Welcome back to Catalyst University. My name is Kevin Tokoff. Please make sure to like this video and subscribe to my channel for future videos and notifications. And a big thank you to my patrons on Patreon for your contributions to my channel.
This video is going to be all about cranial nerve 1 or the olfactory nerve. The term olfactory comes from the term olfaction, which is really just the sense of smell. So anything olfactory pertains to smell.
Now the olfactory nerve is a purely sensory cranial nerve and it's one of only two cranial nerves that is not associated with the brainstem. Recall that the brainstem consists of the midbrain superiorly and then the pons and the medulla oblongata. And cranial nerves 3 through 12 originate in some part of the brainstem, but cranial nerves 1 and 2, the latter which is the optic nerve, do not.
They actually originate elsewhere and then go directly to the brain. So let's talk about a little bit of anatomy right here. So we're looking at a sagittal cut of the head or the skull. And right here is an important bone here for olfaction. This bone right here is called the cribriform bone or cribriform plate.
And above the cribriform plate, there's a cavity up here. This is the cranial cavity. We know it's the cranial cavity because it contains the brain. So that's your cranial cavity. Below the cribriform plate, all of this is the nasal cavity.
Now the nasal cavity contains three parts. This part right at the entrance to the nostril, this is the nasal vestibule, and then pretty much all of this over here, this is the respiratory region of the nasal cavity, and then the smaller part up here is the olfactory region of the nasal cavity that's closest to that cribriform plate. Now over here on the cranial side of the cribriform plate we see the olfactory bulb.
We'll see in a minute that the olfactory nerve is actually the olfactory bulb plus the traps that go posteriorly. But this bulb is actually kind of this engorged or this enlarged region of the olfactory nerve that's most rostral or closest to the nose. Okay, if we zoom in on that olfactory bulb, we see that it has projections that come down here and actually cross through the cribriform plate. These are actually axons of cells called olfactory receptor cells. We'll look at that in more detail on the next slide.
But those axons actually cross through the cribriform plate, and then this part of them actually exists within the olfactory region of the nasal cavity. So in this picture, the individual is smelling a rose, and the smell of the rose is perceived by the person's brain. But there's an issue here.
How are we able to transmit this information from the rose to the other person? to the brain when the rose clearly is not in contact with the person. There's no physical contact there. And the answer is there are little molecules in the rose called odorants.
There's odorants everywhere. Anything that you can potentially smell has odorants. These are little molecules and they can become airborne.
These little green dots right here, these are actually the odorants. And you'll notice here that they become airborne. So they're moving from the rose, they travel in the air. and they're going to have to enter the nasal cavity. So they're first going to move through the nostril, and then they'll end up in the nasal vestibule right here.
They'll then move into the respiratory region of the nasal cavity, and then eventually up to the olfactory region of the nasal cavity. Remember these olfactory receptor cells that I mentioned a minute ago? Well the component of them that resides in the nasal cavity has little receptors on it. receptors for these odorants. And so when these odorants reach the olfactory receptor cells, they bind to those little bitty receptors, and then that information is transmitted through the axon up here across the cribriform plate and into the olfactory bulb.
So a couple pieces of terminology here, olfactory nerve, really refers to both the olfactory bulb right here and the olfactory tract that actually goes posteriorly from that. That's your olfactory nerve. You can actually see from the bulb we have those cells, those axons, that move through the cribriform plate and they create this network of olfactory receptor cells in the olfactory region of the nasal cavity.
Then we have the olfactory epithelium. Now very quickly, in this picture, here's the olfactory bulb up top. It's on the cranial side of this, which is the cribriform plate, and then below the cribriform plate, this is the nasal cavity side down here.
Now you see here a bunch of cells. There's different kinds of cells. These colored cells in green, blue, purple, and yellow, these are your olfactory receptor cells.
You'll notice that they're sandwiched in between these pink cells which are called support cells or supporting cells. And there are some other cell types that exist here that I haven't put for simplicity, but the olfactory epithelium really consists of all of this below the cribriform plate. You have the olfactory receptor cells, you have these pink supporting cells, and then anything else that's there, that is the olfactory epithelium. Now, if we consider the olfactory receptor cells, these ones in green, blue, purple, and yellow, they have important parts. First of all, if we look at the parts that are directly exposed in the nasal cavity, they have these little cilia, these little whip-like things down at the bottom that look like roots.
