Okay, I think we're ready now to explore the chemical senses. Beginning with the sense of olfaction. As I've already told you, this aspect of our consideration of the chemical senses will reveal again the complexity of the human brain. And will provide us an opportunity to explore some really fabulous circuitry that connects the sensory periphery in the olfactory system to the brain. And I think you'll be surprised with some aspects of the specificity of that relationship that reflects genetic instructions that are played out. Both in, in brain space and in time, as the nervous system develops. An all of this, is critical for the foundation of the olfactory sense. So my learning objectives, for you, in this session, in this part of our tutorial, are really three-fold. I'd like for you to be able to characterize the peripheral and central organization. Of the olfactory system. I'd like for you to again focus on this critical issue of sensory transduction that's been central to all of our considerations of sensation so far in the course. And I'd like for you to get a feel for how information is actually encoded. In the olfactory system. I think one of the values in studying the chemical senses is that we discover perhaps some surprising means for coding information in the sensory system that especially an olfaction seem to be quite different than the kinds of coding mechanisms that we've seen so far. In somatic sensation, in vision, and even in audition. Alright, well let's begin with an anatomical overview of the olfactory system. So, as I've already mentioned to you, olfaction begins with airborne molecules interacting with receptors. And those receptors are found in our olfactory epithelium. And our olfactory epithelium is found in the dorsal and medial aspect of our nasal passageway and it's there that we find our receptor cells. We'll have a closer look at those receptor cells in just a little bit, but for now, I just want to place them for you. So our olfactory epithelium then is where sensory transduction occurs. So our sensory cells then, are found within our olfactory epithelium, which is shown at a little bit higher magnification here to the right. And these cells grow an axon. That passes through perforations in the ethmoid bone and we call this region of the ethmoid bone, the cribriform plate. And these axons enter a part of the brain called the olfactory bulb, and there they make synaptic connections with neurons that are in the bulb, and are actually part of the brain. Now, if you remember what the ventral surface of the brain looks like, from our tutorials where I showed you the actual human brain specimen, you might assume that the olfactory bulb and this, this long connection, is actually a cranial nerve. Well, it's not. The olfactory bulb is part of the brain. The cranial nerve that we associate with olfaction represents these axons that pass through the cribriform plate. So this is our first cranial nerve, cranial nerve 1. It's the axons of the primary sensory neurons. That have a cell body in the olfactory epithelium and grow an axon through the cribriform plate to make synaptic connection in the olfactory bulb. This long extension then, that connects the olfactory bulb backward toward the brain, this is called the lateral olfactory tract. So, again, not a nerve, but an extension of the brain. The cells in the olfactory bulb that grow the axons that form this lateral olfactory tract, they are called mitral cells. We'll talk more about that in a little while, but for now I'll just mention that the mitral cells are then the principal projection cells that connect the olfactory bulb to the rest of the brain. And as we'll see these structures in the brain are part of what we call the olfactory cortex. Okay, so here more schematically, we have our olfactory epithelium, here, which is where we find our olfactory receptors. The first cranial nerve connects those receptor neurons to mitral cells in the olfactory bulb. And there are other cells as well, that we'll talk about when we look in more detail at the, cellular structure of the olfactory bulb. So the axons of the mitral cells project through the lateral olfactory tract to a series of targets. That are found in the ventral and medial aspects of the forebrain. Now, collectively we call these targets the olfactory cortex. Some of these structures are quite prominent and clearly are cortical in nature such as the pyriform cortex. here we're spelling it with a y, you'll notice in my handout I spell it with a, with an ipiri. You'll see spellings both ways, so either spelling is acceptable. Don't worry about that. another major cortical structure that is clearly laminated and recognizable as cortex, is the entorhinal cortex. So this is cortex, a little bit more posterior back on the medial aspect of the temporal lobe. It's part of what we call the hippocampal formation. Now these two other components, the olfactory tubercle and the amygdala, these are structures that we considered to be corticoid structures. And so this is simply a way of saying, well, they are derived from the same kind of embryological tissue that formed the beautiful laminated cortex. However, the organization of the cells is not quite so clearly laminar. So from that perspective, we call this corticoid. Now I should emphasize that each of these divisions of. The olfactory cortex are heavily interconnected with one another. While sometimes