Understanding Human Smell and Olfaction

Your body senses can be divided up into general and special senses. General senses are senses that your body can detect that are spread throughout the body. So senses such as touch, pain, temperature, these types of things. You can feel these types of senses throughout most of your body. Special senses, on the other hand, can detect stimuli only in certain organs of the body. So you can't see with your finger, you can't see with your knee, or you can't see with your ear, right? To be able to see, the light has to be detected by your eyeball. So any type of sense that really can only be detected by a particular organ or structure in the body is known as a special sense. So we have five special senses. We have... olfaction, which is your sense of smell, gustation, which is your sense of taste, vision, hearing, and equilibrium. So let's start with olfaction. Olfaction, again, is your sense of smell. And basically the way that olfaction works is you have particles or molecules floating through the air, coming off of an object floating through the air. These smell molecules are what are known as odorants for odor or smell. So odorant molecules are floating through the air. And when you inhale through your nose, these odorants get sucked up with the air into your nose. And inside your nose are a pair of olfactory organs. And it's these olfactory organs that are able to bind to the odorant molecules and they get activated. And then they transmit signals as action potentials to your brain through a series of neurons. And the brain can process that information coming into it. And perhaps you might be able to consciously then smell those odorants or notice those odorants. So let's take a closer look at these olfactory organs. So in the picture here you see this person is trying to smell the rose. So you could imagine little odorant molecules coming off the rose and then you inhale air and that goes with the air. The odorant molecules flow with the air. into the nose into the nasal cavity. Alright, into the nasal cavity here. So in the nasal cavity we have a pair of olfactory organs. The reason we have a pair of olfactory organs is because we have a septum in your nose that divides the nasal cavity up into left and right sides. So because of that septum, we are going to have an olfactory organ on each side of that septum for a total of two olfactory organs. Now I'm going to zoom in on the olfactory organs in just a second, but right now I want to mention to you that olfactory organs are basically made up of two different layers. Now each layer contains different types of cells and structures, but there are two basic layers to an olfactory organ. The outer layer is called the olfactory epithelium. Remember an epithelium is a layer of tissue that forms coverings or linings in the body, so the outer layer is the olfactory epithelium. And then just deep to the olfactory epithelium is what is known as the lamina propria. So if we zoom in on one of these olfactory organs here, oh and by the way, that's this yellow structure that's highlighted down here, okay? Zooming in on this olfactory organ, we can see these two layers, the outer olfactory epithelium here and then the deeper lamina propria right here. Now the lamina propria contains a lot of areolar tissue. Remember areolar is a type of loose connective tissue that can hold things together in the body. But besides this areolar connective tissue, the lamina propria also contains many blood vessels for a good blood supply. It contains nerves and you can't see blood vessels in the picture here but we can see these nerves. So this yellow bundle with the blue axons inside. So here's a nerve, here's another. part of a nerve right here, right? So areolar connective tissue, various blood vessels running through here that we can't see, nerves, and then we also see the base of an olfactory gland, olfactory gland. So here's the base of it right here. The olfactory gland does extend with a duct through the olfactory epithelium to the surface, but the base of this olfactory gland is rooted in the lamina propria. So the function of the olfactory gland is to produce mucus. All right, it's a mucus producing gland. So mucus gets produced down here. and then it will travel through a duct to the surface of the olfactory epithelium right here. So this sort of tannish material that you see, this is the mucus that sort of coats the lining here of the epithelium. So the olfactory epithelium in total is going to contain olfactory sensory neurons. So that is what you see here in blue. All right, these are neurons that we call olfactory sensory neurons. Another name for them are olfactory receptors. Okay so these are the cells that are going to be able to detect, bind to, and detect the odorant molecules, the smell molecules. We also have supporting cells. So in the sort of pink color here these are support cells and then at the base of the epithelium at top, up top of the picture, okay, these are what are known as basal epithelial cells. So let's take a look at the process of smelling. So these purple little circles that you see down here, okay? These are those odorant molecules. Now in order for these neurons here to be able to bind to these odorant molecules, it works much better if they get dissolved in some type of fluid. So when we inhale these odorant molecules here, they're going to hit that mucus produced by the olfactory gland. When they hit the mucus, they sort of dissolve in the mucus a little bit better. That makes it easier for the odorants to bind to these little receptors on the surface of these neurons. Now if you remember your neuron anatomy, you will know that most neurons have