A Sense of Smell

Author: Varun Raju, Learner, Age 14 years.

This is a self-learning project chosen and researched by the learner.

Note – I wanted to learn about something in our body, because that is what we use every day. We use our five senses each day, so I thought it would be interesting to learn about one of them. I chose the sense of smell because I was interested in knowing how we smell and what really happens in our nose. After doing a project on sound, I was fascinated by the way our body works. I was very eager to learn about how our body handles our sense of smell.

My main objectives were to understand:

  1. What are smells are made up of
  2. How does our nose help us smell
  3. How does our brain help us perceive smells
  4. Why do animals have a better sense of smell

After the completion of this project, I wanted to be clear with all the concepts that I wanted to learn about.

Table of contents

What is smell?

What are the structures through which smells pass through?

How do we smell?

The brain – why are smells associated with memory and emotion

How does the sense of smell affect other aspects of the human body?

Why do most animals have a better sense of smell than humans?  

 

Smell is one of the five senses of our body that we take for granted.  We smell things every day, each of them making us react in different ways. Smell is a chemical sense that is mediated by sensory cells called chemoreceptors. Chemoreceptors are receptors that are sensitive to chemicals. Even taste receptors are chemoreceptors, as they detect chemicals too. The senses of hearing and touch use mechanoreceptors, which are sensitive to mechanical pressure. The sense of sight uses photoreceptors which are sensitive to light.

 

Sight Sound Touch Taste Smell
Photo-

receptors

Mechano-

receptors

Mechano-

receptors

Chemo-

receptors

Chemo-

receptors

 

Smells are often associated with memory and emotion, because when you smell something, you get reminded of a thing or you might be disgusted or pleased by the smell. Smell is a very important sense of our body, and helps us be more aware of our surroundings. The human sense of smell is also called olfaction, and the structures through which scents travel through in our body is called the olfactory system. When we talk about our nose, we are all reminded of our sense of smell. In fact, around only 5 percent of our nose is devoted to our sense of smell. The other 95 percent or so is devoted to warming up air we breathe in and filtering it, making it suitable for our respiratory system.

What is smell?

When we think of smell, the first thought that comes to our mind is “what is smell?” We talk about smells every day. The smell of great food, the smell of rain or sometimes the smell of something terrible, like rotten eggs. But what gives everything their specific scent? Why does everything smell different? To understand this, we have to know what smells actually are.

Smells are a mixture of chemicals that are released by certain substances. These chemical compounds can be called odour molecules. When certain chemicals bind, they form a specific smell. The combination of these chemical compounds determine the scent. We all know that coffee has a distinct odour. The number of chemical compounds in coffee is over 800 (as discovered by scientists). This means 800 chemical compounds combined give coffee its unique smell. But not all kinds of coffees smell the same. In another type of coffee, there will be an addition or removal of a chemical compound, making it a different odour. Any addition or removal of any odour molecule can result in a completely different smell. In short, smells are different chemical compounds bound together. Our nose doesn’t directly recognize the smell of coffee. It recognizes the chemical components first. The brain processes the combination of chemical components and then recognizes the coffee odour. We will get into details of our nose and brain later. Here is a table that shows a few of the 800 odd aroma compounds found in coffee.

 

Aroma compound Smell
Methanethiol Rotten cabbage
Furaneol Sweet, caramel
Acetaldehyde Pungent, fruity
Guaiacol Smoky, spicy
Methylpropanal Floral, spicy
2-Furfurylthiol Roasted (coffee)

Different kinds of flowers have very distinct smells. See an image of the different aroma compounds in various flowers here

Smell molecules can be released only by a substance that is volatile. A volatile substance is anything with molecules that can be evaporated. The source of a smell has to be volatile because the smell molecules have to rise from it and mix with the air. Pure steel doesn’t have smell because it is a non-volatile compound and its molecules cannot rise. The strength of a smell depends upon its ability to evaporate. Freshly baked cakes right out of the oven has a much stronger smell than cold cakes. This is because when the cake is just baked, it is hot, and the heat causes the molecules to evaporate quicker. As it cools, its ability to rise into the air decreases, and so less molecules are released.

