Lecture 20 - Special Senses - Olfaction and Taste Lecture Notes PDF

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 20 - Special Senses: Olfaction & Taste Video 1 Introduction Welcome to the next online lecture. In this lecture, we're going to start to explore the special senses. Specifically w...

NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling. Lecture 20 - Special Senses: Olfaction & Taste Video 1 Introduction Welcome to the next online lecture. In this lecture, we're going to start to explore the special senses. Specifically we're going to be talking about taste and smell. So have you ever thought about how we can take molecules that are floating in our air or in our food and turn them into electrical signals that can be interpreted by our brain as a smell or taste. Well, that's what we're going to talk about today. So let's get started. Slide 1 The material for this online lecture module can be found in chapter 15 of your textbook called the special senses. So in this image, we're actually looking at three of the special senses. We can see here this red circle represents the retina of your eye. So that's for our special sense of vision. In the center, we can see a top view or superior view of our tongue. So that's going to be for our special sense of taste. And then over here we can see an image that represents the cochlea, which is found in the inner ear and used for our special sense of hearing. So we're going to be covering all the special senses over the next few lectures. Today we are going to be focusing on this special sense called olfaction, which is smell and taste. Slide 2 So before we get into olfaction and taste, I first wanting to do a quick overview of the different types of sensory receptors we have in the body. So we talked about this a little bit earlier in the course when we talked about sensory receptors, but essentially, they can be divided into two main groups, that general senses and the special senses. Now, when you were a kid, you learned that you had five senses. Smell, taste, sight, hearing, as well as touch. Now you can see those, of course, on this slide here, but you can see it's much more complicated than those five senses. And in fact, out of the five senses, four of them are considered special senses. Touch can be divided into the general senses, along with several other types of sensory information. So what's different between general senses and special senses? Well of course, general senses are going to have receptors that are distributed over large portions of the body. So for example, places like the skin, the muscles, and the joints. Now when we're talking about sensory receptors that are found on our skin, our muscles, and in our joints, these are known as the somatic sensory receptors. So somatic sensory receptors can gather a variety of different types of information, including touch information. So that was the one that you learned as a kid. But they're also gathering information like pressure and proprioception. Now remember, proprioception is when we're gathering information about joint position and muscle tension length, so that we can use that to help our cerebellum make more coordinated body movements, as well as things like balance and keeping us upright. We're also going to have sensory receptors for temperature and pain. So those are all considered to be somatic. And when you think about what touch was when you were a kid, it was on your skin, so many of these receptors are going to be found on your skin, things like temperature, pain, touch, pressure. But things like proprioception are going to be found inside the muscles as well as pressure, for example as well. So they are going to be evenly distributed throughout the body, wherever we are going to have skin, muscles, and joints. Now, of course, along with these general senses that are somatic, we also have some sensory receptors that are found in the visceral domain. So these are going to be gathering sensory information from the inside of the body, and often these are involved in homeostasis. So these are located in our internal organs, and the visceral general senses include receptors that will detect pain, as well as pressure. And we'll talk about how some of those work in a second when we talk about the types of different sensory receptors. Now, if we classify general senses as receptors that are evenly or found in large portions of the body. The special senses are where we're going to really localize our receptors to very specific organs, and that's what makes them special. So this is where we're going to have smell or our olfactory neurons for our special sense of smell, to detect chemicals in the air. Taste, to detect tastes in our food or tastants in our food as we'll talk about today. But then also light information for vision, sound information for hearing, as well as one that you might not think about, but information being gathered about our body's position relative to the ground, as well as things like accelerations and deceleration of the body, also known as balance. So these are all the special senses. Now, if you're interested in learning more about the general senses, you can read about them in your textbook are covered in Chapter 14. But we're going to be focusing primarily in this course on the special senses. So that's what we're going to start to talk about in this lecture. Slide 3 Now when we think about specialized sensory receptors gathering information from