 taste sensation helps us to enjoy foods, detect undesirable foods and to initiate the digestive responses when we put food in mouth that is the cephalic phase of digestion, isn't it? So, how do we detect this taste sensation? Taste sensation is detected by certain cells known as taste cells which are present in taste buds and these taste buds are in turn located on the papilla which are present on the dorsal surface of the tongue. So, let us start from the papilla. See, this is a schematic diagram showing the dorsal surface of the tongue and on the tongue we have three types of papilla. One is the circumvalid papilla which is present on a V shaped area at the posterior side of the tongue. Then there is foliate papilla. This is present on the posterior edge of the tongue and here on the lateral border of the tongue and the last one is the fungiform papilla which is present on the anterior aspect of the tongue. So, three types of papilla are there and simply you remember that circumvalid V is there. So, it is present in a V shaped area on the posterior part of the tongue and foliate. So, how I remember late like that. So, this is present on the lateral edge of the tongue. Now, these papilla, papilla is basically projections on the dorsal surface of the tongue. So, these papilla have taste buds. So, here you see these are the projections and on these we are having these small taste buds. Now, these papilla have different number of taste buds. Circumvalid papilla has around 100 taste buds. Each papilla okay and foliate papilla also has 100 taste buds. However, each fungiform papilla has only 5 taste buds. So, there are less number of taste buds in the fungiform papilla. Now, let us see the structure of these taste buds. You see here is a diagram of a single taste bud and this taste bud is having lot of cells. Basically, these are modified epithelial cells okay because they are present on the surface, isn't it? So, they are modified epithelial cells and they form the different types of cells in the taste buds and these are one is the important taste cells which are basically the receptor cells. So, the sensory transduction that is conversion of the chemical which is causing the taste into an electrical signal is done by these taste receptor cells. Then there are other cells as well, there are support cells and there are basal cells. See, these taste cells have a life span of approximately 10 days. So, they die after 10 days and it is these basal cells whose multiplication causes the production of these taste cells. So, these basal cells are basically stem cells which are responsible for the production of new taste cells. Now, you see these taste cells, they are having certain microvilli. So, each taste cell we see, so I am going to expand this taste cell, they are having microvilli on their surface and these microvilli open on the dorsum of the tongue by means of a taste pore. So, all the microvilli are projecting into the taste pore. So, the taste ends may contact with the proteins which are present on the surface of these microvilli of the taste cells. Now, based on the chemicals which are present in the food, we have different types of taste sensations and basically we have five types of primary taste sensations. So, these primary taste sensations as we all know are saar, salt, sweet, bitter and umami. Saar is caused by acidic foods, okay and the sourness of the food depends on the concentration of the hydrogen ions which is present in the food as the hydrogen ion concentration increases, the sourness of the food increases. Then salty food it is caused by ions and these ions basically are cations like sodium ions and other ions as well, even potassium ions. Now, depending on the cation which is there, the saltiness of that particular food item will be different, you know that normally we are using as NaCl as the salt. But there are food items which are also having saltiness and it depends on the presence of the different cations which are present in that food item. Then sweet and bitter both actually are caused by organic substances and these organic substances vary for sweet it is many substances are there like sugars, alcohol, different types of aldehydes, even ketones they are all sweet. So, don't think that only sugar is sweet, there are other chemical compounds also which are sweet and bitter that is also by organic compounds that is basically the alkaloids, they are having this bitter taste and it is very important because this bitter taste has very low threshold that is important low threshold that means even if it is present in food in very small concentration that can be detected and it has survival value, how? Because this alkaloids are actually present in poisonous substances found in nature in plants. So, that is the reason that evolution of this low threshold of bitter taste sensation has happened. So, we can actually reject the foods which we find bitter. Then finally, there is this umami taste and umami basically means a pleasant taste it's a Japanese word and it means pleasant taste and this taste is caused by food which are having a chemical compound known as L-glutamate. Understanding? So, you see depending on the