The Science of Everything Podcast - Episode 59: Smell

Episode Date: February 18, 2014

A analysis of the sense of smell, beginning with the nose, the olfactory epithelium and the different types of receptor proteins, and progressing through the processes of sensory transduction, transmission of neural signals to the olfactory bulb, and thence into the higher regions of the brain. I also discuss the sense of smell in humans compared to that of other animals, with a focus on the special abilities of bloodhounds. Recommended prerequisite is Episode 38: Neurons and Synapses.

We're all listening to The Science of Everything podcast, episode 59, smell. And I'm your host, James Fodor. Recommended pre-listening for this episode is episode 38, neurons, and synapses. Also, if you've listened to the previous episode on Taste, that will help a little bit, but that's not really very essential. But you do need to have some basic understanding of synapses and synaptic processing and action potentials and things like that if you really to understand this episode. So in this episode, we're going to look at the sense of smell,

more properly known as olfaction. I'll talk about the structures in the nasal cavity responsible for the transduction or the reception and transduction of olfactory sensation. I'll talk about the receptor proteins, the olfactory bulb, the olfactory epithelium, those structures there. I'll talk about how the signals are transmitted to the brain and some of the pathways involved there. I'll also say a little bit about smell in animals and how that compares to humans. All right, so let's get started.

Smell, like taste, is a type of chemical receptor. That means that the sensory apparatus operates by binding to specific chemicals. This is distinct from, say, the vision in which sensory transduction occurs by cells that are responsive to photons, light, or other mechanoreceptors, which are receptive to motion. Smell is a type of chemo reception in which the sensory molecules are receptive to particular chemicals. The chemicals themselves are called odourants. If you remember from the previous episode, the molecules that were detected by the taste buds were called tastens. So in this episode we have odourants. Although taste and smell are separate sensory systems in land animals, they are closely related to each other, as we'll see a bit later.

And, in fact, many water-dwelling organisms often have a single combined chemical sense, which is sort of like taste and smell in combination. Okay, so let's start by talking about the olfactory epithelium, and don't worry, I'm not supposing you already know what that is, I'm going to explain. But smell does not actually occur in the nose. This is probably the single most important thing to understand. The thing that externally we see is the nose, you know, with nostrils and all that, that has nothing really to do with smell. The only thing that that does in regards to smell is bring the air into the nasal cavity. Apart from that, the nose does nothing. Smell actually occurs, or the detection of odourants, actually occurs in a small sheet of cells at the top of the nasal cavity, which is called the olfactory epithelium.

Epithelium just means basically an external or outside layer of cells that's sort of like a thin sheet. So skin, for example, epithelial cells. In this case, the olfactory epithelium is comprised of specialized olfactory neurons, which do the sensory transduction, which means they actually detect the odorants and parts of. on the signals to ultimately see the brain. There are also some other cells that comprise the epithelium, apart from the olfactory neurons. So one example of these are the basal cells. Basal cells are stem cells,

which means they divide to give rise to other cells, so they are capable of division and differentiation. And the basal cells constantly replenish the cells of the olfactory epithelium, replacing them every few weeks. And in fact, the olfactory neurodemeanor, The neuron cells, thanks to the basal cells, are among the only neurons in an adult human, which are capable of regenerating. And so there's not exactly understood why this occurs in the olfactory of therium and not anywhere else,

or not in very many other places in the brain. But it's certainly very interesting because it might shed some light on how we can take advantage of this process to assist in treating neurodegenerative diseases, like Alzheimer's, for example, if we were able to grow neurons in other parts of the nervous system, that would be a huge breakthrough. But in the olfactory epithelium, it's possible thanks to the basal cells, which are constantly dividing. It also means that if the olfactory epithelium is damaged by inhalation, for example, of toxic fumes or physical injury, or certain types of nasal sprays have actually been known to cause damage to the olfactory epithelum, generally it's capable

of regenerating within a few weeks. So usually the damage is only temporary, although in extreme cases it can be permanent. The complete loss of smell is known as a nosemere, but as I said, it's often temporary as the neurons are capable of regenerating. In humans, the olfactory epithelium, again, is a segment of tissue, which is about 10 square centimeters in area. There's actually two olfactory epithelia, one sort of on each side of the nasal cavity, the bilateral structure. Other animals have much larger olfactory epithelia. So, for example, some dogs have 170 square centimeters, according to my sources, of olfactory epithelial. which is 17 times as much as humans have.

