The Science of Everything Podcast - Episode 58: Taste

Episode Date: January 27, 2014

A discussion of how our sense of taste works, including an overview of the basic anatomy of the tongue and relevant brain circuits, a discussion of taste buds and how they work, a review of the five basic tastes and how they differ, and a look at some other interesting topics such as pungency and aftertaste. Recommended pre-listening is Episode 10: The Cell and Episode 38: Neurons and Synapses.

all listening to The Science of Everything podcast, episode 58, Taste. And I'm your host, James Fodor. So in this episode, we're going to look at the sense of taste. In particular, we'll discuss the basic anatomy behind taste, how the different senses in the tongue detect different chemicals, how those signals are then transmitted to the brain, and the different regions of the brain that are responsible for processing those signals. And I'll also have a look at a few other interesting aspects of taste,

like pungency, for example, and aftertaste and a few other interesting little bits and pieces. So, let's get started. So the sense of taste, more formally known as gustation, is a sensation produced when a substance in the mouth reacts chemically with receptors of the taste buds. In order for this to happen, it's generally necessary for the food to be dissolved in a solution. So that's one of the reasons why we have saliva, because food goes in your mouth and you chew it up and it gets dissolved in the saliva. And then that saliva sort of sloshes around over the tongue and interacts with, you know, with the chemical receptors and the taste buds.

So it's quite difficult to taste things if you don't have any saliva or if you have a very dry mouth. So taste is a form of chemoreception, which means that it's the detection of sensors via chemical interactions between molecules. That's similar to, for example, olfaction smell, which is also chemoreception.

And distinct from mechanoreceptors, which are like how we detect pressure and touch and also sound, Sound is detected by mechanoreceptors, so those detect motion, and photoreceptors, which operate by detection of photons, so light, and that's, for example, in the eye. We have photoreceptors. But taste, or castation, is a form of chemoreception. It's important to distinguish between the related, though, distinct concepts of taste and flavor. So in sort of everyday language, we use them fairly synonymously. You say something tastes a certain way, it has a certain flavor, that means basically the same thing. But, In more precise terms, there is a difference between flavour and taste. So, taste refers specifically to gustation, that is, the chemoreception of particular types of molecules by taste buds in the tongue, by specialised receptors that are in the mouth. They're not actually just on the tongue, but they're mostly on the tongue. On the other hand, flavour is a more general term.

It refers to an overall sensory perception of food or other substances as well, but usually food, which is determined in large part by taste, but all. Also by smell and certain other senses as well, these are called trigomoneal sensors, which, for example, detect chemical irritants in the mouth and throat, as well as other things like temperature and texture and things like that. So, flavour is a more general term. It includes taste, but also includes the smell of food, also includes the texture, the temperature, and these various other aspects of food, which are detected by different sensory mechanisms. And we'll have a bit more of a look at some of those later in the episode.

So a flavorist, also known as a flavor chemist, is someone who uses chemistry to engineer artificial and natural flavors, and they often work for food companies, or for chemical companies which then sell their products onto food companies. These people in particular will make a distinction between taste, which is, again, the particular sensory mechanisms, chemoreception behind the sense of gustation, and flavor, which is a more general or gestal's phenomenon. So when we taste food and we say that something tastes like ice cream or tastes like rice or whatever, generally that perception comes from a combination of olfaction, gustation, the trigomanial senses, the temperature and texture, and all of those other things as well. So that's flavour, and it's not purely a result of taste. But in this episode, we're going to mostly focus on taste as gustation specifically, although we'll talk about taste more generally a little bit as well.

So, with that broad background, I'll now begin to go through the basic anatomy of taste from the mouth right through to the brain. You've probably heard about the five basic tastes, or maybe you heard of them as four basic tastes. The four more traditional ones in Western context are salty, sour, sweet, and bitter. Now, it's generally accepted that there's also a fifth basic taste, which is called umami. I'll talk about those in sequence later on. But it's important to understand that the reason there are five basic tastes is because each of those tastes is mediated by a different chemical receptor on the tongue. So in that sense, it's quite similar to the three primary colours.