different chemicals which are present in the food we are having different taste sensation. But remember that even though we are calling that these are the primary taste sensations we can actually detect hundreds of taste various taste sensations we can detect. Why? Because all the taste are a combination of all these tastes. So, there are taste cells which will be stimulated by these different chemicals and actually the sensation which we will have will be a combination and that is being perceived at the level of the cortex. Secondly, you might have heard about the concept that suppose this is the tongue and it is this particular area is responsible for detection of a particular taste and this particular area is responsible for the detection of a particular taste. But that is an old concept you should remember that actually from any area of the tongue we can detect any taste. It is not that a particular area of the tongue is designated for a particular taste. Now, as I told you before that the chemicals which are present in food are dissolved by saliva and then that taste tint that is the chemical makes contact with the microvilli of the taste cell. So, this causes the stimulation of the taste cell and there will be generation of the action potential and all. Finally, it is saliva only which washes away that chemical from the mouth. So, saliva is responsible for the taste if saliva is not there that is in case of zero stomia then there will be decrease in the taste perception. On the other hand, saliva is also responsible for washing away that chemical from the mouth so that we can enjoy the other food as well. So, now let us see how a particular chemical is causing the excitation of the taste cell. So, this diagram I have shown you before this is the diagram of the taste buds having different types of the cells. Now, what happens that when a taste tint or the chemical makes contact with the membrane of the taste cell then there is depolarization of the taste cell. We will see how this depolarization is occurring. Now, this depolarization what it does that in the taste cell there are the vesicles containing the neurotransmitters right. The neurotransmitters will be released and they will stimulate the afferent nerve which are supplying these taste cells. So, this causes the generation of receptor potential in the neuron okay understanding. So, there is depolarization of the taste cell leading to the fusion of the vesicles on the membrane which in turn stimulates the nerve fibers leading to the generation of the receptor potential. Now, this receptor potential is a graded potential meaning that as the intensity of the stimulant or the chemical is going to increase more the chemical in the food substances more will be the magnitude of the receptor potential which is formed and this will lead to generation of the action potential. So, this concept of the receptor potential action potential that I have dealt in sensory physiology in detail in the videos on sensory receptors that you can have a look. I have given the link in the description section of this video also. Anyways, back to it. So, what we were telling that there will be generation of action potential and more the receptor potential more will be the frequency of the action potential. Now, remember here that as long as this chemical is present, there will be generation of the action potentials but initially there will be increased frequency of the action potential and despite the continued presence of the instant the frequency of action potential is going to decrease and hence the action potentials which are reaching to the cortex that is also going to decrease. So, it is going to decrease our perception of the taste as well. This you can very well understand from our common knowledge. Suppose there is particular food we are eating. So, initially we will feel a blast of the taste sensations right but if we keep eating the same food for some time then the taste is gets decreased. So, that is kind of an adaptation happening at the level of the receptor. Okay, but what is the mechanism by which this chemical signal is being transmitted into the electrical signal of depolarization. Now, that varies with the taste sensation or the taste tint. So, for saar and salty, we have seen before that it is basically ions which are involved in this for saar it is hydrogen ions and for salt it is the cations mainly the sodium ions. Now, on the taste cells there is presence of certain channels known as epithelial sodium channels. Actually, whenever this ion is present it can directly enter into the cell by means of these enac channels and that will lead to depolarization. Sodium entry is going to lead to depolarization but there can be entry of hydrogen ions also via these channels understanding. So, hydrogen ion entry also leads to depolarization of positive ion so there will be depolarization. Second, hydrogen ions also act by another mechanism what they do that they block the potassium channels and once potassium channels are blocked. So, here these are potassium channels suppose potassium cannot go out right. So, potassium will start accumulating within the cell and that is also a kind of