Those are the cilia, okay? And then we have the receptors themselves, in particular the cell bodies right here, are sandwiched in between those supporting cells, right? If we follow those up, we have these axons that actually move through the cribriform plate, and they move into these circular structures which are called glomeruli.
Each individual one, there's five here, are called a glomerulus. And you'll notice that within the glomerulus, these axons of the olfactory receptor cells synapse with the mitral cells. So here's a mitral cell. Here in the glomerulus is where it synapses with the olfactory. receptor cells.
So this is a mitral cell. There's actually five mitral cells here. And if we follow the mitral cells past their cell bodies, we see more axons and those axons move towards the olfactory tract. And you can see that these axons kind of all run together ultimately into that olfactory tract.
Okay now you'll notice here that each of these olfactory receptor cells is a different color. What this is meant to show is that each of these cells binds a particular type of odorant. All these little dots down here are your odorants.
So this green olfactory receptor cell through the cilia binds a certain type of odorant, which is a different type of odorant than those bound by the blue one, which is therefore different than the ones bound by the purple one, and so on and so forth. And so you have a bunch of different olfactory receptor cells that all bind a different type of odorant, and that's how we have such a broad array of smells that we're able to... to detect. And so let's suppose we have one of these odorants bind to the receptors on the cilia of this olfactory receptor cell. Well, that information is transformed into an action potential, which travels up here past the cell body of that olfactory receptor cell, across this axon, up through the quiver form plate.
That information is then transferred across the SNAPs to the mitral cell, and it's therefore carried across here from the olfactory bulb into the olfactory. tract and that olfactory tract is going to send information to specific parts of the brain which is now what we're gonna look at here. So right here is my olfactory bulb, it's engorged right over here is more anterior and it narrows into the olfactory tracts which are kind of cut off for the sake of simplicity and those move posterior and you can see that they go to different parts of the brain. Now remember those olfactory receptor cells of the olfactory epithelium are going to synapse with these mitral cells in the glomeruli.
This little circle is the glomerulus, and then this is where it synapses with the mitral cell. There are also other cell types called tufted cells which function fairly similarly. For the most part we're going to treat them the same here, but you'll notice that both of these cell types, their axons, move from the olfactory bulb into the olfactory tracts. Now the axons from both the mitral cells and the tufted cells constitute what we call the lateral olfactory tract.
There's another tract that we're going to look at in the next video that originates from the vomeronasal organ and those go to mitral cells here, but those are not actually a part of the lateral olfactory tract. In any case, let's follow these blue axons. We see that they actually go to five different parts of the brain that are all involved to some extent in smell. We have the anterior olfactory nucleus, the olfactory tubercle, piriform cortex, amygdala, and the enter renal cortex.
Now this first one we're not going to talk about too much but this is the anterior olfactory nucleus. We see that it sends information to the contralateral olfactory bulb. So whenever you sense things with your left olfactory nerve, there's actually some communication with the right olfactory nerve. The significance of that is not completely understood. Now these other four parts of the brain right here, there's a lot of crosstalk.
Because as you can see, they all send their information to really three parts of the brain. And those are the orbital frontal cortex, the frontal cortex, and the hippocampus. The first one we're going to talk about is the orbitofrontal cortex. If you look at all four of these brain parts, notice that they all send information to the thalamus. And then from there the thalamus sends that information to the orbitofrontal cortex, which we actually see in green right here.
The orbitofrontal cortex is part of the frontal lobe, but notice that it's actually very close to where that olfactory bulb is. That would actually be right where my mouse is. Now the orbitofrontal cortex is involved in determining emotions and determining rewards.
as it pertains to decision making with smell. So let me give you an example. So we have a cat named Nimbus, and he goes crazy every time we cook breakfast.
So we often have eggs and bacon for breakfast, and he loves bacon. So he probably smells the bacon. Cats have much better smell than we do. He smells the bacon, and he knows that if he begs, there is a reward involved in that.