And so therefore that provides much greater sensitivity to olfaction. The olfactory cell, so the neurons on the surface of the epithelium, are covered by a layer of mucus, which might sound disgusting, but it's actually vital because the molecules from the air that we breathe in through our nose, and also from air that comes up from the mouth via the pharynx through the back of the throat, the molecules that are in that air dissolve in the mucus, which is, again, coats the epithelium,

and then from the mucus, the olfactory neuron cells are able to bind to the dissolved odorsance, and thereby detecting them. So if it weren't for that mucus, we wouldn't really be able to smell very well. It's very similar to how in the mouth it's necessary first to dissolve food in saliva, and then that allows it to be the molecules to be detected by the taste buds. Similarly in the nose, you first have to dissolve the molecules in the mucus that covers the epithelium. The mucus also contains a variety of other substances, including enzymes and antibodies. The antibodies are particularly important because they help to protect the brain from infection,

so they act to counter-bacteria and other things like that. And it's actually particularly important for the nasal cavity because the nose cavity because the nose actually provides direct access to the brain, as we'll see in a moment. And so it's particularly important to protect that from pathogens. So that's the olfactory epithelium. Now let's move on and we'll zoom in a bit in a sense and talk about the receptor proteins themselves. So how do the olfactory neurons that are on the outer surface of the epithelium? How do they actually detect the odorant molecules? We haven't actually described that yet, so let's do so now. Each receptor cell has small cytoplasmic projections from its dendrite.

So it only has one dendrite, but it sort of divides up into a bunch of small projections that are called cilia. And they are covered by the mucus. So the very outer layer of the olfactory epithelium, the layer at the very edge of the nasal cavity, at the top of the nasal cavity, that is the same layer that is covered with the nucleus. The most outer part of that layer is literally the cilia of the dendrites

of the olfactory neurons. And on those cilia are the receptor proteins that are able to bind to odorants. Each individual receptor cell, so each neuron expresses a single what's thought to be unique

odorant receptor. So that is each neuron has more, I mean, has more than one receptor on it, but only one type of receptor, one specific type. And humans are thought to have about 350 of these different types of receptor proteins in total. Rodents have about 1,000. Humans seem to have the remnant or pseudogenes of many of these older, evolutionarily older, olfactory receptor proteins,

but they seem to have become non-functional. So if we could find a way of reactivating those genes, we would potentially be able to smell a lot more different substances than we're able to now. But anyway, and these 350 different genes that code for these proteins are scattered all across the genome. It forms one of the largest gene groupings that we know of all of these different genes for the receptor proteins. because, you know, for each different protein that you have expressed, you need a unique gene for that. So each of these 350 different receptor proteins has to have its own gene that corresponds to it. It's important to understand, though, quite different to, say, vision and taste, where there's a fairly distinct mapping between types of, let's say, wavelengths of light or types of taste molecules, and the receptors that detect them, that's not the case in olfaction. In olfaction, there are, sure, there are 350 different types of proteins, each expressed on a different cell, more than one cell, of course.

But most of those proteins are able to detect a wide variety of different odorant molecules. So, you know, detect odorant receptor protein number one might be responsive to hundreds of different odorant molecules, and receptor two might be responsive to hundreds of different odorant molecules, and also some of the same ones as as receptor 1. So there's considerable overlap as well. So it's not the case that one receptor only detects one type of odorant,

or even that it only detects 10 types of odorses, and then the second one detects a different 10, and the third one a different 10. No, there's lots of overlap. Many of them detect the same odorant, but then some different ones as well. And of course, we don't know fully exactly what types of odors each of them detect, and there seems to be some individual variability and perhaps genetic variability in that as well.