our environment, what we need to do is take the information from the environment and turn it into an electrical signal. And again, electrical signals are going to be changes in membrane potential. So what we're going to focus on is how we are able to take stimuli from the environment and actually turn that into a change in membrane potential. This is done via a series of different types of sensory receptors. The first type that we're going to highlight our mechanoreceptors. So mechanoreceptors are basically the same thing as what we talked about earlier when we talked about mechanically gated ion channels. Essentially, we're going to change the shape of the receptor in such a way that it triggers the movement of ions through the membrane. So this can happen via compression, bending of the cell or stretching of the cell. These are going to be found in our sense of touch, pressure, proprioception, as well as hearing and balance. The next type of receptors are the chemoreceptors. So chemoreceptors are where we have chemicals that become attached to the receptors that are attached onto a membrane, and that will cause a change in membrane potential. Now this sounds very similar to ligand-gated ion channels. And in some cases it is the similar situation, but chemoreceptors in ligand-gated ion channels are not exactly the same. In fact, you can have a chemoreceptor that's embedded in a membrane in a combined a chemical, but it's not an ion channel. So in the case of chemoreceptors, often what happens is when the chemical binds to the receptor, it sets off a signalling cascade. And that cascade of events inside the cell will eventually open an ion channel, and that ion channel will then allow ions to move through, and we get a change in membrane potential. So it's not always direct, like we see with a ligand-gated ion channel. It's more of an indirect pathway in some instances; and in other instances, it's just simply ions moving through a channel, as we'll see today with some examples for taste. Chemoreceptors are also found with the special sense of smell. So we're gonna be highlighting these today. The next type of receptor are thermoreceptors. So these are going to respond to changes in temperature. Now if we think about these types of receptors, remember they're proteins that are embedded in the plasma membrane of our neuron. So proteins can be changed in terms of their shape by temperature. So temperature changes can, in the case of thermoreceptors, change the shape and basically windup opening the gates, allowing for ions to move into the cell, causing electrical potential changes. The next type of receptors are the photoreceptors. So photoreceptors are really interesting and we're going to focus on them a lot when we get into their special sense of vision. But these types of receptors are going to respond to light stimuli. So how do we take light and turn that into an electrical signal? That's kind of a bit of a complicated process, but we are going to be talking about it with our special sense of vision. And finally, the last type of receptors are the nociceptors. So the nociceptors are specialized receptors that respond to extreme mechanical, chemical, or thermal stimuli. And essentially our brain will interpret signals coming from nociceptors as pain. So when you think about a nociceptor in the way that it responds to the stimuli, it's still using the same mechanical forces, chemical structures, or temperature changes. The difference with the nociceptors is that you need a very large stimulus in order to cause the change. The large stimulus would then be interpreted by the brain as something beyond what the normal capacity for that tissue might be and that it might actually be causing damage to the tissue because the stimulus is so strong. So really it's mechanoreceptors that don't go off with small changes in compression, bending, or stretching of the cells, but very large changes that essentially might do damage to the tissue. And that's why they're activated and interpreted as pain. And the same would be true for the thermoreceptors and the chemoreceptors. So again, today we are going to be focusing on taste and smell. So we're going to talk a little bit more about those chemoreceptors, but we'll be talking about mechanoreceptors and the photoreceptors when we get into the other special senses. So let's take a short pause here and try a couple of practice questions and then we'll come back and talk about olfaction. Video 2 Slide 4 So welcome back. We're going to start by looking at our special sense of smell, also known as olfaction. But before we get into the specialized receptors that are going to detect smells, let's first just look at the anatomy of the nasal cavity. So of course, the nasal cavity is found in the facial region of the skull. So in this image, we're looking at a medial view, which is a sagittal section through the skull. In this case, I'm sure many of you know that in between your nostrils there's a little wall known as a septum. So in the view that we're looking at here, the septum has actually been removed and we're looking at the lateral side of the nasal cavity. So the nasal cavity itself is made up of several different bones which are going to be covering a little bit later in the semester. But on the bottom of the nasal cavity we have the hard palate. And that's essentially going to separate the nasal cavity from the oral cavity, which would be your mouth down here. We'll