The reason we see in three primary colors is because we have three different types of photoreceptors. Well, I mean, there are four types of photoreceptors, but one is the rods, which are only seen black and white. But in terms of color vision, there are three different types of cones which are, responsible for television, and we talked about this in the three episodes on vision. But it's, so it's similar in the case of taste. The reason we have five basic cases is because, as far as we know, there are five basic types of chemoreceptors responsible for gastation. You may have seen these in the context of a tongue map or taste map, which is a map of a

tongue with different regions mapped out as being responsible for salty or sour or bitter or whatever. Now, this tongue map is quite misleading. it's not really accurate. Importantly, it's not the case that different regions of the tongue are exclusively responsible for different tastes. Any given taste is produced by a pattern of sensory activity and sensory perception across the tongue. So all regions of the tongue are responsible for all types of tastes. It is true, however, that different types, that different portions of the tongue are more sensitive to certain types of taste. So it is true that the back of the

tongue and sides of the tongue are relatively more sensitive to bitter, and the tip of the tongue is relatively more sensitive to sweet. And that's because there are more of those types of chemoreceptors on those types of tongue. Remember five different types of chemoreceptors, five basic tastes. So certain regions of the tongue have relatively larger concentrations of certain of those sensory mechanisms. But it's not an exclusive thing. It's not like the tip of the tongue is only sensitive to sweet. It's just disproportionately sensitive to sweet. So you have to take those tongue maps with a bit of a grain of salt because they're not, they're a bit misleading. Okay, so now into some more specific details.

The surface of the tongue is covered with small projections which are called papillet, which are basically shaped like ridges or pimples or mushrooms. There are actually three different types of them which are on different regions of the tongue. If you stick out your tongue and look in a mirror and shine a decent light on it, you'll be able to see these little projections because they're visible to the naked eye. As I said, there are three different types of them, and they're scattered about different parts of the tongue. I won't bother going through the exact names of them and where they are, because that's almost impossible to describe. And it's not too important for our purposes. Each papillai has from a few, like a handful, to several hundred taste buds located around its sides.

So the best way to sort of visualize this is imagine the papillai is sort of a squashed mushroom coming up from the tongue, although they don't all look exactly like mushrooms, but for our purposes we'll just think of it like that. And the taste buds are located sort of on the sides, like the stalk of the mushroom. So they're not right on the top, they're sort of around the sides. The taste buds themselves are roughly circular in shape.

They look kind of like an onion, and they're sort of rolled together and have a little point on the top. And the taste buds are actually not on the surface of the papillae, but they're actually embedded in the sides. So if you imagine a mushroom and around the edges of the stalk of the mushroom, embedded on the surface but still inside the stalk of the mushroom are these little

small onion shape sort of things. Those are the taste buds. This is my sort of crude attempt to explain what this looks like without diagrams, but hopefully you get the basic idea. Now each taste bud in turn is comprised of around 100 taste receptor cells. And these cells are arranged so that the tip of each receptor cell protrudes in a small bulb at the top of the taste butt. So remember I said it's kind of like an onion in the sense that they curl upwards, and you can think about the top as having lots of tips from all of the different cells as they're rolled around the side. Those tips are covered with small microvely, which are small projections of the cytoplasma of the cell. And it's these little regions of the

microvely which are exposed to the, to the outside of the tongue, at the edges of the papillae. And it's these regions of the microvele which actually contain the receptor molecules, which we'll get into in a moment. So just to recap, just to recap, cap, there's sort of three levels of structure here. There's the papillae, which are visible to the naked eye, and those are scattered around the tongue. Not all, the entire surface of the tongue isn't covered

with papilla. Again, if you look at your tongue, you'll see that's only particular regions of them that have large concentrations. They're sort of scattered, fairly dispersed, in some regions, concentrated in others. So first of all, there's these papilla. Then each papillae has, from a few to a few hundred taste buds

embedded around its sides, and then each taste bud, in turn, is comprise of around 100 receptor cells. So that means each papillae will have from maybe a few hundred to tens of thousands of actual taste receptor cells in it. Now, molecules that can be detected by these various taste receptor cells are called tastence. And the way they're detected is by binding to various receptors and or passing through channels in the microvele of the taste cells that form each taste bud. This binding to receptor molecules or passing through the channels in the membrane triggers action potentials in synapt gustatory aphorant axons, which then bundled together