One interesting thing, though, is that the receptor cells are organized across the olfactory epithelium so that cells that express receptor proteins of a particular family are typically found together. So there are 350 different receptor proteins, but they can be grouped into different classes. The most common classification I found was having three different classes. And it turns out that if you map where these are located over the olfactory epithelium, you tend to see that they cluster. So most of the neurons in this region all have type A, class A type receptor proteins, whereas in this region over here in a different part of the epithelium have class B type receptor proteins on their surfaces. So there's a topological mapping it's called, which is interesting. We'll talk a little bit more about later on.

But although I've talked about the receptor proteins, I still haven't explained exactly how they transmit a signal when they detect an odorant molecule. Well, it's quite similar in conceptually actually to how it works in taste. So basically all that happens is the receptor proteins on the membrane of the olfactory receptor neurons bind to an odorant molecule. So the odorant molecule is drifting around. It's dissolved in the mucus. And periodically, it will come into contact with a receptor protein. Now, if the receptor protein is of the right type, the receptor protein will be able to bind to the odorant molecule.

So literally that means they form chemical bonds. with each other and maybe change shape a little bit. And this binding leads to a sequence of reactions, which we didn't get into the details of, a secondary messenger cascade, it's called. Again, this is very similar to what operates in taste. So basically, a sequence of chemical reactions is triggered in the cell, which ultimately leads to ion channels opening up,

causing an influx of cations, particularly calcium cations, so positive ions, and sodium cations coming into the cell. That thereby depolarizes the cell, and triggers an action potential, so then the cell fires an action potential out of its axon. So as long as, well, in a simplistic sense, you might think that as long as the odorant molecule was present, the neuron would keep firing those action potentials because it would continuously be stimulated. That happens to some extent. There is some correlation, for example, between concentration of odorants and the number of action potentials fired.

But in general, the olfactory response only lasts for, the action potentials only fired for a certain period of time. In other words, the olfactory response doesn't last forever. It terminates after a certain time. One reason that can happen is if the odorants diffuse away. They're binding with the receptor proteins is broken for whatever reason, perhaps just through thermal motion, and then the odorant diffuses away, and then there's no more signal to transmit, therefore the action potential cease. Or sometimes the odorant molecules are broken down by enzymes in the mucus. If you remember, I said that the mucus is filled with lots of enzymes and other chemicals. They can break down the odourants after a time, or also the receptor cell can simply undergo adaptation,

which is a general process by which a neuron ceases to respond to a stimulus after prolonged exposure. This is not specific to olfactory cells. It's a widespread phenomenon in pretty much all neurons that if you see something for long enough, or if you stimulate a muscle for long enough, or if you taste something for long enough, pretty much anything like that, hear something for long enough, the number of action potentials that are fired as a result of that will diminish. In other words, the response to a stimulus diminishes over time. It's basically habituation. It's a similar idea. So that happens in the olfactory system as well. It's essentially the same reason. I believe it's called olfactory habituation or something like that is the general phenomenon

by which olfactory adaptation is also called, by which you go into a room and you notice a smell and it's very distinctive, but within a few minutes you don't notice it anymore because you've become adapted to it. You're essentially the particular receptor proteins that bind to the odourants that generate that smell have been saturated and have been firing for so long that those receptor cells have become habituated to that stimulus, and so you don't

really detect it anymore, unless you leave the room and then come back later, and then you can smell it again. Now, at this stage, I'd like to pause in the narrative a little bit, although it's not really too much of attention, but I'd like to pause a little bit and consider a question from one of my listeners which I'd like to address,

and this is a question from Bill regarding smell. He asked, Well, he says, he's always been curious about where odour comes from and how you can smell things, and hopefully I'm addressing that now. But, for example, he says specifically, for example, feces are quite smelly, and it seems like the smell must emanate from the feces. When you smell feces, is some sort of fecal matter actually entering my body through the nose? So that's a very good question. And hopefully you can tell by now, if you've been listening carefully, that the answer is, yes, there's no possible way that you could detect the smell if there wasn't some something, at least, that was entering your body and being detected.