talk about the names of those bones a little bit later. You can also see the nostrils on the nose side. So those are also known as the nares, which will again highlight later. And you can also see the frontal bone. So that's the skull bone that's going to cover your forehead and also over that frontal lobe of your brain. There are some small bones that extend down into the bridge of your nose. The nasal bones again, we'll talk about them more later, but the rest of this is actually made up of cartilage and that's why it's a little bit more flexible. So you can imagine that when you're smelling odorants, they're going to go into the nares of the nostrils into this space. Now on the roof of this space, we have another bone called the ethmoid bone. And in that ethmoid bone we have a region that makes the roof of the nasal cavity that's very, very, very thin and it's filled with lots of little tiny holes called foramina. This area of the ethmoid bone that's really thin is known as the cribriform plate, and the little holes are going to allow the nerve signals to move from the nasal cavity into the cranial cavity. So we're going to look more in detail at this region in a second. But also notice in the nasal cavity, we have these ridges along the side of the nasal cavity; these are known as the nasal conchae. Again, we're going to talk about that more later. But for now I just want to highlight the fact that when you take an a smell, say you're smelling your cup of coffee in the morning, it goes into your nostrils and into this space. These ridges are going to help swirl that air around, not only to force it to come in contact with the mucous membrane so that we can get rid of any dirt or debris, but it also helps to move any odorants in the air toward this region of the roof of the nasal cavity. Because in that region we have something called the olfactory region. And it's in that olfactory region where we have olfactory epithelium and the specialized receptors that we're going to use to detect these chemical signals. So these are representing the fibers that are part of the olfactory nerve, and if you remember back to the lecture on cranial nerves, we talked about the fact that cranial nerve one is actually a two neuron system. So these fibers of the olfactory nerve are the ones that are going to go through this cribriform plate from the nasal cavity up into the cranial cavity, and there they're going to synapse with the second neuron in this system. And where the synapses occur with this second neuron is known as the olfactory bulb. And then of course all the axons from this second neuron bundle together to go back towards the brain through something called the olfactory tract. Now just a note here. It's called the olfactory tract and if you recall from the brain lecture, we said that tracks are bundles of axons in the central nervous system. So this is actually an exception to that, the olfactory tract, is a bundle of axons, but it's actually part of the peripheral nervous system. So we're going to look at where the nervous signals go in a moment, but this is just a nice overview of some of the structures that are surrounding the nasal cavity and are part of that olfactory region. So now are we going to do is we're going to take this region that's highlighted by this box here and blow it up and look at the structures in the olfactory region in a little bit more detail. Slide 5 So this region, this olfactory region, has a layer of cells known as the olfactory epithelium. So again it's epithelial cells, except that what's different about it is it's going to have some specialized receptor cells that are embedded into that epithelial layer. In this image here again, this is the anterior side and this is the posterior side, and we're still looking at that same medial view. So there are three different cell types found in the layer of the olfactory epithelium. The first are our specialized receptor cells, and those are known as olfactory neurons. These olfactory neurons are bipolar cells. So that means again, they're going to have one dendrite in one axon. Off of the cell body the dendrites extends towards the nasal cavity, and in fact, close to the nasal cavity, after it passes the area of the other cells, we get an enlarged area of that dendrite known as the olfactory vesicle. Off of the olfactory vesicle, We have some cilia known as the olfactory hairs. And it's on these olfactory hairs where we're going to find the chemoreceptors that will bind chemicals from our air, and those will be detected as our smells. Now when we're talking about smell, the chemicals that are being detected by the special chemoreceptors found on the olfactory hairs; these chemicals are called odorants. Now in order for the odorants to bind onto the receptors in this region, the olfactory hairs, as well as the olfactory vesicles, are embedded in a layer of mucous. And that mucus layer needs to cover over those olfactory hairs so that when the odorants reach this area, they can be dissolved first in the mucus. And in fact, we actually need the odorants to be dissolved in the mucus in order for them to bind onto the receptors. And this is important in order to activate those receptors or cause a change in membrane potential that will create a signal. Now if you think about this, if you've ever had a really dry nose, you'll know that when near nasal passages super dry, it's actually harder to smell. But on the flip side of things, if you have