Structures through which smells pass through

But have we ever thought about what lies inside and beyond our nose?  All we know is that smells go into our nose. But what happens after that? Like all the other systems in our body, the sense of smell is a very complex design. Let us go a little deeper inside our nose, and explore the structures through which these smells travel through.

After a particular smell has been released and mixed with the air, it travels through the nose. This happens when we inhale. When we breathe in air, the mixed up molecules go inside our nose too. Just after the nostrils, the surface of the nose is lined with tiny hairs called cilia, and mucus. The mucus and cilia help filter the air. Some odour molecules may get stuck to the mucus or cilia, but most make it to the back of the nose. There is an open area behind the nose called the nasal cavity. On the top of the nasal cavity, behind the nose lies a small area, the size of a postage stamp (around a square inch) called the olfactory epithelium. You can see an image that shows the nasal cavity (the open area behind the nose), the connection from behind the mouth to the nasal cavity and the location of the olfactory epithelium (in yellow) here.

The surface of the olfactory epithelium is covered with a layer of mucus. This mucus helps catch odour molecules, but also prevent the olfactory epithelium from getting damaged. The mucus is produced by glands in the olfactory epithelium called Bowman’s glands. Inside the mucus are tiny hairs called cilia. There are thousands of these hairs, each around the size of a tenth (or even less) of a millimeter. The mucus layer has to be of perfect thickness. If it is too thick, it would prevent odour molecules from reaching the epithelium (making our sense of smell weak) and if it is too thin it would leave the cilia vulnerable to other particles in the nasal cavity. The cilia are one of the most important structures of the olfactory epithelium, because the olfactory receptors are bound to them. These cilia are attached to nerve cells called olfactory sensory cells. The cilia are projections from these cells. There are hundreds of these long cells in the epithelium that send projections down into the mucus. Each sensory cell houses an olfactory receptor neuron. This neuron is responsible for converting mechanical signals (caused by odour molecules binding to receptors) to electric signals. All these sensory cells are supported by other cells in the epithelium called sustentacular cells (supporting cells) and basal cells.

The sensory cells have projections that go through a thin bone structure above them called the cribriform plate. The thin projections of the sensory cells are called axons, a type of nerve cell (a cell responsible for communicating with other parts of the human body through electrical signals). The cribriform plate has many tiny holes, which let the axons go through it. Signals are passed on through these thread-like cells. The axons go into a structure above the cribriform plate called the olfactory bulb. Certain axons meet in a round ball of nerves called a glomerulus. A glomerulus is the destination point for every smell. There are many glomeruli in the olfactory bulb that house specific axons. A single nerve extends from each glomeruli. Another cell called a mitral cell, receives the information from a glomerulus and finally sends a signal to the brain through a single nerve or axon. The bundle of nerve projections into the brain is called the olfactory tract. These nerves transfer information across several parts of the brain, which we will get into detail later. The brain then processes this information and causes us to react in a specific way.

This image shows the olfactory epithelium and olfactory bulb. The area which has axons going through it is the cribriform plate and the area above which houses the glomeruli and mitral cells is the olfactory bulb.

How do we smell?

But how do we actually smell? What are the functions of each structure? Every structure in the olfactory epithelium, olfactory bulb and brain plays an important role in our sense of smell. Let us go a little deeper into how our sense of smell works, and how the brain plays a role in perceiving smells.

To understand the process of olfaction, let us take an example that we will use throughout this explanation. Smells always start from their source. Let us say the source is pizza. Normally, when we inhale, we know that we are smelling pizza. But now let us see how we recognize the smell of pizzas, and why it makes our mouths water.