to form cranial nerves, which then carry these taste signals into the brain. So just in case you didn't get what that means, the taste cells themselves are not actually neurons. This is distinct from, say, these cells in our retina, or the cells that are responsible for olfaction, which themselves are actually neurons, which means they carry signals directly to other neurons. The taste cells are not actually neurons. The way they send on their signals is by producing an action potential in a synapsed neuron, a different neuron

which is connected to the taste cell. And these cells then have axons which project and bundled together to form cranial nose which then carry the signals into the brain. But these axons are called gustatory afferent axons. Affirant means they carry the signals out

and gustatory while they're responsible for taste. So the taste cells themselves don't carry the signals. The taste cells are synapsed with gustatory aphrant axons, which then all bundle together, you know, that they sort of trace together from different papilla across the tongue, and form the cranial nerves, which carry the signals into the brain. We'll go into the neuroanatomy of that in a bit more detail later. So hopefully that gives you an idea of the basic anatomy behind the taste receptors on the tongue itself. Now let's move into talking about the five different types of taste receptors. Remember I said five basic tastes, five basic taste receptors, so let's look at each of those in turn.

Sourness is the taste that detects acidity. And acidity, as you might recall from the episode on acids and bases, is essentially just the presence of hydrogen ions or hydronium ions, sort of the same thing. Now, the mechanisms by which these are detected are not really understood very well, particularly because they seem to overlap a fair bit with the mechanisms for detecting saltiness, which we'll get to in a moment. But one model that we have proposes that hydrogen ions that surround the cells inhibit the potassium channels, the potassium channels in the cell membrane, which allow potassium ions to move back and forward across the membrane. So the idea is that these hydrogen ions inhibit the potassium channels, and at the same time, the hydrogen ions also themselves are able to pass through the cell membrane through various other channels, by a combination of inhibiting the hyper-polarization of the cell, because that's what the potassium channels do. The potassium channels allow potassium ions to leave the cell. Potassium ions are positively charged,

which means when they leave the cell, the cell becomes hyper-polarized, which means more negatively charged. Again, if you remember our episode on Action Potentials and Synapses, we talked about these things. So it's sorted by inhibiting the potassium channels, the hydrogen ions inhibit the ability of the cell to become hyper-polarized,

and at the same time, direct intake of the hydrogen ions, into the cell, remember hydrogen ions are positively charged, so that, moving those into the cell causes the cell to become depolarized. So both of these, a combination of these mechanisms, plus maybe some other ones we don't know about, causes the cell to become depolarized, which in turn causes the connected affront axon to fire an action potential. So to summarize that rather complicated explanation, it's not fully understood exactly how

the hydrogen ions cause a depolarization in the taste cell, but we know that they do. and that there are cells that are sensitive to only producing action potentials from hydrogen ions. And so these are the cells that are responsible for detecting sourness. Saltiness is a taste that's produced primarily by detecting the presence of sodium ions. So sourness is basically you're detecting hydrogen ions. Saltiness, you're detecting sodium ions. Remember, sodium is one of the main components of sodium chloride. So when you taste salt, you're actually tasting the sodium component of it, not the chloride component of it.

However, there are also other ions of the alkali metals that also taste salty, mainly because the ions are similar to sodium and so that they can activate similar mechanisms. But basically, the less similar the ion is to sodium, the less salty it will taste. So, for example, lithium and potassium ions are quite similar in terms of size and reactivity to sodium, and so they tend to taste most similar in terms of saltiness. So you don't have to have sodium chloride to taste salt, but that's the the thing that will taste most salty because that's what the mechanisms are sort of best adapted to detecting. The mechanism for detecting salt or sodium ions is really quite simple. The

catars just passed directly through open sodium channels across the cell membrane. Remember, in the case of sourness, it was an issue of potassium channels plus other channels that hydrogen ions can pass through. So it's thought that the hydrogen ions, for example, are able to pass through sodium channels because hydrogen ions are smaller than sodium ions and they're also positively charged. So basically, if the sodium can pass through, the hydrogen can also pass through. So that's why I say that there's some overlap between sourness and saltiness, because basically if a cell is able to detect the presence of sodium ions by having them pass directly through sodium channels and thereby triggering voltage-gated ion channels