So when you see feces or anything, photons that reflected off the surface of the feces enter your retina and are detected, if you hear the feces, and I don't know why it's making sound, but if you are for some reason, then what you're hearing are vibrations in the air ultimately produced in the vicinity of the feces,

and more relevant to our purposes, if you smell them, what you are detecting are odorant, molecules which originated in the feces and have wafted through the air and are passed through your nasal passages or possibly through your mouth and are detected by the relevant receptors that combine to those type of odorant molecules in the olfactory epithelium. So yes, fecal matter is actually entering your body. So that question then leads to a second question in which he says, so it seems that if 800 people were nearby and smelled the same odor, wouldn't that mean that the object was actively shedding mass, even if it was infinitesimal. And so if you kept smelling

an object, wouldn't you build up some quantity of that object in your system? And is that why, if you avoid smelling toxic substances for any substantial amount of time? So again, the answer here is effectively yes. So it's obviously clear that an object that you can smell is actively shedding mass, although the word actively is a bit misleading, but we'll come back to that. Yes, it's clearly shedding mass because there's no other way that you could detect molecules from it unless the molecules came from it and it therefore reduced in mass. However, there's an important point to make here that, as Bill alludes to, the amount of mass that an object like this would be shedding is really infinitesimal.

And let me try and illustrate that. So in the human nasal cavity, we have about 6 million or so. olfactory sensory neurons. And I don't know how many receptor proteins there are in each of those, but I would find it unlikely that there would be more than a few hundred, maybe a few thousand, I don't know, maybe tens of thousands, although that would probably be high. But certainly, certainly not that many. So perhaps we have billions at the most of sensory proteins, so that's individual protein molecules that can detect the odorants in our nose, or in our, I should say in our olfactory epithelium. Now contrast that to how many molecules,

there are in even a fairly small amount of a given substance. So if you remember the concept of the mole, that's M-O-L-E, from various chemistry episodes that we've done in the past, a mole is an amount of substance used in chemistry to talk about chemical quantities. A mole of salt or something like that roughly corresponds to sort of a palmful of something. So it's sort of a moderate, macroscopic amount. So a mole is roughly a palmful. So if you had a palmful of, I don't know, spice or salt or bread or whatever in your hand, there would be one mole of that substance, roughly, which is 6.02 by 10 to the 23, or in a way that may be a bit more meaningful, that's about a trillion trillion. So take a trillion, then times that by a trillion, you get roughly a mole.

So if you have a macroscopic, you know, a small palmful of a substance, you've got about a trillion trillion molecules of that substance. Even if every single one of the receptor proteins in your nasal cavity were bound to an odent molecule from that substance, you would only have maybe a billion of those molecules in your nasal cavity, and of course in practice you would never have anything close to that number. But even if you had a billion of them in your nose, that's still a minuscule of a trillion trillion. Remember, a trillion is a thousand times a billion,

and then times that by a trillion again. A billion is so infinitesically small in comparison to a trillion, and that it's just mind-bogglingly small in effect. So basically, the amount of actual substance that you get in your nose by smelling it, you know, in a normal circumstance, I'm not talking about if you actually rub it in your nose, but in a normal circumstance of smelling it is just very, very small, and it's tiny compared to the mass of most substances that you will be smelling.

Although it is true that humans, and especially some other animals, are capable of sensing very low concentrations. I think humans can detect one part in a trillion of, in terms of concentration in the air of some types of odorant molecules. So our nose is very sensitive. A single receptor detects one odorant molecule, and we don't need very many odorant molecules to be detected in order to distinguish a smell.