dendrites. So yes, this entire blue cell is an olfactory sensory neuron, but this end, all these little branches that you see here, right? These are the dendrites of those olfactory sensory neurons. So these dendrites, these cells do have many many dendrites that are that branch all over the place and they spread throughout the mucus and because the more dendrites we have, the more receptors they'll contain to bind to odorants. So the more dendrites and more receptors you have on the cells, the more odorants you can bind and the better you can smell. Now once an odorant molecule binds to these little cell receptors on the dendrites of these olfactory sensory neurons that's going to create what we call a generator potential all right we call it a generator potential a generator potential if enough odorants bind to the these dendrites and create a big enough generator potential then that will cause this neuron to fire an action potential that will travel towards the brain okay So just because some odorants bind here does not mean that you will actually be able to trigger these neurons to fire an action potential. It does have to be a strong enough stimulus here to make that happen. And again, we call it a generator potential. That is what's created when the odorants bind. Generator potentials, if it's big enough and strong enough, that can lead to the generation of an action potential for the neuron. So if the generator potentials are large enough and... these olfactory sensory neurons do fire an action potential, then that action potential will travel down the neuron into their axons eventually. All right, now it's very fast. So these here are the axons of these olfactory sensory neurons. So these axons are so incredibly long, another name for them are nerve fibers. So when you see olfactory nerve fibers here, That is referring to these individual axons of these olfactory sensory neurons. So it's these nerve fibers or axons. These are the ones that are going to carry the action potentials from these olfactory sensory neurons towards the brain to be processed and possibly consciously smell those odorants. Now, some neurons in your body can live your entire life, however old you are. You have neurons that are... roughly that old. Other neurons in the body though die fairly quickly. So these olfactory sensory neurons are a type of neuron that they don't live very long. So maybe about two months or so is the shelf life of these olfactory sensory neurons. So we have to have, we have to be able to replenish our supply of these sensory neurons because they only live for a couple of months. So that is why we have these basal epithelial cells here in the olfactory epithelium. So these are basal means bottom, and it's kind of upside down, but these are really at the base of the epithelial layer right here. And these are basically stem cells. So these basal epithelial cells, they're just stem cells that are dividing, and when they divide, they can mature or differentiate into new olfactory sensory neurons to replace the ones that only live for a couple months. So those are the two basic layers and anatomy of the olfactory organs. Again, epithelial layer, lamina propria. Now let's take a look at what happens to those action potentials that are created by the neurons after they leave the olfactory organ. So this is what we'll call an olfactory pathway. So again, here's that same picture with a lot of the details removed so we can just sort of focus on the next part. Again, once enough odorants bind here to these neurons, and we get enough generator potentials that are strong enough to create an action potential, then those action potentials will travel down the axons or fibers of these sensory neurons. Now what will happen is we will take the axons of several different neurons and we will bundle them up together. So one of these bundles may contain maybe a couple dozen, 20 or more. different axons from different sensory neurons. And once we bundle them up, they will pass through this bone here. This is the cribriform plate of the ethmoid bone. And they will travel through there again, right? They will travel through the bone here on their way to this bulged structure right here. So these bundles of fibers, they are basically collectively, they form cranial nerve 1, which is also known as the olfactory nerve. So cranial nerve 1 is, again, it's not just this or that or that. All of these little branches together really truly form cranial nerve 1. So we say that you have on this side anyway, on this side, the left side, this is your cranial nerve one, right, the olfactory nerve. Now the olfactory nerve will then travel to and synapse with this bulge structure right here known as the olfactory bulb. Okay and this is where we get the first synapse. All right so those olfactory sensory neurons down here when they fire their action potential will travel up through the olfactory nerve and then synapse here in the olfactory bulb. The olfactory bulb is in your cerebrum, in the brain, so we didn't have far to go before we went right into the central nervous system. Now from the olfactory bulb, these action potentials will then travel through this bundle right here known as the olfactory tract. Remember the difference between a nerve and a tract is a nerve is a bundle of fibers or axons in the peripheral nervous system, whereas a tract is a bundle of axons or fibers in the central nervous system. So down in the nasal cavity, we're in the peripheral nervous system, so we call these things the olfactory nerve. But then up here, we're in the central nervous system, so we would call this the olfactory tract. Then the olfactory tract will... carry those action potentials to, you