and thereby causing local membrane depolarization, which in turn leads to an action potential being fired, if the cell is able to do that, then it's also going to be able to detect hydrogen ions as well. So it's not fully understood how different cells distinguish between whether they're detecting hydrogen or whether they're detecting sodium. And in fact, there's evidence that most cells actually detect a bit of, well, most cells that are either responsible for sourness or saltiness, detect a bit of both. That is, they respond somewhat to both hydrogen ions and sodium ions. And so in a sense, these cells are sort of detecting both sourness and saltiness, but it seems that some cells are relatively more responsive to the sourness part, and some cells are relatively more responsive to the saltiness. part. And as far as I know, it's not fully understood exactly how the different cells determine whether they're being stimulated by a hydrogen or a sodium, or more to the point,

how the different cells differentially respond to hydrogen versus sodium or the other factors that go into that. But there appear to be mechanisms which enable these cells to at least proportionately, so not exclusively, but proportionally specialize in detecting hydrogen ions, and therefore those become salt receptors, detectors, and those that disproportionately detects sodium ions, and therefore those become saltiness detectors. But as I said, the mechanisms there, particularly the sodium channel through the membrane, do overlap to a degree.

That's sourness and saltiness. Basically, in both cases, we're detecting the presence of positive ions. Sweetness is a bit more complicated, and also simple, depending on how you look at it, because it's produced by the detection of sugars, which bind to particular sweet receptors that are located on the cell membrane. So in the case of saltiness and sourness, the action potential was triggered by passage of positive ions across the cell membrane. In the case of sweetness, it's completely different.

So sugars are not positive ions, they're larger organic molecules. They can't pass across the cell membrane in the same way. So instead, their presence is detected by special receptor proteins located on the, well, located actually through the, the cell membrane. These special proteins are called G-proteins or G-protein coupled receptors, and that basically just refers to the class of proteins that these are from. They're quite common in, well, cell biology in general, I suppose, because they mediate the transfer of many signals between different cells. We'll talk about these when I finally get around to doing an episode on the cell membrane and cell signaling. So many signals that are sent between cells

and received from one cell to another are mediated by basically, certain chemicals binding to these different types of G-protein-coupled receptors. And basically what happens is the molecule binds, so that means it forms a chemical bond with the outside of the protein, and then that causes a conformational change or a change in the shape of this protein, which is embedded in the cell membrane. And that in turn triggers a series of other chemical reactions called a secondary and tertiary messenger cascade within the cell, which basically leads to a series of protein interactions, which leads to an action potential.

Or in this case, it leads from an action potential. G-protein-coupled receptors don't always produce action potentials. They usually, that they can lead to all sorts of things, like, for example, changes in gene expression or changes in metabolism. In this case, they lead to the production of an action potential. Because remember, that's ultimately how our brain detects taste. It's through the receipt of an action potential via the cranial nerves that come from the tongue. And so the cell, the taste receptor cells need to trigger an action potential in order to produce that. And so that's what these G-protein-coupled receptors do that are responsible for detecting sweetness.

They bind specifically to sugar molecules, and that binding triggers a confirmation of change in the protein, which then leads to secondary messenger cascade, and through a variety of intermediate mechanisms, leads to an action potential. We don't need to go into the details of what those mechanisms are. It's basically just one protein binding to another protein binds to a different protein. And I'm not sure if I've talked about this in detail, so let me just explain one of me by binding, of proteins. It just means that you have two molecules, they come together and form some bonds with each other, and that causes a change in the confirmation or shape of one or both of the proteins, which

then leads to some other thing happening. Like, for example, you can have an enzyme, if something binds to it, its confirmation changes, which then leads to it being activated or deactivated so that the enzyme either will work or won't work. So that's an example of the way these processes are mediated. Anyway, so that's how sweetness works. The particular sweet protein, as far as I know there's one sweet receptor protein, although there may be more, but there's one main one that we know about, which is made of a combination of T1R2 and T1R3 proteins bound together. That's not particularly important, but just understand that there's one particular protein that is mostly responsible for detecting sugars.

Also, sugar receptors aren't only receptive, do not only bind to sugar molecules, particularly glucose is the main one that it's responsive to. But it's also receptive to other molecules, and this is how artificial sweeteners work. Basically, they just find other molecules that will also bind to these same sweet taste receptors and produce an action potential in these same cells. But the reason you would want to do that is because sugar has energy content. It's metabolized by our bodies to produce energy. And in the West these days, we don't want that because we tend to eat too much food and we don't want the extra calories. So basically, if you can find some molecule that stimulates the same taste receptors and so tastes

nice and sweet but doesn't actually have any calories, so it's because it's not metabolized but in the same way by our body, then that would be a good thing. And that's essentially what these flavorists do, these flavor chemists that I talked about before. They try and find different molecules that will bind to these receptors and therefore stimulate sweet or other tastes. And so if you put them in food, they allow the food to taste nice without contributing to the number of calories present in the food.