a really thick layer of mucus here, it's going to be really hard for those odorants to also reach these olfactory neuron hairs. So in this case, thick mucus can block the odorants from getting in, and if you don't have enough mucus, then you'll wind up not being able to dissolve those odorants to bind onto those chemoreceptors in that location. Now what happens when we have an odorant that comes in, it binds on to the specialized chemoreceptor, that causes a depolarization. That depolarization is then going to move through this bipolar cell up to the region where the axon is located. And of course, the axon is going to bring the information from the nasal cavity into the cranial cavity. First-day travels through a layer of connective tissue and then it's going to travel through a layer of bone. And again, this is a thin layer of bone that's part of the ethmoid bone, known as the cribriform plate. And in that there are a series of little tiny holes known as the foramina, which are plural or singular is foramen. The axon is basically going to travel through the skull case and go into the actual cranial cavity where the brain is located. Once it reaches this region, this is where we had the second neuron in this system, and all of these little synapses that occur from the olfactory neurons to the neuron that's going to carry the information back to the brain, that whole region is known as the olfactory bulb. And from there when we have a synapse in the second neuron, all of those axons are going to bundle together to form the olfactory tract, and that's going to carry the information back towards the brain. So that's the general pathway of how our signals are going to move from the nasal cavity to the brain. Now in the olfactory epithelium, we have two other cell types. The first, are epithelial cells, known as the supporting cells. So these essentially are just going to be helping to hold our olfactory neurons in place. The other cell type is represented here by these little red triangles are the basal cells. Now basal cells are really important because they're involved in regenerating the epithelial layer. Because this layer is in contact with the outside environment, it needs to be replaced often because of damage. And while it doesn't happen all at once, the basal cells will basically regenerate this layer of the olfactory epithelium about every two months. Now another feature here of the connective tissue, which it's not really showing you in this image, is that there are glands found in the connective tissue that make mucous. And this mucous is going to then be the mucous that travels through this epithelial layer to form this mucous layer on the outside surface. So the glands for making that mucous are actually found in the connective tissue layer. So again, just to quickly summarize how everything is working here. Odorants are going to enter into the nasal cavity. They're going to reach this area of the mucous, become dissolved in the mucous, bind onto chemoreceptors that are located on the olfactory hairs. Then once those chemoreceptors are opened, we're going to get a graded potential. That graded potential will get large enough to reach the trigger zone of our bipolar olfactory neuron cell. And then we will get an action potential that goes down the axon towards, the olfactory bulb, through that cribriform plate. These axons are bundled together through these foramina, so it's not just a single axon free toll, you'll actually have multiple. And there in the olfactory bulb they're going to synapse with the neurons that are going to carry the information to the brain. So that's basically how we're going to detect odorants in the air. Now how do we know one smell from another? Slide 6 So again, this is just summarizing some of the things I've already said. Odorants are going to bind to odorant receptors or those chemoreceptors that are found on the olfactory hairs, and that causes depolarization. Now, in order for our brain to interpret the various odorants, there's been lots and lots of research and how this actually works. So we do have several different types of chemoreceptors. So not every odorant is going to bind to every chemoreceptor. But the fact is that are chemoreceptors are not very specific either. So that means one chemoreceptor can bind multiple different types of odorants. But when we think about odorants, they're basically classified based on their shape. So molecules are chemicals that have a similar shape or chemical composition will bind on to similar chemoreceptors. And therefore, it puts them in this sort of scene class of smells or odors. Because remember, all our brain is going to be able to interpret the fact that we activated a neuron in that olfactory region. So every neuron is going to have a different set of chemoreceptors. So it's really going to be about which chemoreceptors are having chemicals bound to them, activating which neurons that are part of the olfactory epithelium, and then the pattern of neurons that are actually activated is what's going to be interpreted by the brain as a specific smell. So how does all of this really work? Well, there's lots of research that's gone into this and there's many different theories on how our brain interprets different odors. But in our textbook talks about the seven primary classes of odors. So this would be molecules that have similar shapes or structures activating a series or pattern of specific chemoreceptors in that