know, further in towards the central nervous system. This information will actually travel to several different places in the CNS. The main most important areas of the brain that it will go to would be the olfactory cortex, your hypothalamus, and your limbic system. All three of those places. So looking here, of course, if the if the signal is strong enough you will you will know what you're smelling. You will smell it. You will know that something is in the air. And you can also compare that with stored information in your memories, and you might be able to identify that smell as well. So that's what we usually associate with, with odorants getting into your nose. You smell something and maybe you can identify it as well. But besides this perception, being able to smell it and identification of the odorant molecule, Because the smell information through action potentials go to those other parts of the brain as well, then smells can also trigger memories and very strong emotional and behavioral responses because we're involving places other than just the olfactory cortex. We're involving hypothalamus and limbic system, which is deeply involved. in things like memories, emotions, and behavioral responses. So example, you smell freshly baked bread, and besides smelling it and identifying that it's bread, it might make you recall a memory of baking bread with your mother or your grandfather or somebody like that, right? Because again, these signals go not just to the part of the brain that can perceive it and identify it, but other areas too that are related to emotions, memories, stuff like that. Also, smelling something really bad, a really horrible smell is very revolting. You have this revulsion, like you want to get away, you're disgusted by it. Again, because these signals are traveling to emotional and behavioral centers in the brain. Now, if the smells are very strong, you're constantly being bombarded by these odorant molecules, you don't want to, you know, you don't want to just completely focus on that one smell, right? You need to be able to handle what's going on in the environment around you, even in the sense of a... even with a strong smell in the area. So what can happen is the brain can also fire signals back to this olfactory bulb right here to decrease the output of the olfactory bulb itself. So basically if you are being bombarded by a very strong smell, then the brain will respond, fire signals to the olfactory bulb to decrease the output of that olfactory bulb so the smell won't smell as strong. Now this will only apply to that one particular odorant. If another odorant enters the air and you start smelling something else, then it will start off smelling really strong until being completely exposed to the second odorant for a long time. Then you will also fire signals to the olfactory bulb, turning down the activity of the olfactory bulb for that second smell. So it depends on the particular smell, but if it's around you for long enough, Then you can sort of turn it down and tune it out a little bit while still being sensitive to other smells that might enter the air. The next thing we're going to talk about is what is called olfactory discrimination. So when we say olfactory discrimination, to discriminate means to be able to make a difference or tell a difference between two things. Usually that gets a negative connotation, but olfactory discrimination is a good thing. So let's define it a little more specifically. Olfactory discrimination is the ability to differentiate between different types of odorants and not only what they are, but how strong or how intense that smell is. So that is olfactory discrimination, being able to tell different odorants from each other and how strong each one is. So humans, we're okay, we're better than lots of other animals, not as good as some. We can discriminate roughly between two to four thousand different types of odorants, but for example, dogs can discriminate between 10,000 to 100,000 more odorants than we can, right? So, you know, we, you notice animals like, like, you know, dogs, for example, they walk around smelling everything because a lot of the way that they perceive the world is, is smell-based, right? We, we mostly rely on vision. Dogs also rely on vision, but, but they much more rely, they rely much more on smell than we do. So usually when you smell something, it's not one particular type of odorant. It's a mix of many different odorants. So the quality of food that you smell, right? Oh, this smells so great. It's that complete overall smell is really just a mix of many different odorants combining in unique combinations. Now, the threshold for being able to consciously detect an odorant, which is what we call sensitivity, that varies depending on the particular odorant. So it might take a little bit of odorant one for you. you to be able to notice it, but it might take twice as much, 10 times as much, 100 times as much of odorant 2 for you to be able to perceive it. So we say that the sensitivity to an odorant, it varies depending on the odorant, how much it takes before you can smell it. Now, as we get older, the ability, the sensitivity to odorants decreases with age. The reason this happens is because those basal stem cells, those basal cells that produce new olfactory neurons. They don't divide as quickly as we age, so the total amount of these olfactory sensory neurons, they decrease in number and sensitivity as we age. We just don't have as many as we get older, and the ones that we do have, it takes a lot more of that odorant to trigger them so we can smell that smell. So we get a decrease in both number and sensitivity to different kinds of odorants as we age.