A pizza is made up of many ingredients. The three main ingredients are flour, cheese and tomato. Each of these three ingredients has their own smell. The scent of tomato is made up of a certain combination of chemicals. Even flour and cheese have their own unique pattern of chemicals. When we mix these ingredients, we get the combined smell of tomato, flour and cheese, which we call the smell of pizza. Let us call the smell of pizza odour 1(O-1) which is made up of chemical 5, chemical 19 and chemical 3021 (C-5, C-19, and C-3021). In reality the scent of pizza may carry many more chemicals, each chemical having their own name. Instead of using the scientific name of each chemical, we will just be calling them C-1 for example, just to make things simpler. So the mixture of C-5, C-19 and C-3021 mix up with the air. When we inhale, these chemicals flow into the nose with the air. When they reach the olfactory epithelium, they stick to the mucus, and move through it until they reach the cilia. There are thousands of cilia. There are tiny receptors on the cilia. These all-important receptors are called olfactory receptors. A receptor is any substance that gets stimulated by something, in this case, chemicals.

The olfactory receptor is also called a G-protein coupled receptor because a specific protein called G-protein is bound to it. When a molecule binds to a receptor, it activates the G-protein and causes it to break away. It is this structural change that causes the receptor to send a signal to the brain. Every receptor is sensitive to one particular chemical. When the right chemical binds to its receptor, the receptor sends a signal to the olfactory receptor neuron, which sends an electric signal to the brain. Let us say that receptors 7, 200 and 587 are all sensitive to chemical 19. When C-19 reaches the cilia, it will stimulate only R-7 (receptor 7), R-200 and R-587. Imagine each chemical has different shapes. C-19 is round and C-5 is triangular. C-19 will only fit into a receptor that allows circles, and C-5 will only fit into a receptor that allows triangles. Like that, for each chemical there are receptors. But in some cases, each receptor can be activated by many odour molecules and one odour molecule can activate many receptors. See the following image in the link to get a better understanding. The purple structure (shaped like a spanner) is a receptor and the circle triangle and rectangle are different odour molecules. In this case, as you can see, the receptor only allows the rectangular receptor to fit in and stimulate itself. View the image at:

Now, R-7, R-200 and R-587, which are stimulated by C-19 will send signals through their sensory cells’ axons. These three axons go through the cribriform plate and meet in their glomerulus. All receptors that are sensitive to C-19 will meet in one glomerulus. This makes it easier for the mitral cell to send one final signal to the brain. The glomeruli are a very important structure in the olfactory bulb. If there were no glomeruli, all three axons (in reality it could be many more) would have to send projections to the brain which is useless, because all of the axons carry the same information. If there were 38 receptors that were sensitive to the same smell, the receptors’ axons would send single projections to the brain, meaning 38 nerves, which would be tough for the brain to read. With a glomerulus, the job is easier. Since all the 38 axons meet at one point, the mitral cell can send one projection to the brain through the olfactory tract instead of 38. This means that the brain finally knows that we have sensed C-19. But remember, we are not only smelling C-19, we are smelling the scent of pizza. Which means even C-5 and C-3021 will be present. The same thing happens. Both the chemicals stimulate their respective receptors which meet at their glomeruli and send a signal to the brain. So it is the combination of C-5, C-19 and C-3021 that the brain recognizes as the smell of pizza.

We were talking about the 800 or so chemical compound that coffee has in it. These 800 chemicals stimulate a pattern of receptors and it is the pattern of these activated cells that the brain recognizes as coffee odour.  In short, what we think of as a single smell is actually a combination of many chemicals acting on a variety of receptors, creating a complex code that we can identify as a particular smell. A slightly different pattern in odour components will show the difference between a black coffee and milk coffee. This image shows some odourants, their basic structure and their odour quality.

Now we have understood how our nose detects smells and how odourants (also called ligands) interact with receptors. But we have not completely finished. In fact, we haven’t reached the main part. The brain. What happens in the brain? Where do all the olfactory nerves carry all the information to?