olfactory region. So our first class of odor is the camphoraceous smell. Now this is the smell of moth balls, so it comes from the camphor tree and that's would be a specific pattern or chemical structure binding onto certain types of receptors. So it's not just one receptor causing this smell, it's a pattern of receptors being activated that would allow for this smell. We also have musky smell, so that sort of the smell that animals give off when they're trying to attract a mate. You'll also see musk in some colognes. Floral smells, of course, are flowers. Pepperminty, with the holidays coming in another month, I'm sure you're familiar with the smell of peppermint. Ethereal, that's a fruit smell, so a sweet smell. Pungent is something that's very strong like a really stinky cheese. And then putrid is more of a rotting smell. So that's that really strong smell of rotting or decaying material. So again, classes are odors. There's lots of different theories onto how many classes of odors actually exist. There might be many more than this. They know that there are multiple different types of receptors. In fact, they believe there's about a 1000 different types of chemoreceptors, and there can be up to 50 different classes of odors. So it's still a complicated process that they're trying to figure out. But when you think about an odorant going into your nasal cavity, think about, say, smelling a strawberry in the summertime. The smells are the chemicals that are coming off of that strawberry go into your nasal passage and activate a series of different chemoreceptors, or a pattern of chemoreceptors. That signal then gets sent to your brain. So now your brain is taking that pattern of electrical signals and associating it with the strawberry. So that the next time that you activate that same pattern of signals, you will then recognize that smell as being a strawberry. Along with the visual information of looking at the strawberry and maybe even some taste information from eating the strawberry. Your brain is putting all of those pieces of information together. So the smells are a lot about what patterns of these receptors we're sending to the brain and how our brain is interpreting those different patterns as a specific smell. So once we've created a pattern of signals and it's going along the olfactory track towards the brain, where does it go? So if you recall from earlier, we talked about smell as being one piece of sensory information that bypasses the thalamus and goes directly to the cortex. So our olfactory track is basically going to a region of the cortex known as the primary olfactory area or the olfactory cortex. This is mostly found in the temporal lobe, but also some portions can be found in the frontal lobe as well. This is where we're going to have the conscious perception of this smell. Now we do have some neurons that will go elsewhere in the brain, namely to this secondary olfactory area. The second olfactory area is deep within the brain near the region of the corpus callosum. In this area we're going to have visceral and emotional reactions to smell. Now that sounds familiar. It's because that's what also the habenula did. And in fact, this secondary olfactory area does connect with the habenula, and they work together to have these visceral and emotional reactions to smell. One other thing I just wanted to highlight before we move on to taste is something known as adaptation. So I'm sure you're familiar with the fact that sometimes when you are in a room that has lots of smelly things, you notice the smells at the beginning, but then they sort of wear off and you don't detect them anymore. Well, there's a couple of different things that can be going on here. One, when there are lots and lots and lots of odorants in the air, you can saturate the receptors in the olfactory epithelium. So when you saturate those receptors, we can't generate more signals in those olfactory neurons. So basically we lose some of the sensitivity of those olfactory neurons. In addition to this, we also get something known as synaptic inhibition. So basically, if you look back to the image on the last slide, you'll notice that there were some other little black neurons that were found in the olfactory bulb. Well, what happens is when the brain's overstimulated by information, it actually can send a signal back to the olfactory bulb and say, hey, block further signals from coming through, we know it's smelly and here. So we have some neurons that are capable of causing inhibitory responses in the olfactory bulb to block the information from being carried to the cortex. So that's how we can adapt to smells and not interpret them anymore. So let's take another pause here and we'll do some practice questions and then we'll come back and start to talk about taste. Video 3 Slide 7 So the next special sense we're going to talk about is the sense of taste, also known as gustation. Like we did with the olfactory area, we first need to look at the anatomy of the structures where these specialized taste receptors are going to be found. Now the majority by far of the specialized taste receptors are going to be found on the tongue. And the tongue of course, is inside the oral cavity. So in this image we're looking at the surface of the tongue. This would be the anterior portion and this is the posterior portion. The majority of this posterior portion is actually deep within your throat, so you can't actually see it when you stick