However, the flip side of that is we don't need very much of the substance to distinguish the smell, so very little of the substance. It's not in the case that much of the substance need enter our nose or anything like that. However, Bill did ask the question, wouldn't you build up some quantity of that subject in your sense? system, of that object in your system, well, as we said, not necessarily because the amount you breathe in is probably very small. It's certainly, the fact that you're smelling it doesn't necessarily mean that you, that very much of it is entering your system. It could be

the case that a lot of it is entering your system if the concentration happened to be very high, but that need to be the case just because you're smelling it. Again, we can detect very low concentrations of some things. The second part of his question, if you recall, is that why you should avoid smelling toxic substances for any substantial amount of time? Basically, yes, although there's more to it than that. So if you're smell, a toxic substance, it's probably coming into contact with your skin as well. Some toxic substances can diffuse through the skin, and that's obviously not desirable. And also, if you're smelling a substance, there's a fairly high risk that it could get in through your mouth, or even

through your eyes. Again, that's a risk that you don't want to take. So basically, the reason you should, I would say, the reason you should generally avoid smelling toxic substances is just because you don't want to get a toxic substance that close access to your body. And how have the risk of it accidentally or inadvertently entering through the skin or nose or eyes or anywhere else or mouth. I think the risk of taking in a toxic dose of anything purely through the nose is probably fairly small for most things, but I'm sure there are exceptions you can find to that. And it is certainly the case, as we mentioned before, that, for example, infectious agents,

viruses and bacteria can enter through the nose. That's why we have antibodies that are found there and the mucus. it's definitely the case that you want to be careful about what you sniff or what you put too close to your nose. Okay, so enough on that little digression. Hopefully that sort of thing was interesting. And parenthetically, if other people have specific questions like that, by all means send them to me. I've asked this many times before, but not too many people have asked specific questions like that. And I'd like to include more of that in episodes if you find it interesting.

So please keep those questions coming. Okay, so let me now talk about the olfactory bulb. The olfactory bulb is actually a projection of the brain. It's literally part of the brain. And it projects out sort of from the middle, lower region of the brain. It sits over the top of the nasal cavity, but projects out from the brain. So it's under the brain but above the nasal cavity. And it's important because the olfactory sensory neurons that project their axons back from the olfactory epithelium, remember,

these neurons project their axons into the olfactory nerve. So you've got neurons that detect the signals from the nasal cavity, and they project their axons back into the olfactory bulb. That's directly where the signals are taken. What happens to the information there? Well, in the olfactory bulb, the sensory neurons synapse with interneurons or secondary receptor neurons that are found there, but they do so in a particular way. It's not random. There's a particular mapping that's found.

In fact, there are sort of spherical clumps of cells, in the olfactory bulb that are called, each of which is called a glomerilus, glomerulus, and within each of each glomerilus, about 25,000 different primary olfactory axons terminate in that glomerulus, and they in turn synaps with about 100 second order olfactory neurons, which then carry the signals deeper into the brain. So just to try and make that a bit clearer, we have the olfactory bulb, which is a projection of the brain.

Within the olfactory bulb, there are a bunch, I think several hundred or maybe a few thousand. I think there are a couple of thousand, actually, of these spherical structures called glomeruli. Within each glomerulus, there are a bunch of primary olfactory axons terminate there, and a bunch of different secondary olfactory neurons take their input, so they have their dendrites there, and then project their axons out into the brain. But there is more input than output. So each of the, each glomerulus has about 25,000 cells going, axons going in, but only about 100 second order olfactory neurons coming out. So there's a centralisation, if you like, of information.

There's like lots of small roads running in, and only a few big roads running out, if you want to think of it sort of that way. And within itchlamarialists and also within the olfactory bulb as a whole, there's a fairly complex internal network and circuitry structure. So there are interneurons connecting different ways. There's back propagations, and there's inhibition between nearby neurons.

There's also some projections that come back from higher regions of the brain, and we don't exactly know what they're for. And we don't fully understand the complex circuit structure here and exactly what processing is occurring, but it is thought that the first sort of lowest levels or first stages of olfactory processing are happening here, even as early as the olfactory bulb, because of this complex circuit structure. We also do know that each lemurilis receives input primarily from olfactory receptor neurons of a particular type. So if you remember before, I said that groups of neurons in different. regions of the olfactory epithelium tend to have receptor proteins from similar classes.