You can feel these types of senses throughout most of your body. Special senses, on the other hand, can detect stimuli only in certain organs of the body. So you can't see with your finger, you can't see with your knee, or you can't see with your ear, right?

To be able to see, the light has to be detected by your eyeball. So any type of sense that really can only be detected by a particular organ or structure in the body is known as a special sense. So we have five special senses. We have... olfaction, which is your sense of smell, gustation, which is your sense of taste, vision, hearing, and equilibrium.

So let's start with olfaction. Olfaction, again, is your sense of smell. And basically the way that olfaction works is you have particles or molecules floating through the air, coming off of an object floating through the air.

These smell molecules are what are known as odorants for odor or smell. So odorant molecules are floating through the air. And when you inhale through your nose, these odorants get sucked up with the air into your nose. And inside your nose are a pair of olfactory organs.

And it's these olfactory organs that are able to bind to the odorant molecules and they get activated. And then they transmit signals as action potentials to your brain through a series of neurons. And the brain can process that information coming into it. And perhaps you might be able to consciously then smell those odorants or notice those odorants.

So let's take a closer look at these olfactory organs. So in the picture here you see this person is trying to smell the rose. So you could imagine little odorant molecules coming off the rose and then you inhale air and that goes with the air.

The odorant molecules flow with the air. into the nose into the nasal cavity. Alright, into the nasal cavity here. So in the nasal cavity we have a pair of olfactory organs.

The reason we have a pair of olfactory organs is because we have a septum in your nose that divides the nasal cavity up into left and right sides. So because of that septum, we are going to have an olfactory organ on each side of that septum for a total of two olfactory organs. Now I'm going to zoom in on the olfactory organs in just a second, but right now I want to mention to you that olfactory organs are basically made up of two different layers. Now each layer contains different types of cells and structures, but there are two basic layers to an olfactory organ.

The outer layer is called the olfactory epithelium. Remember an epithelium is a layer of tissue that forms coverings or linings in the body, so the outer layer is the olfactory epithelium. And then just deep to the olfactory epithelium is what is known as the lamina propria. So if we zoom in on one of these olfactory organs here, oh and by the way, that's this yellow structure that's highlighted down here, okay? Zooming in on this olfactory organ, we can see these two layers, the outer olfactory epithelium here and then the deeper lamina propria right here.

Now the lamina propria contains a lot of areolar tissue. Remember areolar is a type of loose connective tissue that can hold things together in the body. But besides this areolar connective tissue, the lamina propria also contains many blood vessels for a good blood supply.

It contains nerves and you can't see blood vessels in the picture here but we can see these nerves. So this yellow bundle with the blue axons inside. So here's a nerve, here's another.

part of a nerve right here, right? So areolar connective tissue, various blood vessels running through here that we can't see, nerves, and then we also see the base of an olfactory gland, olfactory gland. So here's the base of it right here.

The olfactory gland does extend with a duct through the olfactory epithelium to the surface, but the base of this olfactory gland is rooted in the lamina propria. So the function of the olfactory gland is to produce mucus. All right, it's a mucus producing gland. So mucus gets produced down here. and then it will travel through a duct to the surface of the olfactory epithelium right here.

So this sort of tannish material that you see, this is the mucus that sort of coats the lining here of the epithelium. So the olfactory epithelium in total is going to contain olfactory sensory neurons. So that is what you see here in blue. All right, these are neurons that we call olfactory sensory neurons.

Another name for them are olfactory receptors. Okay so these are the cells that are going to be able to detect, bind to, and detect the odorant molecules, the smell molecules. We also have supporting cells.

So in the sort of pink color here these are support cells and then at the base of the epithelium at top, up top of the picture, okay, these are what are known as basal epithelial cells. So let's take a look at the process of smelling. So these purple little circles that you see down here, okay?

These are those odorant molecules. Now in order for these neurons here to be able to bind to these odorant molecules, it works much better if they get dissolved in some type of fluid. So when we inhale these odorant molecules here, they're going to hit that mucus produced by the olfactory gland. When they hit the mucus, they sort of dissolve in the mucus a little bit better.

