I like to say little bit more about what happens this information is passed from the nucleus of the solitary tract through the thalamus. Then from thalamus up to the cerebral cortex. So with information is processed in our taste regions of the insular. it's integrated with visceral sensory signals that are also processed in this insular cortex. And that infromation then is passed onto what we might consdier to be higher-order associational areas. Especially in the orbital and medial parts of the prefrontal cortex. And I would highlight one very interesting region of the orbital corext of the brain, so here's the orbital surface. It's that part of the brain that overlies the orbits of the eyes. here again is our olfactory bulb and our lateral factory tract just for a little bit of reference. I'd like to draw your attention to this posterior part of the orbital cortex. This we think very well maybe the first place in the cerebral cortex where flavors are represented. This is a site anatomically that's receiving input from our Gustatory pathway, as illustrated in the figure before us. And if you'll recall our discussions about the olfactory pathways. It's also a region that receives input from our primary olfactory cortex, such as our piriform cortex. This is also an area, that is going to be integrating signals derived from our trigeminal chemosensory pathways. As well as, other higher order information from the somatic sensations associated with feeding. So when we put all that together, what we have is flavor, and a sense of the pleasure derived from that food. be it of positive or, or negative, valence. So we talk about the hedonic value of food, as being extremely important, in reinforcing feeding behaviors. And this perspective on the value of food seems to be represented in this part of the brain. This posterior part of the orbital cortex. Well information from, the cerebral cortex is also passed on, into structures like the amygdala. As well as the hypothalamus, and so, we can, appreciate from this anatomy, I think. How information about the foods we ingest, can be distributed to a broad cordical network. That's involved in representing the signals derived from inside of our bodies and integrating that with our conscious evaluations. And judgments about the significance of these stimuli. Well we'll talk much more about these topics when we get into the final unit of the course, when we talk about cognition. The associational cortex in some of the truly fascinating, issues and modern neuroscience, that revolve around our sense of judgement. Our sense, of value, our sense of self. And how much of this is derived. Not just from what we see and what we hear and what we touch in the world that's beyond our fingertips. But also the world within these sensations, including the chemical sensory signals. And our visceral sensory signals that become integrated into our conscious sense itself. So, I hope you'll enjoy that discussion. Stay tuned for that. Okay, now, I think we're ready to turn our attention to the problem of sensory transduction in the Gusatory system. So, sensory transduction again, refers to the challenge of converting mechanical energy, electromagnetic energy, or molecular energy into electrical signals within a nerve cell. So we've seen how that happens in each of the sensory systems. And let's consider now how it occurs in the Gustatory system. So I'll just remind you that the tastants, that is the actual molecules that will activate this system, obviously are ingested. taken into the oropharynx where they can interact with specializations of the epithelium. We actually find these taste buds and specializations that we call papillae. So, at the back of the tongue, we have a row of circumvallate papillae. at the sides of the posterior tongue, we have. They set a foliate papillae . And then in the interior part of the tongue, we have what's called Fungiform Papillae. And these names are just anatomical terms that describe a bit if the far, morphology of these papillae. So don't, don't really be concerned with the precise, configuration of this tissue that forms these papillae. Unless this is of particular interest to you. But otherwise I just want you to know that there are those papillae that are distributed in different ways across the surfaces of the tongue. And then the posterior part of the oral[UNKNOWN]. Now these posterior regions in the esophagus and in the soft palate are going to be intervated probably by branches of the vegus nerve, nerve 10. Here on the tongue, the posterior third of the tongue is going to be intervated by branches of nerve 9, the glossopharyngeal nerve. And the anterior two thirds of some of the tongue including these fungiform papillae. These are going to be supplied By branches of cranial nerve 7. Okay so this accounts for how nerves 7, 9 and 10 provide Gustatory signals into that nucleus of the solitary tract to the brain stem. Well now let's look at the anatomy, the microanatomy of one of these papillae. So, so what we see is that, is that there's a bit of a trench so we can imagine that water soluble tastants, work their way into these trenches. Where they can interact with these taste buds, these specializations that we find in the lateral flanks of these trenches. And it's within those specializations that we find our taste cells. So here's a closer look at a taste bud. So, this is the, the tissue that we find in the tongue. These taste buds include our sensory receptor cells, our taste cells. And they also include a variety of basal cells and supporting cells. Now the taste cells are the ones that, of course, I would draw your attention to primarily. these are sensory neurons that have an apical surface that comes together into something that we call a taste pore. So it's specifically through this taste pore that ingested