So what does he do? Well, he begs. He acts all cute and sweet so that he can get some bacon. How did he know that he was going to get bacon? Because there was smell.
And their past experiences have taught him that if he begs, he's going to get a reward. So the orbitofrontal cortex is heavily involved in that. Determining a reward or a specific emotion in response to a particular smell.
In this case for our cat, it's bacon. The second part we'll look at is the frontal cortex. Again, notice all four of those brain parts send information to the frontal cortex. Now the frontal cortex is the part of the brain we use when we're using critical thought, higher level thinking, higher cognitive functions, but here we're going to talk about those in relation to smell.
It's also involved in comprehension and recall of memories and emotions related to smell and impulse control and problem solving. related to smell. This impulse control is big. So if we think about somebody trying to lose weight and at their office there's a lot of cookies just floating around people's office. People offer cookies.
They might offer donuts. But somebody's really trying to lose weight. Well you know that if you were to eat that donut or eat that cookie It's going to taste amazing, won't it? And so that's kind of your orbital frontal cortex thinking. You know there's a reward for it.
It's going to be wonderful when you're eating it. But you know that if you eat too many of those cookies, what's going to happen? You're not going to lose weight. So the frontal cortex in that case is having to override the orbital frontal cortex because it's got to be involved in that impulse control. You're not going to eat those cookies because you are trying to lose weight.
So that's your higher cognitive function overriding or controlling. that impulse. And then also just the simple comprehension and recall of memories and emotions related to smell.
We'll get into a little bit more of that now with the hippocampus. Now the entorhinal cortex is really the only one of these five brain parts that has any communication with the hippocampus in terms of smell. Here's something to understand.
The frontal cortex is responsible for comprehension and recall of memories and emotions related to smell, whereas the hippocampus responsible for assigning those memories and assigning those emotions to that smell. You can't have a memory of a smell or an emotion pertaining to a smell without first the hippocampus assigning the memory and emotion to that smell. And this actually involves a cluster of different parts of the brain called the circuit of Papes.
I cover this in a separate video, but this is a circuit that involves other structures in addition to the inner renal cortex and hippocampus that are involved in that assignment. of memory and emotion to smells. So what's an example of that? Well, this may not apply to every single person, but you can probably imagine.
So let's suppose you and your grandmother, many years ago when you were a little kid, used to bake cookies, and those cookies had a wonderful, delicious smell. Well, your hippocampus put that into long-term memory, and it assigned that smell to make you think about baking those cookies with your grandmother. So more or less, your grandmother is associated with the smell of those cookies.
And then unfortunately, at some point, your grandmother has passed away. She's no longer with us. Well, one day, you're just doing something random and you smell that smell of those cookies again, or at least something fairly similar to them.
And it conjures up a thought of you cooking with your grandmother, or maybe just a thought of your grandmother herself. It probably also conjures up emotions about your grandmother. It's the hippocampus that's responsible for assigning those memories and emotions regarding your grandmother to the smell of those cookies.
It's the frontal cortex that retrieves that and recalls that. That wouldn't be possible without the hippocampus'function. Now as for the amygdala and the hypothalamus right here, we're going to be talking about those in the next video as it pertains to pheromones and their communication with the brain via the vomeronasal organ.
Let's now talk about how we test the function and the health of the olfactory nerve. We're going to test the left side and we're also going to test the left side. This setup right here is for testing the patient's left olfactory nerve. You'll notice here that his eyes are closed and must remain closed for the entirety of the test. The olfactory nerve that's not being tested, that nostril is manually blocked.
So the patient has his finger blocking the right nostril. Therefore we are not testing the function of the right olfactory nerve. And the test olfactory nerve is tested with the open nostril.
So his left nostril is open, testing the left olfactory nerve. Makes sense. And then the practitioner is going to use a test odor. That's what you see them holding in their hand right here.
They're going to bring that test odor up to within 30 centimeters or less of the nose. This is what one source I found said. but you'll notice that 30 centimeters, if you think about that, is a pretty big distance. This is a lot closer than 30 centimeters.