So they're sort of related and they, different families of the receptor classes, sitting different regions of the tongue, so there's a topological map. Well, it's similar to that in the olfactory bulb, except it's actually more specific. Each glomerulus seems to have a single specific type of olfactory receptor protein that it responds to. So there are, remember, 350 of those different types of proteins in humans, and so there's a few thousand glomeruli. There seems to be a fairly direct mapping there. And this type of mapping is called a chemotopic mapping. So in other words, we have different chemicals mapping to a specific region of the olfactory bulb, but specific glomerulus receives input from all of the different neurons that detect that

particular type of chemical. And it's thought that that's important for the processing of smell, or at least the first part of that process. It's not only that the types of cells that are firing that's been important, it's also thought that there is at least some aspect of temporal coding that's important, so the rate at which different neurons fire action potentials may also feed into how we perceive smells. Now, from the olfactory bulb, neurons project back into a large number of higher brain regions. I won't list all of these here. It's actually quite a complicated story, much more complicated than for vision or for taste, where mostly there's just a couple of pathways.

Here there are many. Basically, the broad pattern, though, is that the neural neurons from the olfactory bulb synaps with a sort of lower region of the cortex, which is called the olfactory cortex. And from there, there are projections which run, there's one sort of set of projections which runs into the thalamus and thence into the orbital frontal cortex. And that is thought to be what the pathway, they're just responsible for the conscious perception of smells, because the orbiter frontal cortex is the region that lights up, you know, in fMRI and and PET scans and such when we get people to smell things. And that's, you know, a region of the near the frontal cortex, which is responsible for conscious thought and things like that.

So that's all consistent with the idea that this region of the cortex is what's responsible for the conscious perception of smell. But that's by far from the only place in the brain that olfactory information is taken. Remember I said that pretty much all or many of the neurons from the of the olfactory bulb project to the olfactory cortex, and from there some of them go to the thalamus, but where do the rest of them go? Well, some of them, some other of the axons, project to other regions of the brain, for example, the hippocampus and the amygdala, which are sort of lower regions, part of the what's sometimes called the limbic system.

It smells quite unique in this sense, because it seems to have a much more direct association with some of the emotional centers of the brain, like the amygdala and other structures. of the limbic system. And so there is hypothesized to be an evolutionarily very old or sort of basic fundamental relationship between smell and emotions, and also emotional memories in particular, because the hippocampus is responsible for a long-term memory formation, or some aspect of it anyway, and there seem to be fairly direct projections from the olfracture cortex to the hippocampus. So we don't fully understand how all those relationships work, but it does seem to be, the information we have at the moment seems to be indicative of a fairly strong relationship there between emotion and fairly basic motivational structures and smell.

And certainly we observe this in many animals where smell forms a very important part of behavioral motivation and can shape animals' behavior in many different ways. That's somewhat less important in humans, although there are some disputes there. I won't get into the whole issue of pheromones, which basically is the system related to olfaction, but it's a little bit different. It's basically a form of chemoreception which leads directly to changes in animal behavior. I might talk about that in a future episode, but I'll just say that it's not established that humans respond to pheromones at all.

I tend to think that the evidence is fairly much against it, although there's some quivocal evidence that they do. But whether we respond to pheromones or not, it's fairly clear that humans at least respond to smell, and that can form an important motivator. But pheromones and smell aren't the same thing, So don't get confused there. They are similar, but not the same.