depolarization because positive ion getting accumulated in the cell. So, sodium ions enter into the cell by enac channels then hydrogen ions act by two mechanisms either they can act directly by enac channels or they can also block the potassium channels thus blocking the efflux of potassium from the cells and leading to depolarization. So, that is for the ions which are causing the sour and salt taste. The other taste sensations that is the sweet, bitter and umami for these we have different types of receptors that is the G protein coupled receptors or they are also known as metabotropic receptors. So, these ion ones are known as what? Ionotropic receptors, ionotropic receptors and these are metabotropic receptors. So, the chemicals that is the organic compounds which are there they will bind to these receptors they will lead to the activation of these G protein coupled receptors ultimately causing downstream events. So, either there will be increase in CAMP or decrease in CAMP or depending on the type of the G protein coupled receptor and hence ultimately the product is going to cause the opening of the different ion channels causing the entry of the sodium ions. So, this is an indirect way of action by metabotropic receptors ultimately leading to depolarization ok. So, sensory transaction occurs based on the chemical which is there fine. Now, let us go on to the final part that is the taste pathways. So, taste pathway like other pathway consists of three neurons. So, there will be first order neuron, second order neuron and third order neuron. Now, you see that there are different nerves which are carrying the information one is the facial nerve. So, here you see seventh nerve is there it carries the information from the anterior to third part of the tongue. So, basically these exons are supplying the tastes which are present in the anterior to third part of the tongue. Then we have the ninth cranial nerve that is the glossopharyngeal nerve and it is supplying the area that is from the posterior part of the tongue that is the circumvalid papilla from there the information is going via the ninth nerve ok. And again behind the posterior most part actually these taste buds I have missed before these taste buds are also present on the pharynx they are present on the tonsillopillars which are there they are present there also then even in the epiglottis ok they are present and in the soft palate as well. So, from these areas it is the tenth cranial nerve which is carrying the taste sensation. So, three nerves are involved seventh cranial nerve from anterior to third part. Ninth cranial nerve glossopharyngeal nerve from the posterior one third part of the tongue and the tenth cranial nerve from the posterior most part right. And all of these reach the NTS in the brain stem nucleus tractus solitarius and they go and synapse there. So, you see here synapse is shown. So, what is the first order neuron? From the tasers the cranial nerves carrying information to the nucleus tractus solitarius. So, all these are forming the first order neuron. Now from nucleus tractus solitarius the information is going to the thalamus and the nucleus here is ventropostromedia nucleus of the thalamus. So, this is the second order neuron and you see one specialty here that it is on the ipsilateral side that is there is no crossing over. So, in the taste pathway there is no crossing over. It goes to the postromedia nucleus of the thalamus and synapses there. Then from postromedia nucleus of the thalamus it goes to the gastiatric cortex which is basically present in the parietal cortex but little deep right. So, it goes into the gyri of the parietal cortex in the anterior insula and frontal operculum anterior insula and frontal operculum in the parietal cortex. So, that is the third order neuron which forms a taste pathway fine. But remember one thing here that this is a simple taste pathway. However, this pathway makes contact with other pathways as well. You see that the flavor of food which we get how it is happening? It depends on both the smell and the taste. If the smell of the food is not enhanced much then the flavor of the food we feel is lesser right. Secondly that there is activation of the salivation as well that is happening because some information from this nucleus tractor solidarius within the brainstem it is going to salivatory nuclei as well okay salivatory nuclei and from there there is activation of the efferents to the salivary glands and initiation of the salivation. So, that is the reason that when we keep food in mouth there is initiation of the salivation. Now, only thing which is left is certain abnormalities and there are certain terms which you should know. One is egusia that is absence of the ability to detect the taste then there is hypogusia. Hypogusia is decrease in the ability to taste then there is dyscusia dyscusia or also known as paragusia which is unpleasant perception of taste unpleasant perception. So, taste is something else but we are tasting something else that is dyscusia or paragusia. So, that was the complete physiology of taste sensation. Thanks for watching the video. 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