Okay, so that's sweetness. Now moving on to bitterness. Bitterness is actually the most sensitive of all the tastes, so you can taste it, you can detect a bitterness taste even without very much substance at all. It doesn't require very much of the relevant chemicals to stimulate the receptors because they're very sensitive. Many people perceive it as an unpleasant, sharp or disagreeable taste, although some people like it as well. But certainly it's not considered to be obviously attractive in the same. way that, you know, sweetness and saltiness are. The reason for this is because many toxic substances are bitter, particularly poisonous substances from various plants. And so we have developed

as, well, we as humans, but also mammals in general, have detected, have evolved a special sensitivity to and aversion to, bitter tastes, basically as a way to, as a way to help detect, and then expel these substances when we come into contact with them. If you imagine the shape that your mouth forms, when you taste something that's very bitter, your sort of tongue sort of sticks out and your lower jaw sort of clenches a bit. You can easily see how that's an adaptive mechanism for expelling the substance from your mouth. So that doesn't mean that you automatically spit it out, but it means that your body sort of has a kind of a reflex reaction to prepare yourself for expelling that substance. It's also, incidentally, a very similar response that we see in terms of

the motor actions and the facial expression that we exhibit when humans feel the emotion of disgust, and there's some interesting connections between the particular, between the connection, basically, between a bitter taste and the emotion of disgust, which we might look at in more detail if I do an episode on emotions. But anyway, bitter tastes are mediated by a large class of receptor proteins called the T2Rs, or if you remember the sweetness was produced by T1Rs. So these are related but different receptor molecules. The basic idea, though, is similar, so sweetness is mediated by, what I remember, those transmembrane proteins called G-protein coupled receptors.

In bitterness, it's the same. Except the receptors are slightly different, so they're not the same, but they're in the same class of G-protein-coubled receptor molecules. So in a similar way, the initial binding of the chemical triggers a conformational change in the protein, which then generates a secondary messenger cascade, leading ultimately to the release of neurotransmitter and consequent firing of an action potential by the affront gustatory axon. Same processes before, basically. In fact, it's quite interesting, in the sweetness, bitterness, and umami tastes, everything

after the binding of the taste end to the chemical receptor molecule seems to be exactly the same across all the cells. So all of the secondary messenger cascade and the firing of the action, potential and all that seems to be precisely the same. So the only difference between sweetness and bitterness and umami tastes is the particular, is which taste cells and therefore which neurons are producing the signals. So in that sense it's very similar to, well, many other sensory perceptions as well. It's not, your brain can't tell the difference between one action potential and another,

to the brain they look the same. The only way that can distinguish which is which is where it came from. And this is how you can get various interesting phenomena as well, like synesthesia. Now, the other point just to make about bitterness is that unlike sweetness, where there's mostly one receptor, maybe there's some others, but there's one main one, there's a class of like 20 or 30 different bitterness receptors, which are all T2R proteins, so they're all similar, but they all bind to slightly different molecules. That's why we can detect a wide variety of bitter substances and a wide variety of toxic substances, because bitterness taste receptors aren't just responsive to one particular molecule, like, for example, sample, sourness, saltiness, and to some extent sweetness are there, they're responsive to a much broader range of molecules. And also, as you said, much more sensitive. So that means active in the presence of a lower concentration of the relevant substance. So now moving on to the fifth and last taste, which is umami. So this taste is relatively recently discovered.

And the reason it's been widely accepted as a fifth taste is because recently we have actually discovered that it has a specific chemical protein receptor that is responsible for detecting it. So previously people talked about it as being, you know, particularly people from Eastern cultures where this sort of taste was considered to be, and still is considered to be particularly important to Eastern cuisine. And so there was talk about it, perhaps counting as a fifth basic taste. But in order to count as a basic taste, generally we would consider it to be necessary to have an actual different chemical mechanism present.