molecules would interact with the ethical surface of our taste cells. And its at that ethical surface, that interact with the taste and molecules. Leading to the depolarization of those taste cells. The release of neuro transmitter, and and a synapse between the base of the taste cell. And the axons of those three cranial nerves, nerves 7, 9 and 10. Okay so, now let's focus in on this apical surface of taste cell. And understand how sensory transduction happens. Well, for taste cells sensory transductions happens through a variety of different mechanisms. And as I'll tell you in just a moment. The receptors that are expressed help to define the response of that taste cell. And cells that are tuned for particular molecular forms. Thereby giving rise to particular Gustatory sensations, are distributed. And overlapping, but somewhat biased regions over the surface of the tongue and in the posterior region of the oropharynx. Well, I'll tell you that story in just a moment. But first I want us to focus on the interaction between our tastant molecules that are going to be presented to this taste pore. The apical surface of the taste cells. And we know that there are lot of different flavours up there right? Well so psychometrically we can categorize them basically into 5 principle flavours. There are tasted molecules will induce these flavour perceptions. And those five basic categories are salt acid or sour, sweet, bitter, and just in the last couple of decades. I think psychologists around the world have agreed that there's a fifth category that we call Umami. And Umami refers to the taste of monosodium glutamate and probably other related amino acids. It's derived from a, a Japanese word that means delicious. This is probably why some of us enjoy a little bit of monosodium glutamate in our diet. I hope you all appreciate now that too much monosodium glutamate can be a very bad thing, it can induce [UNKNOWN] toxic injury. but in small amounts it is sufficient to activate this taste pathway that gives rise to this aparently for most of us very pleasurable sensation. Okay, so the point here is that there are different categories of tastants that interact with different taste receptors. And these receptors have quite distinct mechanisms for transducing this information. For the salt, and the acids or the sour tastants, they interact with Ion channels that have receptors on the extracellular face of these receptors. So, when the tastant interacts, an ion channel opens. And positively charged cations can enter the cell, depolarise it. And that leads to the activation of voltage gated channels that ultimately allow for the entrance of calcium at the basal region of the cell. And in a calcium dependent manner. There's the exocytosis of vesicles that contain the neurotransmitter serotonin. There may be other neurotransmitters involved. But, this is one that we know about. And seratonin will interact with receptors expressed at the terminal of that apherin fiber. With sufficient release of neurotransmitter, there will be a depolarization that is sufficient to generate an action potential, in that afferent fiber. And then communicate the signal on into the central nervous system. Well, other categories of tastants, specifically those tastants that we perceive as being sweet, being bitter. And again this delicious amino acid taste that seems to be distinct in various ways. These tastants interact with G protein-coupled receptors. Now, we've talked quite a bit about this category of receptors. So, I hope it's now familiar to you. It's a seven transmembrane protein that is associated on it's cytoplasmic surface with G proteins. And these G proteins go on to activate second messenger systems. And these second messenger systems can interact with a variety of other systems within the cell. Some of which will lead to the depolarization of the taste cell. Others will release calcium from intercellular stores. So whether calcium is coming in through voltage gated ion channel or being released from vesicular compartments. the increase in Calcium in the basal region of the taste cell is potentially sufficient to lead to the exocytosis of neurotrasmitter. So, in this particular taste cell we're illustrating the presence of multiple receptors. And, and that may be the case. But for many taste cells there just may be one receptor that is expressed by that taste cell. And this figure gives us more detail than we need to know. But it just, gives us a closer look at the transduction machinery associated with the receptors. So again the salts and the acids, they interact with receptors that are on ion channels. And as these receptors are gated then sodium ions or protons can pass into the taste cell and lead to it's depolarization. other tastants, such as the sweets, the amino acids, the umami tastants, and bitter interact with G protein coupled receptors. And we know a little bit more about the detail of these G protein coupled receptors. Some of them are model monomeric that is, there's one receptor that interacts with G protein sum or dimeric. And even heteromeric meaning that there are different types of receptors. In this case the tr2, the tr3 or the tr1 and the tr3 that come together to form the functional receptor that interacts with the tastant. In some of these receptor systems, the downstream effector is a trip channel. Specifically, a trip and five channel that allows for the influx of calcium into the post synaptic cell being gated by a second messenger system. in the case of preceptors for amino acids, and for bitter we think that second messanger is an IP3 message. So, you can see that in various means, there are a diversity of mechanisms that allows for the depolarization of the taste cell. And the release in a calcium dependent manner of the neurotransmitter at the basal end of the taste cell.