That makes it easier for the odorants to bind to these little receptors on the surface of these neurons. Now if you remember your neuron anatomy, you will know that most neurons have dendrites. So yes, this entire blue cell is an olfactory sensory neuron, but this end, all these little branches that you see here, right? These are the dendrites of those olfactory sensory neurons. So these dendrites, these cells do have many many dendrites that are that branch all over the place and they spread throughout the mucus and because the more dendrites we have, the more receptors they'll contain to bind to odorants.

So the more dendrites and more receptors you have on the cells, the more odorants you can bind and the better you can smell. Now once an odorant molecule binds to these little cell receptors on the dendrites of these olfactory sensory neurons that's going to create what we call a generator potential all right we call it a generator potential a generator potential if enough odorants bind to the these dendrites and create a big enough generator potential then that will cause this neuron to fire an action potential that will travel towards the brain okay So just because some odorants bind here does not mean that you will actually be able to trigger these neurons to fire an action potential. It does have to be a strong enough stimulus here to make that happen.

And again, we call it a generator potential. That is what's created when the odorants bind. Generator potentials, if it's big enough and strong enough, that can lead to the generation of an action potential for the neuron.

So if the generator potentials are large enough and... these olfactory sensory neurons do fire an action potential, then that action potential will travel down the neuron into their axons eventually. All right, now it's very fast. So these here are the axons of these olfactory sensory neurons. So these axons are so incredibly long, another name for them are nerve fibers.

So when you see olfactory nerve fibers here, That is referring to these individual axons of these olfactory sensory neurons. So it's these nerve fibers or axons. These are the ones that are going to carry the action potentials from these olfactory sensory neurons towards the brain to be processed and possibly consciously smell those odorants.

Now, some neurons in your body can live your entire life, however old you are. You have neurons that are... roughly that old.

Other neurons in the body though die fairly quickly. So these olfactory sensory neurons are a type of neuron that they don't live very long. So maybe about two months or so is the shelf life of these olfactory sensory neurons. So we have to have, we have to be able to replenish our supply of these sensory neurons because they only live for a couple of months.

So that is why we have these basal epithelial cells here in the olfactory epithelium. So these are basal means bottom, and it's kind of upside down, but these are really at the base of the epithelial layer right here. And these are basically stem cells. So these basal epithelial cells, they're just stem cells that are dividing, and when they divide, they can mature or differentiate into new olfactory sensory neurons to replace the ones that only live for a couple months. So those are the two basic layers and anatomy of the olfactory organs.

Again, epithelial layer, lamina propria. Now let's take a look at what happens to those action potentials that are created by the neurons after they leave the olfactory organ. So this is what we'll call an olfactory pathway.

So again, here's that same picture with a lot of the details removed so we can just sort of focus on the next part. Again, once enough odorants bind here to these neurons, and we get enough generator potentials that are strong enough to create an action potential, then those action potentials will travel down the axons or fibers of these sensory neurons. Now what will happen is we will take the axons of several different neurons and we will bundle them up together. So one of these bundles may contain maybe a couple dozen, 20 or more. different axons from different sensory neurons.

And once we bundle them up, they will pass through this bone here. This is the cribriform plate of the ethmoid bone. And they will travel through there again, right? They will travel through the bone here on their way to this bulged structure right here.

So these bundles of fibers, they are basically collectively, they form cranial nerve 1, which is also known as the olfactory nerve. So cranial nerve 1 is, again, it's not just this or that or that. All of these little branches together really truly form cranial nerve 1. So we say that you have on this side anyway, on this side, the left side, this is your cranial nerve one, right, the olfactory nerve.

Now the olfactory nerve will then travel to and synapse with this bulge structure right here known as the olfactory bulb. Okay and this is where we get the first synapse. All right so those olfactory sensory neurons down here when they fire their action potential will travel up through the olfactory nerve and then synapse here in the olfactory bulb.

The olfactory bulb is in your cerebrum, in the brain, so we didn't have far to go before we went right into the central nervous system. Now from the olfactory bulb, these action potentials will then travel through this bundle right here known as the olfactory tract. Remember the difference between a nerve and a tract is a nerve is a bundle of fibers or axons in the peripheral nervous system, whereas a tract is a bundle of axons or fibers in the central nervous system.

So down in the nasal cavity, we're in the peripheral nervous system, so we call these things the olfactory nerve. But then up here, we're in the central nervous system, so we would call this the olfactory tract. Then the olfactory tract will... carry those action potentials to, you know, further in towards the central nervous system. This information will actually travel to several different places in the CNS.