The brain – Why smells are associated with memory and emotion

When we smell something we may get reminded of a particular person, place or event. Smells also cause you to react in different ways and cause different emotions. How does this happen? Why do smells remind you of things and cause you to react? This ability is a very important one. If you are near a forest fire, you will smell smoke. You smell smoke and get reminded of a fire. And the smell of smoke is dangerous, so run away! This is how the sense of smell can be helpful.

The nerves take scent information to many parts of the brain. The first stop is an area behind the olfactory bulb, called the piriform cortex. The piriform cortex works to identify smells, by processing the combination of chemicals, and recognizing the smell. From there, smells go to the thalamus, a structure somewhere in the middle of the brain that serves as a relay station for all sensory information coming into the brain. From the thalamus, smells are transmitted to two other parts of the brain. It sends smell information to the orbitofrontal cortex, where it is integrated with taste information. It also sends information to the insular cortex, which is believed to be involved in consciousness and plays roles which are linked to emotion, your body’s control and its awareness.

The piriform cortex also sends information to three parts of the brain’s limbic system – The entorhinal cortex, hippocampus and amygdala. The entorhinal cortex plays an important role in memory and navigation. The hippocampus along with the entorhinal cortex processes memory and the amygdala processes emotion. This is why smells trigger certain emotions and bring back memories. When you smell something, the hippocampus stores the information and the amygdala stores the way you reacted to it. When you smell something new, you link it to an event, person, a thing or moment. Your brain creates a link between the smell and the memory, and it stores the same emotion with which you reacted to the specific smell. Let us say you smell chlorine for the first time. It would be near a pool, and you don’t like the smell. The brain stores all this information. The next time you smell chlorine, the link is already there, and will trigger the memory (pool) and mood (bad smell). This is why smells are so closely associated with memory and emotions.

This image shows all the parts of the brain that are concerned with smells. In the image – PC: Piriform cortex, Amyg: Amygdala, Ento: Entorhinal cortex, Hipp: Hippocampus, Thal: Thalamus, Insula: Insular cortex, OFC: Orbitofrontal cortex.

The table below shows the structures which smells pass through in order.

Nostrils → Nasal Cavity → Mucus

layer →

Cilia and

receptors →

Sensory

cells →

Axons → Glomeruli

in bulb →

Mitral

cell →

Olfactory tract → Piriform

cortex →

Thalamus → Orbitofrontal cortex → Amygdala → Hippocampus→ Smell is

perceived

 

How does the sense of smell affect other aspects of the human body?

If any of you have noticed, when you have cold, your sense of smell is not very effective. A cold means that there is an excess of mucus stuffed in your nose. This mucus can stop many odours from reaching the olfactory epithelium. But a cold doesn’t only make your sense of smell less effective, it also makes you sense of taste less effective. You cannot taste things as well as you would without a cold. Why would something in your nose stop you from tasting something in your mouth?

Your mouth is connected to the nasal cavity through a passage behind your mouth. This means that things that go into our mouth have access to the olfactory epithelium. When you eat something, your taste receptors help you taste. But the smell from whatever you eat travels to the nose. When you smell something from what you eat, the smell can trigger emotions and memories, and enhance the foods flavour. If you are eating roast chicken, the smell travels to the brain and you know it is roast chicken and you like it. Smells can travel from the back of the mouth and up into the olfactory epithelium. But if you have a cold, it wouldn’t be able to pick up those smells properly. Try closing your nose and eating your favourite food, it wouldn’t taste as good.

