Neuroanatomy of smell (olfaction)

Ajouté : 2026-06-04

[00:00:00][music] >> The way smell is wired up between our noses and our brains is It's not quite It's not like other cranial nerves. It's quite different.

[00:00:11]It's quite special. When I was reading more about the limbic system for last week's video, smell plays a big part in that, and the wiring and the circuit are quite interesting. Let's have a little look at how the neuro- neuroanatomical circuits take the sensation of smell from the nasal cavity and take it back to the brain.

[00:00:28]And there's a few interesting little facts and ideas in here.

[00:00:33]Shouldn't take too long.

[00:00:41]Okay.

[00:00:43]The nose.

[00:00:45]So, inside the nasal cavity, uh we see mucosa. The skin has become mucosa. It's a mucous membrane. It has mucus on it.

[00:00:55]I think you know what I'm talking about, right? And we've got quite a lot of surface area here, but the region involved in olfaction, involved in smell, is the bit up here. Now, this mucosal membrane is an epithelium, and in here we find epithelial cells, but what we It's an olfactory epitheliums.

[00:01:12]In here, we find um olfactory sensory neurons.

[00:01:19]You could say that the neuron cell bodies of cranial nerve one and people do say this are in the olfactory epithelium here, and their axons then project back towards the brain, which is a bit weird cuz of all the other cranial nerves, they have their their neuronal cell bodies in the brain stem, and they project outwards.

[00:01:39]But, we'll we'll follow it back and we'll see how this all wires up. So, in this olfactory epithelium, you have olfactory sensory neurons. These are the neurons that are specialized to detect odorants. They have specialized receptors to detect different volatile chemicals in the air that get inhaled into the nasal cavity as we breathe in, right? So, and and this is specialized as well. So, we have So, each neuron can only detect one type of smell, one volatile chemical.

[00:02:11]So, we have lots and lots of different sensory olfactory neurons in here that detect lots of different chemicals. So, it detect lots of different smells or maybe a smell is a combination of those volatile chemicals. Anyway, so the olfactory epithelium has olfactory sensory neurons. And then it has a lot of support cells. Just as we see support cells, glial cells in the central nervous system supporting neurons, these support cells in the olfactory epithelium are supporting the neurons. So, these cells need to be present and in good health to support the function of the olfactory neurons.

[00:02:46]And with an upper respiratory tract infection like COVID, for example, it's often these support cells that get damaged and affect our sense of smell.

[00:02:54]So, we have support cells in there. And then we have basal cells. We have other cells as well, but those These are the three main types. The olfactory sensory neurons, the support cells for the neurons, and the basal cells. Now, the basal cells, like other basal cells in epithelia, are the stem cells, the progenitor cells. As you lose cells from the surface of your your mucous membrane, as you often do, uh new cells can be made. So, it's constantly being repaired.

[00:03:21]Now, these I think this is a great fact.

[00:03:24]These basal cells in the olfactory epithelium can differentiate and become olfactory sensory neurons. So, you can make new neurons in your olfactory epithelium. Those basal cells could also differentiate and become um the support cells.

[00:03:44]Um there are other cells in here making the mucus and what have you, right? But that's the olfactory epithelium.

[00:03:49]Now, those olfactory receptor cells, olfactory sent olfactory sensory neurons, olfactory receptor neurons, whatever you want to call them, they are bipolar. Meaning they have a cell body and they send out a long axon and they also send out a longish dendrite. So, they send the dendrite down through the um olfactory epithelium and that dendrite then gives off lots of little branches which get called cilia.

[00:04:16]They're not motile, but they're lots of little branches, maybe 10 or 20 branches. And those branches spread out into the surface of the epithelium, so they're increasing the surface area of receptors of that neuron, making it more sense more sensitive to those volatile chemicals. And then the other axon, the main axon, goes up and it goes through this bone here.

[00:04:44]Um Oh.

[00:04:51]Here is >> [clears throat] >> Here is another Here is another nose, right? So, if I take the midline septum off, look, there's that very well vascularized mucosa.

[00:05:03]If I take this off, look in there. Look there. This is showing you how all of those axons from those olfactory sensory cells pass up through This is the ethmoid bone here, through the ethmoid bone into this structure here. This is the olfactory bulb. Now, these these axons here are the olfactory nerve, cranial nerve one.

[00:05:32]But, typically in anatomy, we don't really think of it like that. But, in the textbooks, this is how it's often described. So, these nerves here are the olfactory nerve are cranial nerve one.

