Let's take a look at the pathways that these signals take from where they're created in the tongue to the brain. So we call these the gustatory pathways. In the taste buds, all of those gustatory epithelial cells or those taste receptors, right, in your taste buds, they're all innervated by cranial nerves. But there's more than one cranial nerve that can carry the taste information to your brain. So towards the back here, Okay, so where you see the epiglottis, now towards the back here, any taste information from cells back here is going to be carried to the brain by cranial nerve 10, which is your vagus nerve.
So to the back part of the tongue in pharynx, that is going to be innervated by cranial nerve 9, which is glossopharyngeal. And then the front two-thirds So the anterior two-third part of the tongue is going to be innervated by cranial nerve 7, which is your facial nerve. So whether those taste receptors are sending signals through the vagus nerve, cranial nerve 10, glossopharyngeal nerve, which is cranial nerve 9, or the facial nerve, which is cranial nerve 7, they are all then going to carry those action potentials through these nerves.
to the same area in the brain. All right, so all this taste information from here essentially is going to be transmitted to the medulla oblongata, right, to the medulla oblongata. Now from the medulla oblongata, we synapse there and then we will send that information to your thalamus and then from your thalamus we will then transmit those signals to the gustatory cortex. in your cerebrum and it's only when these signals get to the gustatory cortex right this part of the cerebrum right there that is where the information gets processed in a way that allows you to perceive or recognize the taste and then try to identify that taste all right but this is the basic pathway right tongue through one of three cranial nerves to the medulla oblongata and then from there to the thalamus and from the thalamus to the gustatory cortex where we can perceive and identify the taste.
Just like we can discriminate different scents from each other, we also have the ability to discriminate different tastes from each other. So we will call this gustatory discrimination. We have several different types of taste sensations or tastes that we can perceive. Originally, people identified what we call the four primary taste sensations. And so those are sweet, salty, sour, and bitter.
Sweet, salty, sour, bitter. These are the four primary taste sensations. So these are what we call primary taste sensations because every taste bud that you have can detect all of these primary taste sensations. All right.
So it's kind of a universal thing that a taste bud could do. Every taste bud in those papillae can detect all four primary taste sensations. So once we identified those, Later on in time, we actually discovered two more different taste sensations after that. So instead of changing the number from four to six, we say that we have four primary taste sensations that are detected by all taste buds.
And on top of that, we actually have two other taste sensations. So the first past the four primaries, the first one we have after that would be umami. All right.
Umami essentially is how savory the food is, savoriness. So we see a lot of umami taste in things like meat, broths, vegetables, mushrooms, fermented products like soy sauce, tofu, pickled things, so things that get fermented. So it's savory, right? And you could imagine tasting meat or like chicken broth or cooked vegetables and mushrooms, right?
It has a certain unique kind of savoriness to it. That's umami. So the way that umami works is it's really a couple of different types of molecules that can create an umami type of taste. And so umami comes from nucleotides that are present in food.
and specifically the amino acid glutamate. So different kinds of nucleotides that form nucleic acids like DNA and RNA, and specifically the amino acid glutamate, can activate these umami receptors and give us that savoriness to our food. So one reason umami isn't considered a primary taste sensation is because the amount of umami receptors vary depending on what type of taste bud it's at. We tend to have a ton of umami receptors.
in those valate papillae at the back of the tongue, all right? Those up to 12 valate papillae we have on the back of the tongue, those in particular contain tons of umami receptors. All right, whether you believe it or not, another taste sensation is water, all right? So we do have water receptors as well. Now, you know, you're going to say, well, water really doesn't have much of a flavor, right?
That's true, right? That is true. But... We use these water receptors less for tasting water, more to activate them to let our body know that we're drinking water.
So when you drink water, water hits water receptors, okay, here, like sort of back of the tongue, kind of in the pharynx area. And then those receptors that get, those water receptors that get activated in the back part of the tongue and pharynx, right? Instead, that information is going to be sent to other parts of the brain.
to let us know that you're drinking water and that's going to help us kind of control the amount of water in the body and also help control the amount of blood the blood volume in the body as well okay so that basically if you have low blood volume right and we want to make more blood that's good for your body to know that you're drinking water water's incoming so we can use the incoming water to make more blood and increase our blood volume so we can have the right amount of blood right Just like with smells, different tastes or taste sensations can require different concentrations or different amounts for you to perceive them. So we'll call this idea taste sensitivity. The amount or the threshold that a particular taste receptor needs to be able to detect that particular taste.