your tongue out. Most of what you can see ends at about this level right here. Now while the specialized taste receptors are primarily found in the tongue, there also are some of these receptors in other portions of the oral cavity, specifically on the roof of your mouth, on your lips and in your throat as well. So know that you can have some of these specialized receptors elsewhere, but we're going to be focusing primarily on the tongue because that's where most of them are going to be located. The other region of the tongue that we haven't talked about yet is this really posterior region known as the epiglottis. So this is basically on the base of the tongue and it's deep within your throat. You can't really see it when you're trying to look in the mirror, but your epiglottis is the structure that covers over your windpipe when you're swallowing. So we will talk more about this structure in term two with digestion. But for now, just know that this is the structure at the base of the tongue because there will also be some of our specialized taste receptors found in that region as well. So the surface of the tongue is covered with small epithelial projections called papillae or papilla (singular). These epithelial projections differ depending where on the tongue you're looking. And they also differ in their function and the amount of taste receptors that they have in them as well. So there are four main types of papillae. The first type is known as the vallet papillae. So the vallet papillae are by far the largest of the papillae. And if you stick your tongue out really far and you look in the mirror, these are the really large bumps that you see at the back of your tongue. In fact, they form a V on the posterior portion of the tongue. So the largest of all of the epithelium projections, in fact, we have anywhere from eight to 12 of them in total, So you can sort of see them back here, it's showing about eight of these vallet papillae. And the reason they're called vallet papillae is because they make this V shape and the posterior aspect of the tongue. But notice what the papillae actually look like. So the papillae, as I said, are epithelial projections. So the outside layer is epithelium, and then it sort of folds in on itself to make these little pockets. In the pockets is where the taste receptors are going to be found, and for our special sense of taste and the taste receptors are found in the taste buds. So depending on the type of papillae will have a varying number of taste buds. In the vallet papillae, as I said, we only have about eight to 12 total vallet papillae, so not very many, but they do have a large number of taste buds. In fact, they can have anywhere from a 100 to 300 taste buds within each papillae. So they do contain a large number of those taste buds. So that's the vallet papillae. The next type of papillae are the foliate papillae. So the foliate papillae are found on the sides of the tongue, so the lateral surface of the tongue. Their named the foliate papillae because they take on the shape of a leaf or foliage. They're structured a little bit more differently and they're quite a bit smaller than our vallet papillae. And we have a larger number of them, but you can see that they still form these little pockets. They solve a layer of epithelium and they still contain taste buds. Now in the foliate papillae, this type of papillae, the taste buds do start to degenerate with time or age. So we have a larger number as children and then they start to deteriorate over time. So this papillae won't be quite as active when you're older because the taste buds within them will no longer exist. By By far, the largest number of papillae are going to be on the surface of the tongue. So the first type that we're going to talk about that's on the surface of the tongue are the fungiform papillae. So the fungiform papillae get their name from the fact that they look like little mushrooms. So again, structurally, they're all fairly similar in the fact that they create these little pockets, that's why there are projections of the epithelium, but they will have varying numbers of taste buds. And in fact, the fungiform papillae only have about five taste buds per papillae, but there's so many more papillae in total that overall they're going to have a larger number of the taste buds in general, mixed in or scattered amongst the fungiform papillae we have the last of the papillae called the filiform papillae. So the filiform papillae gets its name from the fact that it's shaped like a flame. Now what makes the filiform papillae different is that it doesn't have any taste buds. In fact, the filiform papillae, its main job is to create the rough surface of your tongue, and that's going to help you to manipulate your food when you're eating. So these two are basically going to be scattered throughout this superior surface of the tongue. Now if you wanted to look at this in an electron microscope, this is what it looks like. So the filiform papillae are basically these rough shaped little projections, whereas the fungiform papillae are much broader, more mushroom shaped and they will have the taste buds, whereas these filiform papillae do not. So overall, with all of the papillae and all of the taste buds, on average, we have about 10 thousand taste buds, but as I said, this will decrease with age, especially in that region of the foliate papillae. Also the other regions as well, but the foliate papillae