceptably sweet
Combinations of gustants
Resulting perception is hard to explain
Aging – reduction in taste perception.

21. – high threshold to NaCl Cigarette smoking
Deprivation from salt
– Low threshold to NaCl
Dehydration
– high threshold to NaCl
Cigarette smoking
Diminish taste perception to all kind of taste
Increase bitter taste
Local anesthesia
Salt & sweet tastes are reduced.

22. Sense of Smell
The olfactory membrane lies in the superior part of each nostril.
Medially, the olfactory membrane folds downward along the surface of the superior septum; laterally, it folds over the superior turbinate.
In each nostril, the olfactory membrane has a surface area of about 2.4 square centimeters.

23. Olfactory Cells
The receptor cells for the smell sensation are the olfactory cells. There are about 100 million of these cells in the olfactory epithelium interspersed among sustentacular cells.
The mucosal end of the olfactory cell is provided with olfactory cilia that project into the mucus that coats the inner surface of the nasal cavity.
These cilia react to odors in the air and stimulate the olfactory cells.

24. Among the olfactory cells in the olfactory membrane are many small Bowman’s glands that secrete mucus onto the surface of the olfactory membrane.

25. Mechanism of Excitation of the Olfactory Cells
The odorant substance first diffuses into the mucus that covers the olfactory cilia.
Then it binds with receptor proteins in the membrane of each olfactory cilium, this leads to:
1. Activation of the receptor protein.
2. Activation of G-protein.
3. Activation of cAMP that opens large numbers of sodium channels.

26. Introduction of Na ions will generate an action potential that excite the olfactory neuron and transmit into the central nervous system by way of the olfactory nerve.