Otherwise, it's basically just a combination of the other four. tastes. So, you know, chocolate chip is not a basic taste, although it tastes different to many other things, because it's just a combination of the other tastes and those other factors we talked about before. But recently, it was discovered that there is a particular protein, so again, this is one of those G-coupled receptor proteins, that is responsible for the detection of umami. And I should probably explain what it actually is. I haven't said that yet. It's usually described as a savory or sort of meaty taste, and it's the result of detecting the amino acid glutamate. often in the form of monosodium glutamate or MSG.

You may have heard of MSG as a flavouring that's used a fair bit, particularly, as I said, in various Eastern cuisines. So basically, the component of that that the umami cells detect is actually the amino acid glutamate. The word umami itself comes from Japanese, and it basically means good flavor or good taste or delicious. It's also sometimes translated. So the basic mechanisms are similar,

but it has a particular type of the G-couple, a G-protein-coupled receptor which is able to detect the amino acid glutamate. Okay, so those are the five basic tastes. Now that we've essentially described where those signals come from, where those neural signals come from, I'll discuss how they're conveyed into the brain. So in humans, the sense of taste is conveyed

by three of the 12 cranial nerves. A cranial nerve is just a nerve that protrudes directly from the brain, as opposed to all of those that come out from the spinal column. So there are 12 cranial nerves, and they're understood in quite a lot of detail, three of them are responsible for the sense of taste, or that is responsible for carrying the sense of taste into the brain. The facial nerve carries taste sensations from the anterior, two-thirds of the tongue.

Anterior means the forward two-thirds, so that's near the tip of the tongue. The glossopherangeal nerve carries taste sensations from the posterior, so the back third of the tongue, and a branch of the vagus nerve, so part of the vagus nerve, carries some taste perceptions from the back of the oral, cavity. So there's actually taste receptors on various other parts of the oral cavity as well. Not just the tongue, but most of them do come from the tongue. So three cranial nerves carrying those signals. So remember, these axons are synapsing with the taste receptors and then all tracing back, bundling up into these three nerves. Remember, nerves are just bundles of axons wound up together and insulated, and the signals are then carried through these nerves into the brain.

So where do they go in the brain? Well, the first place they stop off at, or synapse with, is region of the medulla, called the gustator and nucleus. The medulla is a region of the very lower brain. So it's the part of the brain that just sits above the spinal column. And if you're familiar with the triune theory of the brain, which is an old and very simplified, but still kind of useful model of the brain, the medulla is part of the reptilian brain. Basically what that means is just a very primitive part of the brain. So this is interesting because it tells us that taste perception is received as input by one of these very lower, primitive. parts of the brain. So this part of the brain is not responsible for conscious thought.

It's well below that. That's distinct from vision, which goes directly back into part of the higher brain, the cerebrum, and doesn't project into any of these lower regions. Taste and smell, however, do. And this is sort of, this all feeds into the fact that taste and smell are both very much more closely related to much more sort of basic mechanisms, like, you know, basic survival mechanisms, things like particularly eating and also sex, and survival mechanisms, like, for example, detecting of poisons. And this is also thought to be related to how, remember I talked about before, bitter taste is related to the motion of disgust. So, you know, we don't know the precise mechanisms by which all of these relationships are worked out,

but we do know that taste and smell project into very lower regions of brain, that is, you know, medulla, very primitive regions. And so, in that sense, are sort of very directly related to many of these more primitive, more evolutionarily early types of behaviors like, you know, feeding and sex and survival and things like that. Anyway, so there's three cranial nerves synaps with a region of the medulla. Now, where do they go from there? So these neurons in turn have axons that project into the thalamus, which is in the full brain, so that's a much more evolutionary, evolutionarily recent part of the brain. And thalamus acts as a relay center.

So it's sort of a sensory integration center. It receives all of the sensory input from many different types of sensory modalities. So then from the thalamus, neurons in the thalamus then project their axons into various regions of the cerebral cortex, which is the outside of the brain responsible for most of the higher brain functions like vision and conscious thought and language and so on, including particularly the primary gustatory cortex. The primary gustatory cortex is the main brain structure responsible for the conscious perception of taste, and it is located basically just to the real, to the posterior region of the frontal lobe and just above, or superior to the temporal lobe.