[00:05:43]But, their cell bodies are in the olfactory mucosa and the axons are taking action potentials that way, of course, because this is a sensory neuron. Now, the olfactory bulb sits Here's the face.

[00:05:57]Take the cranial cap off.

[00:06:00]Sits in these these two depressions in here.

[00:06:04]Um and you can see lots of little tiny tiny tiny holes in there. This is the cribriform plate.

[00:06:12]So, that shape the bone has formed around the olfactory bulbs and those tiny So, it looks like a sieve. So, the cribriform plate shows you all those little neuronal axons passing through the bone, through the ethmoid bone, to run between the nasal cavity and the olfactory bulb up here.

[00:06:30]Um This is the um What's it called? The the the the the rooster's the um crista galli. You know, the rooster's that sticky up bit there, the crista galli the crista galli. Um okay.

[00:06:45]So, these are our kind of our first-order neurons detecting volatile chemicals in the air and sending action potentials up to the olfactory bulb. In the olfactory bulb, we find mitral cells, kind of a second-order sensory neuron. Now, these these are neurons.

[00:07:03]So, these mitral neurons are receiving inputs from these olfactory sensory neurons.

[00:07:12]But, each mitral cell is probably receiving inputs from maybe hundreds or a thousand um sensory olfactory sensory receptor neurons.

[00:07:26]Now, because we have Oh, this is how So, because each olfactory sensory neuron reacts to only one volatile chemical, all of those olfactory sensory neurons that react to that same volatile chemical, they all go to the same or similar same mitral That's the best way of thinking about it. They all go to the same mitral neuron, that same secondary or second order neuron, right? So, you have a whole bunch of neurons, they detect the same smell, they then send that action potential all the way up to the same mitral neuron, and then that mitral neuron sends that action potential back to the brain. And you So, then you have different mitral cells in the olfactory bulb, which are responsible for receiving different smells. Does that make sense? I think it's kind of cool. A very very cool and specific organization.

[00:08:26]And that's what gets called the olfactory glomerulus. All of those sensory olfactory sensory neurons that detect the same smell, they all come together and congregate um on the same or similar mitral neurons, and that that congregation is the olfactory glomerulus.

[00:08:45]So, these mitral cells are then taking those action potentials back to the brain. There are other neurons in here.

[00:08:50]There are other circuits, which are kind of fine-tuning smell a little bit, but that's the the crux of it. So, if that's the olfactory bulb, um brain And that's the olfactory bulb, one on either side, the axons of those mitral cells run back to the brain in the olfactory tracts, which are running on the inferior surface of the frontal lobe here. Now, these olfactory tracts typically, we end up calling them the olfactory nerve, cranial nerve one. Is that fine? It probably is. Um but you can see there's a bit more to it. These are actually the second order neurons carrying smell back to the brain. And largely, these action potentials are running back to the limbic system.

[00:09:42]Um This is one of those confessions.

[00:09:47]Um Here's the thalamus.

[00:09:50]The thalamus is a sensory relay that all sensory information has to pass through to get to the brain. And famously, we say that all sensory inputs have to go through the thalamus to get to the cortex except for smell.

[00:10:06]Which which isn't actually true like most You know, like the thalamus also has a whole bunch of other jobs, but we call it the sensory relay.

[00:10:15]It's mostly true, but um if you want to know more about the limbic system, have a look at last week, but we can say that the um the olfactory tracts, so they're called tracts because they're considered extensions of the they're considered parts of the central nervous system.

[00:10:34]Now, a nerve is a peripheral nervous thing, a tract is a central nervous thing. Anyway, these tracts carry information back to the limbic system, let's say the olfactory cortex. And it's not just us that's saying that, that's what it's called. So, if this is the temporal lobe here, this in here is the uncus. In there is the olfactory cortex. The olfactory cortex is a bit of an umbrella term which pulls in various structures of the limbic system like the olfactory tubercles and the amygdala. If you want to know more about the amygdala, look at last week. So, the olfact- This is why we say that smell doesn't go through the thalamus because it goes into the limbic system. Now, what's happening there is smell is one of those senses that drives behavior.

[00:11:21]You smell Oh, you smell a food that you like and that makes you hungry. You feel hungry, you think about going to get food, your stomach starts to rumble because the blood flow through your body is starting to change. You may be producing saliva. So, smell drives behavior and also if we smell something horrible, something really disgusting, you you go somewhere else. You leave that environment because there's probably something not very good for you nearby.