we tend to focus on the pyriform cortex really it's this entire extended network of cells that constitute our olfactory cortex. Well in addition to intrinsic associational connections, these divisions of the olfactory cortex also have projections to other parts of the brain. And this includes the thalamus, the hypothalamus, and especially important, is the orbital and frontal parts of the prefrontal cortex, that is the part of the frontal lobe that sits just above the orbits of the eyes. As we'll come to discuss, as we move along through this set of tutorials. It's here in the orbitofrontal cortex that information from all of our chemo sensory systems is combined with somatic sensation and, and even visual sensations. And it's in this part of the brain that our concept of flavor appears to be represented. As well as a sense of the rewarding value of food. Now, of course in addition, food can be a powerful trigger for memory and for recall, so it makes sense that the entorhinal cortex has direct access to the circuitry of the hippocampal formation where new declarative memories are formed. Now, let's look at these brain regions from an anatomical perspective. And I'll go ahead and show you my brain model. just to remind you in a bit more realistic fashion what we're actually looking at here. So I think you can recognize nicely the olfactory bulb, and the lateral olfactory tract that connects the olfactory bulb back to the olfactory cortex. And even in this brain model, as you may remember from looking at the actual human brain, it does look like a nerve. But again, I would emphasize, that the olfactory bulb is really part of the human brain. Well, the lateral olfactory tract then connects back to the inferior, and posterior part of the frontal lobe, right near the junction of the frontal lobe and the temporal lobe. That's where we find the beginnings of our olfactory cortex. Well now, to look at our figure from our textbook, that cortex is illustrated here in the green shade, and the first division that we encounter of that cortex is the olfactory tubercle. So, this is really a ventral part of the striatum. So, it sits just below the main bulk of the basal ganglia. We'll get to talk about that in the next unit but just to help you localize this region it's just dorsal and a bit anterior to the level of the optic chiasm. Which we see right here on the ventral aspect of the brain. Well, the rest of the olfactory cortex flows into the medial part of the temporal lobe. So, there's a junctional region where the temporal lobe attaches to the inferior and posterior part of the frontal lobe. It's there that we find this division that we call the pyriform cortex. So this illustration, perhaps, is just a bit too generous. Indicating a more medial and, perhaps, a more ventral position of the pyriform cortex. But most of the pyriform cortex is actually tucked underneath this little region where the temporal lobe joins the frontal lobe. Well, that leaves us with the rest of this medial. parahippocampal gyrus, and that's where we find the cortical divisions of the amygdala and the entorhinal cortex which receive the inputs from the mitral cells of the olfactory bulb. Well now that you have a bit more information about where the olfactory cortex is present. I want to make just a couple of general points about the organization of these olfactory structures, that are worth noting as we discover this anatomy. First of all, I want you to notice, that there is no thalamic relay between the sensory structures, that we might consider to be more peripheral, such as the olfactory bulb. And the cortex. Now, in all of the other sensory systems that we've evaluated so far, you'll recall that there is a nucleus of the thalamus that receives ascending or incoming sensory information and projects that information into cortex. And this we think is, is quite important for how the sensory systems operate. The thalamus, is something of a gatekeeper, to the flow of incoming sensory signals to the cerebral cortex. Well this doesn't seem to be the case, in the olfactory system. There is a connection to the thalamus, but it happens after information has been processed in the cerebral cortex. So, there's no obligatory thalamic relay between the olfactory bulb and the olfactory cortex. So, the second point worth highlighting is that as we detail the organization. Of the cortex itself and consider the topic of sensory coding, what will be very clear is that there is no known map of the sensory environment. And there may not even be a clear map of the sensory periphery at the level of cortical processing. Now there are very precise rules that help us understand how our sensory cells are related to specific neurons in the olfactory bulb. The challenge in understanding olfaction, is that those rules are not conserved as the olfactory bulb projects back into the olfactory cortex. So at least no one has yet demonstrated in any compelling fashion, that there is something like a topographic map of the sensory environment in the olfactory cortex. In the way that we've seen it for a body map, in the postcentral gyrus, a map of the retina, or the visual environment in the occipital cortex revision, or even a map of[UNKNOWN] or map of the basilar membrane of the auditory environment. So, it's as if the rules of coding seem to be different in the olfactory system and that makes it very much worth exploring and understanding.