decay at a more rapid rate and tend to be gone by adulthood. Slide 8 So this is just summarizing some of the information I just mentioned. So the vallet papillae are the largest of the papillae. They form that V-shaped border between the anterior and posterior tongue. They're the largest but the least numerous, only about 8 to 12 in total, and they have many, many taste buds. The fungiform papillae are irregularly scattered on the surface of the tongue. They also have taste buds. The filiform papillae are the most abundant of all of the papillae. They're on the superior surface of the tongue scattered amongst the fungiform papillae, they don't have any taste buds and their job is to create that rough surface that we need in order to manipulate our food when we're chewing. And then finally, the foliate papillae are on the sides of the tongue, or the lateral aspect of the tongue. They are the most sensitive of the taste buds, but they do decrease in number with age. And in fact, some research shows that they're gone when you reach adulthood. Slide 9 So when we talk about taste buds, what actually is a taste bud? So in the image here we have a foliate papillae and then it's taken one of the taste buds. Also notice that the taste buds are found in these regions that are along the side of the papillae that actually helps to protect some of these taste buds from damage from food while you're chewing. So we're just going to blow up one of these taste buds and look at it in little bit more detail. So this is an individual taste bud. Now, within, as I said, each papillae, you'll have a varying number of taste buds, so the vallet papillae have about a 100 to 300 taste buds, whereas the fungiform have about five taste buds. But within each taste bud, just like we had in the olfactory epithelium, we have a number of epithelial types of cells mixed with our neural cells. So this is our epithelium right here. And as we mentioned before, we're going to have stratified squamous type of epithelium in our mouth because that provides that great layer of protection. But then embedded in that epithelial layer is the taste buds. So there are three different cell types. We have the taste cells or gustatory cells. These are the ones that are actually going to create the nervous system signal. And then we have the supporting cells, which again are just going to help hold everything in place. And also we have the basal cells, much like the olfactory epithelium. So those basal cells are also going to help regenerate this epithelial layer or these taste buds. And in fact, the basal cells replace the taste cells and the surrounding epithelial cells approximately every ten days. So there's a lot more damage in the oral cavity to these cells compared to the olfactory region which gets replaced every two months. The structure of the taste cells within the taste bud is such that a portion of the taste cell extends through an opening in the epithelium towards the oral cavity. And that structure is known as a gustatory hair or a taste hair. These are essentially microvilli. And this is where our chemoreceptors are going to be located for our special sense of taste. These taste hairs project through an opening known as the taste pore. Now within a taste bud, we will have approximately 50 different taste cells. You can see how the taste cells are connected to the neurons that are going to bring that information back to the brain. So this is just depicting the nerve fiber or of a sensory neuron. So if we take this arrow and blow it up over here, so it's not actually pointing down, it's over here. And look at our taste cells in more detail, we can look at the structure of the taste cell. So again, these are our taste or gustatory hairs, those are microvilli. So that's going to differ from our olfactory neuron cells, which have cilia at the end to make their olfactory hairs. But essentially both of them have hairlike projections that contain the chemoreceptors. Then we have the cell body, and notice that there's no acts on in our gustatory or taste cell. Now that is another thing that makes it different than an olfactory neuron, because it has no axon. Essentially, the taste buds are so short that they don't actually need an axon. So in this case, what happens is when our chemical binds onto the chemoreceptors here, it causes a graded potential, and essentially that reaches the end of the cell and releases neurotransmitter directly. So it's essentially we've gone from the cell body right down to the presynaptic terminal of a neuron. We kind of skipped the axon portion in a gustatory cell or a taste cell. So there are some similarities between olfaction and taste. They both use chemoreceptors. They both have supporting cells and basal cells. They both have specialized cells that are going to gather information from the environment, chemical information. They're both going to send a signal to a neuron that will carry that information back to the brain. But the main thing that's different here is that our gustatory cell has no axon and it's microvilli for our taste hairs. So those are some of the differences, but otherwise they have a lot of similarities as well. So essentially how this is going to work is we're going to have chemicals that are introduced into the oral cavity with our food that when we break it up, it's going to dissolve in our saliva. So in the olfactory epithelium, remember our odorants, dissolved in that mucus, but in the oral