So if you kind of can visualize that, basically it's sort of right in the middle of the side of your head. If you just take the side of your head and point to the middle, middle, top to bottom, middle, back to front, In there, that's roughly where the primary gustatory cortex is, distinct from, for example, the primary visual cortex, which is right at the back of the brain. Now, studies of the gustatory cortex in rats, for example, have shown that these neurons in the primary gustatory cortex exhibit complex responses to changes in concentrations of a taste-at.

So, for example, the same neuron might increase its firing rate in response to changes in the concentration of a particular tasteant. Whereas another neuron in the primary gustatory cortex, might only be responsive to a certain intermediate level of concentration of the same tastent. And other neurons seem to exhibit activity in response to various, very complicated combinations of activities of multiple tasters. So basically, it's thought that specific taste, you know, chocolate chip ice cream or chicken or whatever, are the product of complex patterns of activity across many neurons in the gastricor cortex,

each receiving input from different types of taste and also smell receptors. This is called population coding. So it's not like there's a particular neuron or particular bunch of neurons in your brain, which when activated, cause you to taste chicken. It's more that there is a particular pattern of activity across many neurons, which when a pattern of activity like that is activated, you taste chicken or taste chocolate ice cream or whatever. And those patterns of activity, in turn, are the results of complicated patterns of signals that ultimately come from the five basic taste receptors across the tongue. And also, as you said, incorporating signals from olfaction and from texture and temperature and other things as well. So that's basically the story of how we detect taste right from the taste receptors through to the primary gustatory cortex. Now I'll just talk about a few other little interesting tidbits that I picked up in my research.

And the first thing I'll talk about are some other types of taste. Well, they're not taste in the strict sense of the word because they're not, They're not chemical receptors that are responsive to tasters. However, they're certainly related to the sense of taste, and so I'll talk about a few of them. One which I would certainly be very remiss not to discuss is pungency. Or this is what we call hotness or spiciness. So it's the burning sensation produced when you eat things like chili peppers.

This burning sensation is produced by trigomenil, or trigomenil, I'm not sure I've pronounced that word, nerve action, together with some normal taste reception. So there aren't particular taste buds or taste cells that are responsible for tasting spiciness. Basically, this sensation that you get of pungency is caused by a phenomenon called chemistesis, which is not actually taste in a technical sense, because it does not arise from the actual taste buds or the taste cells. It's produced by the action of a different set of nerve fibers, which carry it to the brain. So it's not carried to the brain by one of these three cranial nerves that we talked about before.

it's a different mechanism. Basically, these foods like chili peppers activate various nerve fibers directly, and we detect this. This sensation is interpreted as hot or as spicy or something like that. Basically, the actual sensory receptors that are being stimulated are somatosensory receptors. So these are pain, temperature, pressure fibers on the tongue, which are being activated in a different way. So if you touch your tongue, you know, you can feel that, and that's detected by these mechanoreceptors, various somatosensory receptors.

And you can also feel pain and temperature and other things like that if you provide the correct stimuli. But obviously, touching your tongue or putting an ice-key bone in your tongue, none of those things produce the sensation of pungency. That's because foods like chili peppers activate the nerve fibers, those same nerve fibers, but in a more direct way and also in a different combination of them. So it might activate all of them and not just one of them. we sense that type of activation of these somatosensory nerve fibres, we interpreted as the sense of pungency. So it's interesting to note that many parts of the body, many parts of the body with exposed membrane, so that would include the nose, for example, but no taste receptors under the fingernails as another example of the surface of the eyelid or an open wound. So any

exposed membrane that does not have taste receptors will produce similar sensations of heat or burning, or chemistesis, as it's called, when exposed to hotness agents or pungent agents. So, I mean, that doesn't mean you can taste spicy foods with your fingernail, but it does mean that you will receive a similar sensation. So, if you want to do an experiment, get a chili pepper and shove it up your nose and see what kind of sensation you have. I haven't done it myself, so I don't know exactly what it will be like, but I imagine you will experience a fairly similar pungent, hot, spicy sense to that.

to the case in which happens in which you, to the case in which you put the chili pepper on your tongue, although obviously it won't activate any taste receptors, so it won't be exactly the same, but it should be fairly similar. And that's because it's not an actual taste, it's chemistesis. So it's the chemical sensitivity of mucus membranes as a result of stimulation by pain, touch, and thermal perception. There is an interesting measurement of the degree of pungency of chili peppers, and it can also be used for other spicy foods, but it's mainly used for chesty,

It's called the Scottsville scale. It's measured in Scottsville heat units. And basically it's a function of the concentration of a chemical called Capsaicin, which is the active ingredient in chili peppers. So in addition to this sense of pungency, there are other types of sensations that can be produced by stimulating various non-taste receptors on the tongue. So one example of this are temperature receptors that are on the tongue. So these can be activated by various molecules.

