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Current time:0:00Total duration:12:31

Video transcript

we have five main tastes each taste depends on a particular receptor that's localized somewhere on the tongue so the five different things that we're able to taste our bitter compounds salty compounds sweet compounds sour compounds and one more thing known as umami and this is basically the ability to taste a particular molecule known as glutamate so glutamate now these five tastes all depend on a particular receptor so let's imagine that here we have a tongue and this is the front of the tongue this is the back of the tongue we basically have tastebuds sprinkled throughout the tongue so these tastebuds are kinda all over the place and for the most part they are very very highly localized in the more anterior aspect of the tongue so they're more localized in this anterior aspect of the tongue more so than in the back there are three different types of tastebuds there are taste buds that kind of look like this and these are known as bunch of form taste buds I'll write that down bunch of form taste buds and these are the ones that are mostly found over here in the anterior part of the tongue there are also taste buds that look like this where the little buds are over here and these are known as full ea8 taste buds and these taste buds are mostly found over here on the side of the tongue and finally there are taste buds known as circumvallate taste buds which look a little like this and the taste buds themselves are actually over here on the sides and these circum bay light taste buds are mostly found back here in the back of the tongue now each one of these taste buds if we actually zoom and so if we kind of zoom in to a individual taste button what we would find we would find a little pour and this pour would have a bunch of different types of cells so it would have like one cell over here it would have another cell right next to it it would have third cell over here and so on and basically each one of these cells is in is responsible for one of these five tastes so for example we can take this pink cell over here to be able to detect salty things and for example we could imagine that this whitish cell would be able to detect sweet things and so on and so forth so the general idea is that each taste bud so this thing would be a taste bud each taste bud contains all of the different taste cells so it every single taste bud is able to detect bitter salty sweet sour and umami compounds and these taste buds are found all around the tongue mostly in the anterior part of the tongue so another way to say this is that we're able to taste all five of these different tastes everywhere in the tongue so it's not as if bitter compounds can only be tasted in this region and salty compounds can only be tasted in this region and sweet and sweet compounds only in this region that's not the case this is not the case so it's not like that and instead we basically are able to taste all five of these different compounds throughout the entire top and mostly most of the taste buds are concentrated over here in the anterior part of the tongue so what I want to talk about next is something known as a labeled lines model so here we have a taste bud and as I mentioned before we have all of the different types of cells and each one of these cells is specialized to a particular one of the five tastes and what you can see down here what you can see down here is each cell has a little axon projecting from it and what is interesting is that these axons actually remain separate all the way to the brain so all these get projected and they eventually reach the brain and they actually synapse on different parts of the brain so let's imagine that this is the gustatory cortex so let's imagine that this is the part of the brain that receives input from various taste cells well each one of these different axons will synapse on different parts of the brain so here we've so here we've got this axon it's going to go it's going to synapse over here and every other acts on every other taste cell that is this color blue so let's just say this is a sweet cell let's say this is a sweet a sell every single sweet taste cell will send its axon to the brain and it will all end up in this one region of the gustatory cortex so we can say this is the sweet region of the gustatory cortex I keep using this word gustatory and and taste can also be thought of as our as our sense of get station so this station similarly all the cells that are green and we can say that green could be bitter so we can say the green cells are our bitter cells bitter taste cells there they're really bitter they will all synapse and one part of the cortex which we can imagine to be the bitter part of the cortex it's very mad at the world so this is basically how taste cells send their projections to the brain they actually send them through dedicated axons and there's no mixing and this is known as the labeled limes model so let me just write that down here labeled lines model and again what the labeled lines model is saying is that each one of these cells has its own dedicated labelled line and this distinction is basically carried on all the way to the cortex itself now let's look at an individual taste cell it's so let's look at an individual taste cell so here's the axon this is the cell nucleus and little hairs that kind of project out into the tongue so what happens is let's say that this is a sweet cell a sweet taste cell so it's sensitive to sweet molecules so let's imagine that we've got a little molecule of glucose so let's imagine you're eating cake of some glucose will I will hit the tongue and some of the glucose will find its way over to a sweet cell and how does it how why is this cell sweet world sweet because it actually has receptors in the membrane so as little receptors in the membrane that are sensitive to glucose and other sweet molecules so when the glucose binds to the cell it triggers a cascade of events that eventually allows the cell to depolarize and send an action potential all the way down its axon and to the brain now I mention that we have five different tastes so we have sweet umami bitter sour and salty these top three taste cells over here have similar receptors and these receptors are known as G protein so g-protein coupled receptors so basically what a g-protein coupled receptor is let me just go ahead and draw it out it's a inner membrane protein so this is the protein and then let's imagine this is the cell membrane so this protein is a receptor and the receptor binds to a ligand so let's imagine that we have glucose in this case so let's imagine that a molecule of glucose comes in hits the receptor the receptor will undergo a conformational change that basically causes a g-protein which is coupled to it so this is AG protein over here so when the like binds to the receptor it causes the g-protein to dissociate so this bond is broken and the g-protein goes off and if the g-protein can do a couple different things inside the cell so this is inside the cell over here this is outside the cell so basically the g-protein can go off and do a few different things now one of the things that the g-protein can do when it gets dissociated from this receptor is it can actually open some ion channels so the g-protein can go in open ion channels so the opening of ion channels via a conformational change can actually cause can cause the cell to depolarize and fire an action potential so fires and action potential so on the other hand these two taste cells sour and salty rely on ion channels so instead of relying on a g-protein coupled receptor they actually rely on a receptor so let's imagine this receptor over here so in the case of salty let's imagine that a molecule of NaCl so a little salt molecule comes in it'll bind to this ion receptor and then cause the receptor to open up so the receptor will open up and it will allow positive ions outside the cell to flow in when positive ions flow inside the cell it causes the cell again to depolarize and fire an action potential and that action potential goes to the brain so let's look at what would happen if we put a salty receptor inside a sweet cell so here we have a sweet cell and the reason that this cell is is sweet is because it will have receptors in its membrane that bind to glucose so let's imagine that we took a salty receptor so as I said previously the salty receptors are ion channels and let's imagine that we put a little salty receptor here now if you remember the labeled lines model basically says that a particular cell so in this case a sweet cell has an axon that will eventually reach the brain and when it reaches the brain the brain since the axon is coming from a sweet cell will detect the compound that activated the sweet cell as sweet so what would happen if we put a salty receptor inside the sweet cell well if we have NaCl so some some salt comes in and it will activate this receptor and when this receptor is activated opens up and positive ions outside flow into the cell this sweet cell depolarizes and causes an action potential to fire and by the time the action potential reaches the brain the brain isn't able to to differentiate between a sweet molecule or a salty molecule and a sweet molecule so both should both salts and sugar will activate the sugar cell but since the brain has already decided that hey this is a sugar cell and every time this cell normaly activates it's usually something sweet it's going to think that the salty compound is actually sweet it's going to detect it as being sweet so if you were actually if you were to put a salty receptor and a sugar cell you can actually trick your brain into thinking that something that is salty such as NaCl sodium chloride was actually sweet