The main most important areas of the brain that it will go to would be the olfactory cortex, your hypothalamus, and your limbic system. All three of those places. So looking here, of course, if the if the signal is strong enough you will you will know what you're smelling.

You will smell it. You will know that something is in the air. And you can also compare that with stored information in your memories, and you might be able to identify that smell as well. So that's what we usually associate with, with odorants getting into your nose.

You smell something and maybe you can identify it as well. But besides this perception, being able to smell it and identification of the odorant molecule, Because the smell information through action potentials go to those other parts of the brain as well, then smells can also trigger memories and very strong emotional and behavioral responses because we're involving places other than just the olfactory cortex. We're involving hypothalamus and limbic system, which is deeply involved.

in things like memories, emotions, and behavioral responses. So example, you smell freshly baked bread, and besides smelling it and identifying that it's bread, it might make you recall a memory of baking bread with your mother or your grandfather or somebody like that, right? Because again, these signals go not just to the part of the brain that can perceive it and identify it, but other areas too that are related to emotions, memories, stuff like that.

Also, smelling something really bad, a really horrible smell is very revolting. You have this revulsion, like you want to get away, you're disgusted by it. Again, because these signals are traveling to emotional and behavioral centers in the brain. Now, if the smells are very strong, you're constantly being bombarded by these odorant molecules, you don't want to, you know, you don't want to just completely focus on that one smell, right? You need to be able to handle what's going on in the environment around you, even in the sense of a...

even with a strong smell in the area. So what can happen is the brain can also fire signals back to this olfactory bulb right here to decrease the output of the olfactory bulb itself. So basically if you are being bombarded by a very strong smell, then the brain will respond, fire signals to the olfactory bulb to decrease the output of that olfactory bulb so the smell won't smell as strong. Now this will only apply to that one particular odorant. If another odorant enters the air and you start smelling something else, then it will start off smelling really strong until being completely exposed to the second odorant for a long time.

Then you will also fire signals to the olfactory bulb, turning down the activity of the olfactory bulb for that second smell. So it depends on the particular smell, but if it's around you for long enough, Then you can sort of turn it down and tune it out a little bit while still being sensitive to other smells that might enter the air. The next thing we're going to talk about is what is called olfactory discrimination.

So when we say olfactory discrimination, to discriminate means to be able to make a difference or tell a difference between two things. Usually that gets a negative connotation, but olfactory discrimination is a good thing. So let's define it a little more specifically.

Olfactory discrimination is the ability to differentiate between different types of odorants and not only what they are, but how strong or how intense that smell is. So that is olfactory discrimination, being able to tell different odorants from each other and how strong each one is. So humans, we're okay, we're better than lots of other animals, not as good as some. We can discriminate roughly between two to four thousand different types of odorants, but for example, dogs can discriminate between 10,000 to 100,000 more odorants than we can, right? So, you know, we, you notice animals like, like, you know, dogs, for example, they walk around smelling everything because a lot of the way that they perceive the world is, is smell-based, right?

We, we mostly rely on vision. Dogs also rely on vision, but, but they much more rely, they rely much more on smell than we do. So usually when you smell something, it's not one particular type of odorant.

It's a mix of many different odorants. So the quality of food that you smell, right? Oh, this smells so great.

It's that complete overall smell is really just a mix of many different odorants combining in unique combinations. Now, the threshold for being able to consciously detect an odorant, which is what we call sensitivity, that varies depending on the particular odorant. So it might take a little bit of odorant one for you.

you to be able to notice it, but it might take twice as much, 10 times as much, 100 times as much of odorant 2 for you to be able to perceive it. So we say that the sensitivity to an odorant, it varies depending on the odorant, how much it takes before you can smell it. Now, as we get older, the ability, the sensitivity to odorants decreases with age. The reason this happens is because those basal stem cells, those basal cells that produce new olfactory neurons. They don't divide as quickly as we age, so the total amount of these olfactory sensory neurons, they decrease in number and sensitivity as we age.

We just don't have as many as we get older, and the ones that we do have, it takes a lot more of that odorant to trigger them so we can smell that smell. So we get a decrease in both number and sensitivity to different kinds of odorants as we age.

Transcript for:Understanding Human Smell and Olfaction

Transcript for:
Understanding Human Smell and Olfaction