#lecture 5: continue general sensation in oral cavity

#16.3.2011

#done by: abu bakar, fahmi majid, adalia ain, fatin izzati

Last time we started with an interesting topic which is general sensation of oral cavity. And we talked about different sensation: mechanosensation, thermal sensation, proprioreception, motor distribution, and we also talked about the chemical sensation represented by taste.

(Slide 11) is actually the different distribution of regional taste in tongue. All right, so, actually we have four different major type of taste perception. We can taste salty things, sweet things, acid things and bitter things. Generally, we can taste salty at the most anterior part, followed by sweet, acid and posteriorly bitter sensation. These are general terms. Sometimes, we can taste sweet and salty at the same time. Not necessarily everything should be only specific to one of these four tastes. Sometimes, we taste something that is difficult to describe its taste. For example, avocado, it is difficult to describe, is it sweet? Is it sour? Is it salty? So, these are the combination of these four major tastes. That is why it is difficult to tell which taste it is represented to. Generally, salty and sweet are represented at the anterior part. Acid and bitter at the posterior part, and at the most posterior part is the bitter sensation which is at wall of pharynx.

Because of sweet and salty perception located generally at the anterior part, the nerve responsible for sensory of these taste is facial nerve.

You learnt facial nerve in head and neck anatomy. Facial nerve is responsible of innervating the anterior two thirds of the tongue. Facial nerve only innervates taste buds. So, the sensory of the taste bud (anterior 2/3 of tongue) are carried through the cauda tympani (lingual) of the facial nerve. Cauda tympani is one of the branches of the facial nerve that receive information of taste buds at the anterior two thirds. So that is why the facial nerve area is at the anterior area of the mouth. Acid sensation is not really a part of the anterior mouth. But, bitter sensation is innervated by branch of

glossopharyngeal nerve, which is the nerve that innervates the posterior part of the mouth. Taste buds located at the posterior third such as circumvallate papilla, and foliate papilla. These are the taste bud that is innervated by glossopharyngeal nerve.

Dorsal of the tongue is more sensitive to acid and bitter taste. The most sensitive stimuli of the taste bud are the acid and bitter. In other words, put a drop of acidic water, and put drop of sweet water in your mouth. Which one will be the major taste? Acidic water, So that’s why acidic and bitter are more sensitive than sweet and salty.

Most of old people complain of decrease taste sensation... But, they found that the bitter taste is not affected. Ya3ni, they suffered decrease sensation for taste salt, sweet, and acid sensation, but not for bitter sensation. Why? It is thought that because of decrement of taste buds. However, the circumvallate papilla is responsible for bitter sensation, which is not affected by these old people!

What you should know, many of these old people wear denture. Because of wearing denture, they covered the palate. So because of they cover the palate, the taste perception decrease. As you know, the palate is very important for the taste sensation. The bitter sensation is not affected because taste buds responsible for that sensation located at the posterior part of the mouth. Not at the palate, as you can see in the slide from anterior, salt, sweet, acid and posteriorly the bitter taste.

These are the distributions of different tastes in the mouth. This is region taste sensation for the palate (above) and this is the region taste sensation for the tongue (below).

Remember that the taste bud which is the fungiform and filiform papilla at the anterior part of the tongue is supplied by facial nerve and responsible for conduction of salt, sweet, and acid stimuli. But the taste bud at the posterior part of the tongue which is circumvallate and foliate papillae, all the taste buds located posteriorly are supplied by glossopharyngeal nerve which is usually respond to bitter stimuli.

Taste buds cells actually are epithelial cells which are modified epithelial cells. These epithelial cells can conduct nerve fibers. These cells are subject to turn over 2-30 days. We have four types of taste buds. We can find nerve fiber in the taste buds. This is taste bud (slide 17). But each nerve fiber is non-mylinated. They lose their myeline sheath after entering the taste buds. They end adjacent to epithelial cells. Their nerve

terminal is very close to the epithelial cells. They may end in invagination of type I cells or coiled around type II or synapting with type III. That is why some people think that type III cell is actually neuron because it makes synapse with the nerve terminal.

Taste pore:

We also have opening in the taste buds which is the taste pore. And this is the taste pore which is continuous with the saliva here. This taste pore contains the pore substance including cell. This is tip of the cell. Can you see it? Some of these cells have microvilli to increase the surface area. It also contains gel like material. It contains ascorbic acid and enzyme.