Okay, a quick point about signal processing, or how we actually extract information about smell from all of these neural signals that are flowing around these different parts of the brain. You might wonder how it's possible for us to smell so many different unique sense when, in fact, we only have 350 different types of receptors. Well, the answer is that it's thought

that this is possible through what's called population coding. We talked about this in the episode on Taste. Population coding is basically, a method of storing information in which, processing information in which information about what is being smelled is not stored, it's not contained in the activity or action potentials from a

single neuron, or a single, even a single bunch of neurons, but it's broadly distributed across a wide group or population of neurons. And so particular patterns of activity will correspond to different tastes. And so you can have many different types of tastes

emerging from only a fairly small number of basic receptors. In fact, this is precisely what happens with taste. We only have five basic types of taste receptors, and the activation of these five in lots of different combinations, and to differing degrees, contribute to the vast and complex repertoire of different tastes that we can experience. Well, it's similar with smell, except that we have an even larger number of basics receptors

to work with in the case of smell. So it's this sort of population coding that we think is what's responsible for a human's ability to perceive many different types of tastes. As I mentioned before, it's also thought that temporal coatings, so the rate at which action potentials are firing may also play a role in distinguishing some odors. Okay, so that essentially brings us to an end of what I wanted to say about the neural and physiological structures of olfaction. I just wanted to say a little bit about smell in animals before we close, because I just found this information, and I thought it was quite interesting. So, as I've said before,

many animals have a much keenest sense of smell than humans do. Humans have, broadly speaking, humans have very good sense of vision, particularly color vision, compared to most animals, but a fairly poor sense of smell compared to most animals. Most mammals in particular have quite good sense of smell, humans being a sort of relative exception, whereas most birds do not have a very good sense of smell. A lot of fish also have a fairly good sense of smell, and many fish also, or water-dwelling animals in general, have a combined taste and smell, chemoreceptic sense, as I mentioned earlier. dogs in particular have quite a good sense of smell

it's estimated that in general dogs have an olfactory sense about 100,000 times as acute as a human's you may have heard figures like this before it's very important to understand what they mean the way they test this is basically by repeatedly reducing the concentration of an odorant molecule and seeing if the animal or the human can detect it

so you present them with a high concentration and then reduce it a bit maybe by factor of two or a factor of ten see if they can still detect it if they can you reduce it more and you reduce it more, until you reach the stage where it's too dilute for them to detect the urgent molecule. There's not enough of it left to register. And that's how sensitive the organism is said to be to that particular type of smell. And so in doing experiments like this, dogs, on average, come out as being able to detect concentrations 100,000 times lower than humans are able to detect. That doesn't mean that they smell everything 100,000 times as intensely as a human

does. So it's not like chocolate smells 100,000 times more like chocolate or more chocolatey than to a dog than it does to a human. That's, or feces smell 100,000 times worse to a dog than a human. That's not a good way of thinking about it. Probably these things smell, well, they wouldn't smell exactly the same. But I suspect the intensity of something like that is not dramatically different. It's just that they can detect much, much lower concentrations than a human can. So that's dogs in general, but there are a type of dogs that are specifically bred for having a particularly acute sense of smell. The general term for this is scent hounds, and the most famous example are bloodhounds, which were developed in Britain, in the early modern period. There's a little bit of controversy exactly how, what their origin was.

But anyway, they've been around for a few hundred years, and they have the keenest sense of smell of any dogs, as far as I know. They have noticed that are some 10 to 100 times, sorry, 10 to 100 million times more sensitive than humans. So that's about 100 times more sensitive even than most dogs. again, this doesn't mean that every smell is ridiculously overpowering to them, it means they can detect much lower concentrations of things that humans can. They've been known to be able to detect trails of peoples from only a few cells. So that means a very, very small number of molecules are actually entering their nose there. So that's very impressive.

And that's essentially why they were bred. They were bred basically to be hunting dogs, and now they're used in police investigations and a variety of other things. And they're specially trained to be able to detect and follow a train. trail sense. There are some amazing reports of the feats that they're capable of doing, like, of following a sense that are trails that are several days old, even across water and vast distances and other things like that. So it's their very dense concentration of olfactory receptor neurons that is, well, that plus their very strict training, which allows them to do this. A bloodhound's nasal chambers are much larger than those of most other breeds, and they contain, they have about 4 billion receptor cells. which, if you remember, is about a thousand times more than a humans you have around 5 or 6 million. So a much, much larger number of receptor molecules, which allows them to be so good at following these types of scent trails.

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