Some people cannot smell at all. The inability to smell anything is called anosmia. It is a complete loss of smell. This is mainly caused by nasal congestion, which is any excess of mucus or allergies in the nasal passage which blocks out the olfactory epithelium. The sense of smell is most keen between the age of 30 and 60, and starts to decline after. This loss of smell after a certain age can be caused by dying receptors. An infection in the nose can cause the mucus to swell up, stopping smells from getting through. Any exposure to harmful toxins can damage the receptors. For most people, anosmia is a temporary deficiency of smell caused by a stuffy nose. Later on after the nose is cleared, their sense of smell returns. When people are asked what sense they would be prepared to do without, the sense of smell is on top of their list. This shows that people are unaware of the importance of smell. Smell is essential for our sense of taste, if affects one’s memory, and helps us make decisions. Anosmia can have a huge effect on someone’s life.

We know that many things stink, or smell bad. Even we, humans, sometimes stink. Why do we take showers? Every day, when we go out, we collect some amount of dirt, sweat and bacteria on our skin. All these things can release bad smells called body odour. To stop this from happening, we clean ourselves every day to wash away the dirt etc. from our skin. When we don’t bathe, we use fragrances like deodorants and perfumes to overcome the bad smell. We especially stink when we sweat. Is there something bad about sweat? In fact, sweat by itself is basically water and salt, and doesn’t smell at all. Sweating is the body’s way of remaining cool, by letting water evaporate. There are tiny bacteria and germs on our skin. When your sweat evaporates, it carries these bacteria and germs from the skin into the air. It is the bacteria that release a bad smell, or so called body odour.

The odour molecules that evaporate from objects are called esters. Esters are chemical compounds. All the chemicals we talked about before (C-19 etc.) are known as esters. The ester that gives bananas their smell is called isoamyl acetate, and the primary smell of an orange comes from octyl acetate. Esters can now be made artificially, and that is where artificial flavours and smells come from. Many of the same chemical compounds in different things, but, as mentioned before, the combination of compounds give it its unique smell. Scents are basically chemical bonds. When different atoms bond to each other they form a particular smell molecule. An addition or removal of any molecule can create a completely different smell.  Even our taste receptors detect similar chemicals. These chemicals that make us taste, also release smells, and those are the smells from our food that travels to the olfactory epithelium.

Why do most animals have a better sense of smell than humans?  

We know that we rely on our sense of smell to navigate, to remember something, or to stay away from something. Humans are not the only creatures that use their sense of smell. In fact, most animals’ sense of smell is much more important to them than it is to us. Most animals have a much better sense of smell. Let us take the common example of a dog. We know that dogs have a much better sense of smell. The olfactory epithelium in a human is about the size of a postage stamp, or 1 sq. inch. If you could unfold this area in a dog, the surface area would be around 60 sq. inch. This means that dogs (and most other animals) have many more scent receptors, therefore making their sense of smell more precise. A dog’s brain is also specialized for identifying scents. The percentage of a dog’s brain that is devoted to identifying smells is about 40 times larger than that of a human’s. The average number of scent receptors that humans have is around 5 million (this may not be true. Since it cannot be counted, scientist have given an average number based on other information), while a bloodhound would have about 300 million! Click this link to see a table of the number of receptors that different breeds of dogs have.

Other than the number of receptors, olfaction in animals work in pretty much the same way as humans. Bears are thought to have the best sense of smell compared to any other animal. Bears’ acute smell evolved in order to find food, mates, keep track of their cubs and avoid danger. A bear can detect animals from 20 miles away. Even though a bear’s’ brain is one-third the size of a human’s, their olfactory bulb is 5 times larger and they have many more receptors. Many animals cannot live without their sense of smell, so you can see why it is so essential for them.

So smells have a long distance to cover before they are actually perceived. They stimulate a pattern of receptors which fire a signal to the olfactory bulb, which sends information to different parts of the brain. All this happens well under a second. It is amazing how all this can happen so fast. Next time when you smell pizza, just think about how the scent has to be picked up by certain receptors, organized in the olfactory bulb, interpreted by the brain, and you will be amazed. Our sense of smell is a very important aspect of the human body.

Hopefully now, after reading this, the next time we catch a whiff of something we won’t take our sense of smell for granted!

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