Type I cell (dark cell)

They stained dark on staining (eosin), they are located at the periphery of the bud, not in the centre. They make up 2/3 of the cell. They end at the bud pore with microvilli, Separated by basal membrane from the pore substance. They have apical granules. They seemed to be secreting something. These (the cells) are claimed to secrete pore substance. They think the pore substances contained within the pore area are synthesis and secreted by type I cell. That is why they have the granules. Now basal end reach down to the basement membrane. This is the base of the cell and it is limited by the basal membrane. The cell has lateral extensions extending across the other cells & verve fibrils. They have invaginated apical nucleus. Notice that the nucleus in white in color & is invaginated, not rounded and folded.

Type II Cell (Light cells)

These are light cell. It appears light when staining. They contain thicker microvilli. They contain a number of vesicles. They have smooth-walled apical nucleus. *dr remind to study this well because in the exam there will be questions about these cell* they make up to 20% of bud cell, less abundant than Type I cells. They do not reach apical basement membrane and do not show any synaptic-like features. As you can see in the slide (17), these usually do not reach the basement membrane at the base and also do not reach the other side. They also do not need synapses with neuron.

Type III cells (20)

These cells make synapses with neurons. They are slender peg-like cells. No microvilli on this cell and they appear to synapse with adjacent nerve fibers. These cells thought to be of the neurological origin.

Type IV cells (Progenitor cells)

These cells make up or form the other three cell types, Type I, Type II, and Type III.

*dr pointed the cells in figure*

Sensory Mechanism of Taste (21)

Let’s talk first about the terminology.

Sapid: any substance stimulating taste sensation. Food must be in solution because taste sensation depends on chemical perception that is why it must be in solution. Theory, lets remove all the saliva in one patient, we give him sugar, he will not taste it, in order for us to taste anything, this thing must be dissolve in water, or dissolve in liquid that is saliva. If we have saliva, we have good taste. Ask people that have poor amount of saliva, their taste is not good. These people cannot taste properly because they do not have enough saliva to dissolve sapid material. Sapid react with pore content and buds cells. These bud cells are called bud receptor.

Gustation: process involve in taste perception (22)

-Old theory: Inhibition or potentiation of the pore enzymes. Sapid material may actually inhibit or potentiate some enzyme that is present in the pore of the taste bud, and this actually may change the receptor cells and lead to conduction of impulse.

-More acceptable theories: changes in membrane permeability of receptor cells or nerve fiber resulting from binding of excitatory substances. The binding of excitatory substance to the surface of the cell change the permeability membrane of the cell and lead to production of <<taste impulses>>. The evidence is linear relationship between cons. of sapid and frequency of action potential in the gustatory nerve. As you increase the cons. of the sapid materials the amount of action potential in the nerve also increases. *dr repeats the theory once again*

Gustatory pathways/ autonomic pathways: (23)

Through autonomic pathways, taste is controlled by autonomic that is sympathetic and parasympathetic. This cell is small myelinated and slowly conducting. Remember, that although these cells are myelinated, these nerves when they enter the taste buds, they lose the myelination and become non-myelinated. They make synapse at the nucleus of tractus solitarius. Tractus solitarius is an area of the brain stem responsible for taste

perception. 2° neuron crosses midline in the medial lamniscus in the thalamus, and 3° neurons at the lower part of the postcentral gyrus. Postcentral gyrus is the area responsible for sensation, the somatosensory cortex.

(24) Tongue is located on the lower postcentral gyrus. The third order neuron from the thalamus to the cerebral cortex. This is the second order neuron from the brainstem but crossing the midline. And this is first order from the tongue to the tractus solitarius which is a nucleus in your brain, nucleus in your brain responsible for passing the first order neuron to the second order neuron.

Classification of taste (25)

Henning: describe 4 type of taste, salt, sweet, sour and bitter. Used in testing taste sensitivity. If we want to test taste sensation of a person, we must use one of these tastes to see the sensitivity of a person. They did not talk about the modalities of taste but the extremes of the taste types. As I said, this is not the only four tastes that you taste; those four tastes present the extreme of taste modalities. That why it is called the corner of tetrahedron. You can taste anything which is a combination any of these 4 tastes inside the tetrahedron, but this tetrahedron present the extremes of taste.

The most seems as salt, sweet, sour, or bitter but u can taste anything which is from any combination between these four different modalities.

Alkaline or metallic tastes are not among taste categories! This taste is alkaline but this is not among the four tastes. Our metallic taste .This is not among these. This is different.