[00:11:45]So, smell drives behavior and that's what the limbic system is doing. It's taking sensory input and it's driving behavior.

[00:11:52]Um also, smell is associated with memory, right? I'm sure I'm sure you've got smells that will help will make you remember things that will bring back memories. Ah, you think, "I haven't thought of that for years." just cuz you smell this thing. So, and memories are um a whole bunch of sensory inputs encoded into long-term memory. And again, this is a largely a temporal lobe and a hippocampus and a limbic system thing.

[00:12:16]So, smell being associated with memory, this is also limbic system. Um So, smell comes from the olfactory tract, goes into the olfactory cortex, gets involved limbic processes, but it also passes into the uh the thalamus and gets relayed through the thalamus to the frontal cortex, the um orbitofrontal cortex. If you think that the eyes are here, this is the frontal lobe sitting above the orbits. So, the orbitofrontal cortex. Now, this is a higher center of the brain.

[00:12:56]This is probably the region of the brain where we our conscious becomes aware of that smell.

[00:13:04]Um we it's a it's like a a smell association cortex.

[00:13:09]We know what that smell is. We associate it with something. I can smell a barbecue. I can smell bacon.

[00:13:17]I can smell toast, whatever.

[00:13:21]Do you know what I mean? Uh and this is probably also where we are then consciously aware of that smell, whereas the limbic system is kind of driving autonomic automatic behaviors. So then we can also say that where all factory inputs go into the all factory cortex in the limbic system we could call that the primary all factory cortex and this region here of the frontal cortex we could call the secondary all factory cortex kind of the the higher functions.

[00:13:46]That's what I mean by or that's what we generally mean when we say that all sensory inputs go through the thalamus except for smell because they go straight into the limbic system. It's not true because smell is also relayed through part of the thalamus to get to the frontal cortex.

[00:14:07]Sorry.

[00:14:08]One of the main reasons this anatomy is so interesting, I think it's interesting just for the sake of interesting, but what one of the main reasons is because of loss of smell.

[00:14:18]Damage to any of these structures can lead lead to a loss of smell, anosmia, or a change in smell.

[00:14:27]And this has been popular in recent years cuz a lot of us experienced that with COVID. But something that's very interesting is um a blow to the head can cause a loss of smell. And what's happening here is because the brain is floating inside the cranial cavity and cerebrospinal fluid um a blow to the head means that the brain can continue to slide forward and it can drive those all factory bulbs um forward or backwards and tear the all factory neurons, the all factory sensory neurons that are passing through the cribriform plate and into the all factory bulb causing a fairly sudden loss of smell. It might be masked by inflammation or sorts of other things, but a a complete loss of smell.

[00:15:14]And for some people that will never come back. They will always be anosmic. But for some people that sense of smell will come back.

[00:15:22]And it might take years.

[00:15:24]And now that I've described the anatomy to you, you can see how that might happen.

[00:15:30]Um it's the progenitor cells, the basal cells in the olfactory epithelium, can differentiate and produce new olfactory sensory neurons that can then send their axons up through the cribriform plate. Now, neurons when they're damaged, they do try to regrow, but they often have to grow a really long way. They grow really, really slowly. They don't really know how to get there. And once they get there, the thing they were innovating is often atrophied and and and is much smaller and is broken down. But here, these neurons don't have to grow very far. And they're guided by the cribriform plate.

[00:16:11]They're guided by those little bony tubes. So, they are guided to grow back up here. And once they pass through the bone, they meet the olfactory bulb. But now you can also see the complexity of how do those neurons, those axons, find those mitral cells? How do they reconnect properly and reconfigure that that olfactory um pathway?

[00:16:36]So, you can see how And there are other things as well. Scar tissue or other damage might prevent this from happening. But you can see how a sense of smell can return after a loss of smell due to a blow to the head because of these progenitor cells, because of this bony redirection, because they don't have to go very far. But there are still some difficulties and hurdles to overcome.

[00:16:56]Isn't that interesting? I've always thought it was in the other way, but it's in in that direction. So, um that's the neuroanatomy of olfaction, generally speaking. And that's why the sense of smell can sometimes come back after that particular type of injury.

[00:17:13]I hope you found that as interesting as I did. I doubt it, um cuz I really like this sort of thing. But, uh, there you go.

[00:17:20]Fun facts to share with friends.

[00:17:22]>> [laughter] >> All right. Speak to you next week.