And so this is, for example, responsible for the fresh or minty sensation that's produced when you consume things like ethanol or menthanol or various types of mints. So this is caused by activation of the same sensory mechanisms that detect cold, but without the actual presence of cold. So that's why it feels similar to cold, but not quite the same, because you don't have the actual change in temperature. So the sort of coolness is only a perceived phenomenon. There's nothing cool about them. It's just there activating some of those same neurons that are also activated. if you put an ice cube on your tongue. So it's interesting how, in the case of both punctancy and sort of mintiness,

that those sensations are produced by activating existing sensory receptors on the tongue that are responsible for things like pain and pressure and temperature, but in slightly different ways and slightly different combinations, which then yield slightly different perceptions of what the sensation is actually like. Okay, one final little phenomenon I want to talk about, which is called aftertaste, and this is probably something that all of you have experienced, at one point or another, after-taste is a taste or a perception of taste of a food or beverage that is perceived immediately after that food or beverage has been removed from the mouth.

Now, this is a little bit of an odd phenomenon, because remember, the way we detect taste is by food molecules dissolving in saliva and then interacting with the receptors on the taste buds. But if there are no molecules to interact with the taste buds, because they've been removed from the mouth, then exactly how are we tasting anything? It seems like a bit of a contradiction. And aftertastes, and it's not just because there's a small residual left sort of in the saliva or over the surface at the tongue, that might partly explain it, but because you can still detect aftertaste if, for example, you have a drink of water after you have consumed something, which would generally wash away most of the residual food molecules that are dissolved in the saliva,

but you can still often sense an aftertaste even if you have a drink of water. So it's an interesting little puzzle here how we can possibly taste something after there's not actually any active ingredient left in the mouth to taste. Obviously the answer is there is actually some sort of tasteant's molecules still in there somewhere because otherwise you wouldn't be tasting anything. The question is exactly how does it stay there or how is it interacting? Now we don't fully understand the precise neurobiological mechanisms of aftertaste, but we have some ideas. There are some hypotheses. So one hypothesis is that certain taste and molecules are able to diffuse across the receptor cell membranes, either directly across the membrane or maybe through various channels or other things like that.

And only certain chemicals would be able to do this. But once these chemicals were able to cross the cell membrane, so they're actually inside the cytoplasm of the taste cell now, they would be able to directly activate the signaling pathway, so the proteins of the signaling pathway, inside the cycloplasm in the intracellular space, thereby leading to the production of action potentials. But this intracellular process would presumably be a fair bit slower

than the external cell activated process, because first the components have to diffuse across the cell membrane, and then they have to interact with the intracellular proteins in a more indirect way. And so this is why the sensation would occur sometime after the... Sometime after when the actual food was present in the mouth, because it's essentially a delayed response. First, the tastants have to diffuse across the membrane, then they have to interact with the cellular proteins.

So this would only be possible for certain chemicals that were able to both diffuse across the cell membrane and then interact with the intracellular protein mechanisms in the correct way. So only certain types of chemicals will be able to do this, particularly as thought bitter chemicals, which is why aftertastes are often bitter, not always. This is one hypothesis for how aftertaste might occur. So basically, it's by tasteant molecules interacting in unexpected or unusual ways with the taste cells,

thereby eliciting action potentials and neural responses after most of the taste molecules have actually been removed from the surface of the mouth. Okay, so that's all I have for this episode. Hopefully you found it interesting. Maybe change the way you perceive of taste and the way you enjoy food. I suppose it would be kind of weird if you were eating while listening to this episode, actually. I hadn't thought of that. Hopefully that didn't detract from your experience. or maybe it added to it, I don't know.

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or anything about the podcast that could be improved, maybe there's a complaint about sound quality, I don't know, whatever. I'd love to hear from him. You send me an email. My address is Fods12 at gmail.com. That's F-O-D-S-1-2 at gmail.com. Thanks again for listening, and I'll talk to you next time.