Taste buds vary in sensitivity to respond to one of the taste modalities there are not the same. Some taste buds only respond to one taste modality. Other taste bud may respond to more than one taste modality. Ya3ni most respond to more than one, ok? But usually one modality is common. Ya3ni for example the taste buds located at the tip of the tongue are responsible for tasting salty things. But does it mean that when u put something salty at the back of the mouth u won’t be able taste it? No! We still taste it. But the taste bud at the anterior region or the tip of the tongue, the most dominant taste for them is salty. The taste bud at the back of the mouth the most dominant taste for them is bitter and so on. Some taste buds may respond to only one taste. But usually most of our taste buds respond to all of these modalities but one of these should be dominant.

Acidic Taste (Slide 26)

When we eat something acidic like lemon juice let say, as we know we have hydrogen ion. Because of this acidity and they have (H ions) blocked potassium channel and this which will lead to depolarization. This is actually the explanation of what happens in the

conduction of nerves. Inorganic acids taste metallic; as they are inorganic they taste like metallic! Organic acids taste fruity like lemon juice for example.

Salt Taste (Slide 27)

For salty things, the mechanism of action for salty receptor is different. This leads to stimulation of receptor by cations. Like for example NaCl. It is the cation which called gustant type. This is the example type of taste perception which is salty perception, so this response by cation (ya3ni response by molecule).

NaF tastes less salty than NaCl. In low concentration sometimes you may taste sweet instead of salty. Ya3ni sometimes if the concentration of salt is very low you might taste sweet instead of salty. For example potassium chloride and potassium sulphate if you actually provide these in low concentrations, you will taste them sweet instead of salty.

The possible theory is that the cations like sodium (Na), they open the Na channels and this leads to depolarization and the beginning of action potential.

Sweet Taste (Slide 28)

The gustant type is sucrose not glucose. Chemical similarity between sweet & bitter sapid materials, usually the explanation of perception of sweet is similar to the explanation perception of bitter. But salty is different, acidic also. The other substances may act like sucrose gustant type include

Some amino acids Lead acetate Chloroform Saccharin Cyclamate

Sucrose is not desirable. It is used in diet beverages ok. You cannot gain weight if you drink something diet BUT remember that sucrose is VERY CARIOGENIC. Ya3ni still remember sucrose is not desirable because it’s more cariogenic.

Non cariogenic sweeteners include sugar alcohol, sorbitol and xylitol. This sweetener does not cause caries!

Common chemical feature of sweet taste-producing materials… we have covalent hydrogen bond which is loosely attached to a second group with a long distance. This binds to a mirror image conformation on the receptor membrane, closure of K channels, and then depolarization. Don’t worry about the details but remember the closer of potassium channel and depolarization, what happened in the first section of sweets.

Bitter Taste (Slide 29)

In bitter taste, the gustant type is quinine which has a therapeutic value (in medicine) Used in many alkaline drinks. This includes the other stimulant sapid material. For example from the slide:

Alkaloids

Glycosides

Picric acid

Nitrus & sulphide groups

Mg sulphate

Calcium oxide

Bitter tasting substances are harmful/ poisonous. That’s why taste is used as warning in animals. So that’s why taste is important, it’s a warning in animal. Ok? in animals because they don’t have mind to think about what they eat, anything bitter straight away they think that it is poisonous and they spill it out.

Common chemical feature of bitter taste producing substances… the cellular mechanism of stimulation is not well-understood. But it is similar to sweet; they have covalent hydrogen bonding which is loosely attached to a second group with a shorter distance.

Flavor (Slide 30) النكهة

Now we will talk about flavor. What is the difference between flavor and taste (al-nakha w al-ta3m)? It is important. Flavor is more than taste. It includes the taste. But it is more than taste. The total sensation induced when a particular foodstuff is introduced into your mouth, its called flavor. Flavor is a total sensation. The smell of something that comes into your mouth is part of the flavor. So, flavor is not a taste. It includes taste and it’s more than taste. Anything that comes in your mouth, the total sensation even the emotional sensation is flavor. For example you eat something sweet, you are happy with it. Happy is flavor. But taste is the chemical reaction of these taste materials.

So what does taste include? Taste is a major component of flavor but taste does not mean flavor the whole thing. Flavor depends on stimulate that simultaneous stimulation of; in order to perceive flavor you have to think about general many factors integrating together:-

i. Olfaction

This means smell. If olfaction is blocked, taste of some flavor can be difficult. One example you have common cold (influenza) you will have difficulty in detect flavor. Why?