All right. So the way that we evolved, our taste receptors tend to be much more sensitive to unpleasant types of... of tastes.
So I'll give you a couple of examples of this. So you see here in the picture, we have all these citrus fruits, right? So we're about a thousand times more sensitive to sour tastes, like acids, basically, than we are to sweet and salty, right? So let's think about this. Why would you be a thousand times more sensitive?
It takes a thousand times less of a sour acid to perceive it than something that's sweet or salty. Well, acids are acids. So if it's too concentrated, too acidic, then that can damage your mouth and pharynx. So we want to be really sensitive to a very sour kind of a taste because too much of it could damage your body. And so we want to be able to get rid of it or spit it out if it's just too much or try to get it out of the body, right?
Bitter also, we're very sensitive to bitter tastes. So we're about a hundred times more sensitive to bitter. than we even are to acids.
So we're really sensitive to acids or sour. We're about 100 times more sensitive to bitter chemicals than we are to those sour acids. Now, the reason we're even more sensitive to bitter things than sour acids is because a lot of toxins that could harm us are bitter.
They have a bitter taste to them. So we evolved the ability to, if we get something fairly bitter, it could be a toxin from like a plant or something like that. So instinctually, It's like, oh, this is a bad taste, so we want to spit it out. So hopefully before we consume too much of it and poison ourselves.
So there is a pretty big variability in taste sensitivity among individuals. So there's a huge genetic component to how well you're able to taste something. So if you see people, you know, chefs, they'll take a tiny little...
taste of something, a little smidge or a little teeny tiny spoon, and they can kind of break down all the different ingredients and that little tiny thing that they tasted, right? They have a really good taste sensitivity. So for certain people, it doesn't take much of a particular chemical for them to perceive it and taste it and even identify it, right? And again, that's under genetic control largely. Now as we age, just like with smell, our sense of taste decreases as we age.
That is due to the number of those taste buds and receptor cells declining with age. We just have fewer of them as we get older, and so our sense of taste isn't as good. This can cause problems in the elderly population because they just don't want to eat.
And they'll say, I don't want to eat this because the food has no taste. and you taste it and you're like, no, this tastes fine. I can taste it.
It's seasoned perfectly. I love it. Well, they're just, it takes more, more taste for them to be able to detect it. And so food often tastes very bland. Or if elderly people cook the food themselves and season it themselves, for us, they tend to over season it and it's very, very salty or whatever that taste is that they added.
But for them, it's perfect again, because they're not quite as sensitive to taste. Now, Taste and flavor often get used synonymously, but flavor is a little bit different. So when I say flavor, when we talk about flavor in anatomy and physiology, flavor is defined as the distinctive quality of a food or a drink.
So that includes both the taste that you're getting from those taste receptors in your taste buds. Yes, absolutely. But it also includes... smells that you're getting that are being detected by those olfactory receptors in the olfactory organs, and what we call taste-related sensations.
So taste-related sensations aren't those taste sensations that we talked about, like sweet, sour, bitter, umami, those kinds of things, right? Those are true taste sensations. But we have also taste-related sensations. So these would be things like if the food is spicy or hot, if the food has a minty kind of cool quality to it.
Does the food have a really dry or astringent kind of flavor to it? Is there a metallic taste in it, like the metallic taste of blood or something? What about the temperature of your food? What about the texture of your food? Is it mushy and soft or is it crunchy?
Is it sharp, right? So these things like this, the spiciness of it, if it's minty or cool, does it taste dry, metallic? What is the temperature of our food?
What's the texture of our food? All these are not tastes themselves, detected by taste buds. but these are taste-related sensations.
So all three of these things come together to form the actual quality of your food, the flavor of your food. Taste, taste-related sensations, and smell information coming into your nose, right? So without your sense of smell, you are several thousand times less sensitive to flavor than having it, right? In other words, if you get a cold and your nose is all stuffy, All that stuffiness and all that extra mucus in your nose might block a lot of those odorants from hitting those olfactory organs in your nose.
And if you can't smell that well because it's all plugged up with mucus, then your food just doesn't taste as good, right? It tastes very bland. You can't taste anything, kind of a deal, right?
So olfaction makes you, like, it makes a huge difference in flavor of your food. Several thousand times more sensitive to flavor than just taste alone. So it is a big deal. Smell is just as important as taste to flavor.