cavity these chemicals are known as tastants and they are dissolved in our saliva. So again, they're going to bind to those chemoreceptors were going to get a depolarizing response in the gustatory cell that will send a signal through the cell, release the neurotransmitter via the synaptic vesicles and then we'll get an excitatory response sent to the brain. Slide 10 So like smell, we have different chemoreceptors that are going to detect different tastes in the oral cavity or in our taste buds. But unlike smell, we only have five different receptor types. So this will give us the five different tastes. So we have taste buds for salt, sour, bitter, sweet, umami. And for those of you who have never heard the word umami before, it's basically loosely translated from Japanese into savory. So this is going to be activated when you take in typically amino acids or proteins. So these are basically the different tastes. And again, they are much more specific, unlike what we saw with odorants, where we could have odorants binding to multiple different receptors creating patterns. In this case, we're only going to activate one of these or multiple of these five different tastes depending on what we've eaten. Now the way that we activate these receptors is highlighted in your textbook, but you don't need to know all the details. The main thing is that for salt and Sour, there's actually ions that are part of those molecules. So salt has sodium, and sour has hydrogen ions. So by eating those foods, those ions can actually move through ion channels and cause depolarization. For bitter, sweet, and umami, we have a G protein signalling pathway that gets activated and essentially that's going to activate an ion channel to open and cause depolarization. So again, you don't need to know all the details of that, but just know that we are very specific in our tastes, whereas our odorants are much less specific in terms of the chemical receptors. So again, just to summarize our taste or gustatory cells contain the taste or gustatory hairs. And that's where these chemoreceptors for salt, sour, bitter, sweet, umami are located. And that's where depolarization is actually going to occur. So again, remember that we have about 50 different taste cells within a taste bud. Each taste cell will only have one type of chemoreceptor, but within a taste bud you can have multiple different types of receptors. However, usually within a taste bud you will favour one receptor over all of the rest. So within one taste, it will be mostly salt or it will be mostly sour, it'll be mostly bitter and so on. So it will be between taste buds where you'll be able to detect the different taste, not usually as much within one single taste bed. So our chemicals in our special sense of taste or known as tastants, those are dissolved in saliva and those are what are going to bind to those chemoreceptors or enter in as ions to depolarize those taste or gustatory cells. Slide 11 So in terms of the neural pathways for taste, we start with, of course, the gustatory receptor cells, or those taste cells. So those are going to be found primarily on the tongue, but we're also going to see some on the epiglottis region. And as I mentioned, there will be some in the lips and even the roof of your mouth. So the gustatory receptor cells are going to connect directly to neurons of three of the cranial nerve. So cranial nerve seven, the facial nerve covers the anterior 2/3 of the tongue. We have the glossopharyngeal nerve, cranial nerve nine, that covers the posterior 1/3 of the tongue. And then we have the vagus nerve, and that's going to cover the region of the epiglottis and the throat. From there, the information goes from these cranial nerves to the medulla oblongata. So you can see that it's synapsing here in the medulla oblongata, and from there, everything that sensation except for smell goes to the thalamus. So then we're going to reach the thalamus and we'll have another synapse point there. From there, it's going to carry the information to the taste area that's part of that insula region of the brain. Remember that fifth partial kind of Lobe, the insula, that's where we're going to have our taste area in the cortex. And again with our special sense of taste because we have very specific receptors, our interpretation of this information will be based on the number of each of these types of receptors that are activated. So let's pause here and try out some more practice questions. Video 4 Conclusion So that's where we're going to end this online lecture today. We talked about our special sense of smell or olfaction. We talked about the specialized region of the nasal cavity, the olfactory epithelium, where we have the sensory receptors that are going to gather information about odorants in our air. We also talked about the special classifications of the odorants and how our brain can interpret that information. We also talked about our special sense of taste or gustation. In this case, we talked about how tastants or the chemicals found in our food, are interpreted or turned into electrical signals via and very specialized receptors found on our tongue in those regions known as the papillae. Then we talked about how we can interpret the various signals coming from those receptors to give us the different types of tastes. In the next online lecture, we're going to continue with the special senses by looking at our special sense of vision. So until then, take care.