Welcome back to our study of the Chemical Senses and in this tutorial, I'd like to talk to you about the sense of taste or in more formal terms what we call a Gustation. So this conversation about our chemical senses again illustrates the complexity of the human brain. And it allows us to, once again, discover circuitry in the brain that is genetically determined and provides the foundation for function in the central nervous system. And the circuits are modifiable through life experience and that's the topic we'll get to in more detail when we consider the changing brain in Unit 5. My learning objectives for discussion on gustation are first to characterize the peripheral and central organization of the gustatory system. I want us to again focus on the issue on sensory transduction and understand how that occurs with in our sensory neurons in our taste buds. And thirdly, I'd like for you to be able to discuss the means by which information is coded within the gustatory system. Well, let's begin with a broad anatomical view of our gustatory pathways. So, gustation, or a sense of taste begins in the oropharynx. And what we find are sensory cells associated with an anatomical structure that we'll consider in more detail in just a few minutes, called the taste bud. And these, taste buds, allow for the presentation of ingested molecules. Many of them are water-soluble, some of them are lipids. but, in any event, they interact with the apical surfaces of our taste cells where they generate electrical signals through a process of sensory transduction. So we're going to want to understand exactly how that occurs. Well, at the base of these sensory cells, there's a synaptic connection with an afferent axon and that axon comes from one of three cranial nerves. And so, those cranial nerves in question are nerves VII, IX and X. So, there is a ganglion cell associated with each of these cranial nerves that grows a peripheral axon out to various regions in the oropharynx where we have taste buds. And as we'll see these are of course on the tongue as I think you all know but, there are also taste buds on the epiglottis. And taste buds that perhaps or even more posterior back in the Pharynx. So these axons of cranial nerves VII, IX and X, project into the brain stem. And once they are in the brain stem they synapse with a nucleus of the brain stem. That we call the nucleus of the solitary tract. So, we're going to want to see this nucleus in just a bit of detail. that nucleus then receives this first order information and in that nucleus, we have cells that grow axons that project from the brain stem on up into the thalamus. So in the thalamus then, is where we would find our third, order neurons in this pathway. So the nucleus as a solitary track would be second order, and then the thalamic neurons are the third order. An once again, our thalamic relay is in the ventral and posterior part of the thalamus. But not the exact same cells that we considered when we talked about somatic sensation. For gustation, it's a nearby population of cells and a slightly different part of this broader ventral posterior thalamic complex. But we can just consider this part of the Ventral posterior complex of the thalamus. Well, those third order neurons then provide input into the Insular cortex. there are probably several zones within the insular cortex that receive this input, so these are regions that we might call primary gustatory cortex or sometimes we just call them the insular taste regions. Insular taste cortex is how it's labeled in this figure and then, in a more rostral position, the frontal taste cortex which is really still just part of the insular cortex. As you will recall from our studies of brain anatomy, the insula is that part of the cortex that ends up being folded and covered by the gyrus that forms the upper part of the temporal lobe. And then the gyri that form the lower part of the frontal and parietal lobes. Specifically, the superior temporal gyrus from the temporal lobe, and from the frontal lobe the inferior frontal gyrus, the inferior part to the precentral gyrus. And then just a little bit into the parietal lobe, the inferior part of the postcentra gyrus. So, this is where we find our taste receptive neurons, at the level of the cerebral cortex, for the first time in cortical processing. Well, to carry the story onward, I think we will appeal to more schematic representation so that you can understand perhaps little bit more about what happens to this information about taste. So in this schema, we again show our cranial nerves. So here's Cranial nerve VII, Cranial nerve IX, Cranial nerve X. Representing taste buds that are distributed in different parts of the oropharynx. All of this information converges in the brain stem in this Nucleus of the solitary tract. Well, I'd like to take just a moment and actually show you that nucleus. So if you have your version of Sylvius accessible, you may want to pull that up, or otherwise just follow along here. Well, I've just opened my copy of Sylvius to the brainstem Atlas, perhaps you've done the same. And what we're looking for now, is the nucleus of the solitary tract. Well, this is a rather long, rod-shaped nucleus that spans the junction of the pons and the medulla. So to find that nucleus, we need to, get at the appropriate sections. So I'm just going to click down from the level of the mid-brain into the pons. So now we're in the upper pons, through the middle of the pons, and as we get into the region of the junction between the pons and the medulla, this is actually the upper part of the medulla. What I'm going to be looking for, is something that we sometimes described as a bullseye, that is a dark spot of heavily myelinated fibers surrounded by a lighter region, which would be gray matter. So, if we look right in this dorsal lateral aspect of the tegmentum of the brain stem, we see something that well, if you use your imagination, maybe you can imagine this as being something of a bull's eye. So if I clear my drawing for just a moment and let Sylvius do the work, here. This central dark region, is what we call the solitary tract. This, represents the, central process of those first order afferents that have entered the brain stem through Cranial nerves VII, IX and X, and are making their way down to the appropriate level of the nucleus. Now the nucleus, actually surrounds the tract. So, notice how, anatomically this is a bit different than our, perhaps more familiar tract in nucleus, which is our spinal trigeminal tract, and spinal trigeminal nucleus. And the trigeminal system, the tract is just lateral to the nucleus, which is just medial. But in the solitary system, the tract is completely engulfed by the cells that are receiving the synaptic input from those first order afferents. Now, there's another important distinction I'd like to make about this nucleus of the solitary tract. there are really two major parts to it. There is a rostral part and a caudal part to this nucleus. The part of this nucleus that is receiving gustatory information is actually the rostral part. So we would encounter this division of the nucleus at roughly this level. The upper part of the medulla and then, on up into the caudal part of the pons, and that's pretty much where we lose it. So we've, we've lost the nucleus now that we are at least a few millimeters into the pons. But as we get very close to the junctional region, now we see this nucleus of the solitary tract, and there's our solitary tract, still more or less engulfed by the gray matter. So the rostral part of this nucleus is the gustatory relay. Now there's a caudal part to this nucleus, and if we follow this nucleus down into the medulla just a little bit further we would continue to follow this for some distance. The caudal part of this nucleus is actually receiving input from the viscera. It's a visceral sensory integrator and it makes perhaps good sense that our sense of taste would be received in a gray matter structure that also has a caudal division that receives input from out viscera. Because taste and visural sensations are awfully intimately intergrated. imagine, experiencing the wonderful flavors of a palatable meal. and having those sensations integrated within the sensory signals that are being derived from that gastrointestinal system that does the work of digestion. Well, sometimes it, it works the other way, right? Sometimes we have a flavor, a gustatory sensation that is signaling the presence of a harmful taste, a harmful substance that we've just ingested into our mouth. So, it's important to get rid of that harmful substance. We do that through various means, we can spit out from our mouth. The morsel that we may have just taken in, or if we've already swallowed it, then we actually would benefit from vomiting out that substance. Well, it's as if the nucleus of the solitary tract is a key center for integrating both taste and visceral sensation. And the nucleus of the solitary tract has important outputs to circuits in the brain stem that govern some of these actions, like gagging or coughing or spitting, and even vomiting. Well, we'll talk more about our visceral sensory and visceral motor functions when we get deeper into unit four. So stay tuned for that. Okay, so just to summarize again this anatomy. The nucleus of the solitary tract is an important integrator of gustatory signals in the rostal division of the nucleus, which is what we're saying here at this level. Whereas the caudal division is an important integrator of visceral sensory signals.