Because you will lose the olfaction part of the flavor. You only have the taste part. Taste part is not similar to flavor part/ taste perception is different from flavor perception. Flavors include olfaction.*dr repeat once again.*

ii. Touch receptors in the palate

For example when you eat something you can feel the texture of the thing. The texture or the mechanoreception of the food that you’re eating is also part of the flavor.

For example, when we put alginate impression, this alginate material is making contact with the palate or the tongue. So that’s why you can feel the texture of them. And your tongue can have extra information about the texture of this material inside your mouth. So this is part of flavor, not taste.

iii. Reactions to irritants on the lingual surface

The reactions to irritants on the lingual surface, When you eat something like hot or something chilly; perception of chilly or the perception of hot things, it’s not taste but it is a flavor.

So taste does not mean flavor. Again as I said if we eat something chilly, chilly things do not actually activate taste buds but they activate nociceptors. That’s why you feel pain. The amount of pain that you feel with the actual taste of this material is the flavour. Ok?

Q: about hot

Hot not present as taste. Dr. said that there are some people can withstand chilly things coz they have high level of noxious stimuli. It has nothing to do with taste. We don’t taste chilly but we actually receive chilly things as flavor not as taste. Please remember this adaptation of taste perception decrease perception of sweetness food when eaten after another.

Q: if we eat something sweet as chocolate it relieve depression/ release the pressure?

A: actually it doesn’t relieve depression straight away but it is part of psychiatric (psychology). I don’t know the chemical reaction. Generally when you eat carbohydrate you can elevate your mood. I call it as a part of flavor. When you eat something sweet your mood will be adjusted so this feeling is a part of flavor of that food. I don’t really know how it works. Other student added that it might relate to serotonin level but dr. don’t have any extra information

We continue. I’ll give you an example after eating kinafah and then you drink sweet tea, tea will be sweet or not. No. Why? Coz the taste buds got adapted. All the taste buds in your tongue got adapted to the sweetness which is very concentrated. That’s why you won’t feel that the cup of tea that you actually drunk after the kinafah or the dessert you won’t feel it as very sweet (tea) although it is very sweet.*dr said you all like sweet tea but he doesn’t. ;)*

Adaptation if stimulation continued… More central level of being accustomed to different levels of sweetness… Actually this getting used to sweet things and not feeling the sweetness of another thing drunk after it is central process not peripheral. Yakni it is not a problem with taste buds it is something that is being actually active in your brain. Central level of being accustomed. It is not peripherally.

Sweetness is considered as “acquired”… Absent or weakly presents in infants… Some people said that sweetness is something acquired. It’s not something that congenital. That’s why infants, they have decrease amount of sweetness. Ok

Finally the factors that affect taste perception. How can taste perception be affected? *dr. will ask about this in the exam*

Oral temperature - cold, when you drink or you eat something cold you will feel reduction in receptor ability. Ice cream very cold but still it is very sweet. Melt it

and make it hot and drink it. You will not be able to drink it because it will be very or extremely sweet. When you make something cold you also decrease its sweetness… So that’s why the temperature is very important. Ice cream need to be sweeter than hot desserts to be acceptably sweet.

Combination of gustants – sometimes we eat something not actually sweet or sour or bitter. It is combination of these things together resulting perception is hard to explain. As example when you eat avocado, it is hard to tell this sweet or sour or salty so that’s why some things actually difficult to explain.

Hormones - also can affect the taste perception Age – reduction in taste perception. As we actually grow in age we will not be able

to taste things as when we were young Genetic factors- are important that’s why we taste things differently Deprivation from salt- this leads to low threshold of NaCl. Dehydration – you are dehydrated. In this case when I give you something salty

you will not taste it as salty. This should be very high in salt in order for you to taste it as salty

Cigararette smoking – diminish taste perception to all kind of taste but they have increase bitter taste (heavy smokers). Ya3ni they have decrease taste to everything else but they have increase taste to bitter.

Local anesthesia – salt and sweet tastes are reduced. Taste sensation is abolished FIRST to bitter sensation then salt then acidic and then sweet.

We would love to thank our Malaysian colleagues for their lovely efforts in lectures note writing …

“All people Die but few of them actually live” hope you all have amazing lives Good luck to all Correcting team