You can do variations of the test where you can have it at certain distances. You just don't want to exceed 30 centimeters. But generally when you're just doing a quick cranial nerve screen of the olfactory nerve, you're going to have it much, much closer than 30 centimeters. This is probably closer to three centimeters actually.
This is a really short distance right here. That's about the distance you want, this from the open nostril. You don't want to touch the patient when you're doing the test because it can give them some manual cues, which you don't want. We're only testing that nerve.
And then the test odor should really be something that is familiar to the patient. Examples might be coffee, mint, lemon. If you think about those items, first of all, they are familiar to most people, and people should be able to recognize those.
But they're also things that you can easily name. They don't have some weird, complicated name that maybe a select group may not have heard of. Another thing that you can also use is a 70% isopropyl alcohol swab. You can see those down here at the bottom of the screen. The reason why you might want to do those is because, first of all, most people are aware of the smell of alcohol, rubbing alcohol, hand sanitizer kind of things, right?
They don't have to say 70% isopropyl alcohol. alcohol would suffice, but most people are familiar with it and could name it. But also these alcohol pads are very common in clinics. They come in these little packets. You just rip open the packet and take the pad out, and that pad would replace whatever this object is in the practitioner's hand.
And you, the practitioner, are assessing, first of all, can they detect the smell? And if so, can they identify it? identify it by its name.
And generally speaking, there are three possible results here. There's normosmia, hyposmia, and anosmia. So normosmia is a negative test. This is where the patient has normal smell.
So you put whatever the test odor is up to their nostril, they can detect it, and they can name it, okay? And you would do this on both sides. And then there's two positive results, the first being hyposmia. Hypo usually means less or below. So we're not talking about an absence of smell, we're talking about diminished smell.
So it's still there to a little extent, but it's diminished. And we're usually comparing left versus right. So we could see hyposmia, perhaps, if there's a patient who somehow got a little bit of damage to, let's say, their right olfactory nerve.
So in that case, we would expect the left olfactory nerve to be a negative result, normosmia. But then the right side, which had a little bit of damage, may have a little bit of diminished smell. So... hyposmia.
The other positive result is anosmia, which is basically where the patient has no ability to smell on one side or both. And again that can manifest in one of two ways. You could either have one side that has normal smelling ability but then the other side has completely lost it. So that's a unilateral anosmia. Or maybe both sides actually are anosmic and the person has absolutely no ability to smell whatsoever.
Now, there's several different causes of hyposmia and anosmia. Those include rhinitis, any trauma to the face. So, for example, if you're dealing with anyone in like mixed martial arts, boxing, and they take a big blow to the nose, there is a possibility that with the crumpling that occurs in there of the bones, you could actually have damage to the olfactory nerve.
And if that's the case, it could either cause hyposmia or anosmia. It can also be a side effect of some medications and also... and in recent times, COVID.
A lot of times people who have COVID lose their senses of smell and taste, which if you're not familiar with taste, it's linked very closely to olfaction. A lot of times even when people have allergies or a cold, they lose both their senses of taste and smell to some extent, and that might actually be hyposmia. So it may not actually even be due to actual nerve damage. It could just be the person is having a runny nose, they're stuffy, They got a little bit of an illness. So those are things you also want to rule out before coming to a conclusion about the olfactory nerve.
Before we conclude the video, I want to mention one last thing, and it's the identity of the test odor. In some cases, you do have to be culturally sensitive and aware of your specific patient, because there are some people that may not be familiar with what are otherwise common odors. So I did my very first clinical rotation in an area where there was a very, very high population, about 95% Latter-day Saints or LDS. And in the faith of LDS, they do not consume coffee. And so the smell of coffee for some of those people may actually be unfamiliar.
And so if you're aware that you're dealing with some group of people where they may not be exposed as much to one of these odors, it's probably best to not use that because they may get a false positive. And so you might be better off with something like mint or lemon. So just some food for thought there.
But hopefully this video gave you a good understanding. of the structure and function of the olfactory nerve, and then also how you test it. Thanks for tuning in. Please like, subscribe, and check out my Instagram for cool science and not science stuff.