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If I enter a room with certain odor, I can sense the odor. However, if I stay there for some time I cannot sense it anymore. A new entrant to the room can still smell it or I have to leave the room for some time and upon returning I can sense it again. Does our nose detect only if there is change in odor?
Senses in general adapt to continuous stimulation due to various processes. One such process is simply the exhaustion of the reserves of a receptor, or secondary neuron, due to a depletion of neurotransmitters. Entering a room activates a fresh pool of chemical receptors, thereby generating a clear sense of smell. However, this dims due to adaptation. Exiting and re-entering the room after a while revives the neuronal response. More central adaptive responses may also play a role. A comparable effect is seen in the tactile sense. For example, the awareness of wearing a ring or watch fades quickly. Changing its position revives the percept. So yes, sensory systems like olfaction and touch are more responsive to differential stimuli.
Your sense of smell helps you enjoy life. You may delight in the aromas of your favorite foods or the fragrance of flowers. Your sense of smell is also a warning system, alerting you to danger signals such as a gas leak, spoiled food, or a fire. Any loss in your sense of smell can have a negative effect on your quality of life. It can also be a sign of more serious health problems.
One to two percent of North Americans report problems with their sense of smell. Problems with the sense of smell increase as people get older, and they are more common in men than women. In one study, nearly one-quarter of men ages 60–69 had a smell disorder, while about 11 percent of women in that age range reported a problem.
Many people who have smell disorders also notice problems with their sense of taste. To learn more about your sense of taste, and how it relates to your sense of smell, read the NIDCD's Taste Disorders publication.
How does the sense of smell work?
Smell is a very direct sense. In order for you to smell something, molecules from that thing have to make it to your nose. Everything you smell, therefore, is giving off molecules -- whether it is bread in the bakery, onions, perfume, a piece of fruit or whatever. Those molecules are generally light, volatile (easy to evaporate) chemicals that float through the air into your nose. A piece of steel has no smell because nothing evaporates from it -- steel is a non-volatile solid.
At the top of your nasal passages behind your nose, there is a patch of special neurons about the size of a postage stamp. These neurons are unique in that they are out in the open where they can come into contact with the air. They have hair-like projections called cilia that increase their surface area. An odor molecule binds to these cilia to trigger the neuron and cause you to perceive a smell.
According to the book Molecular Biology of the Cell:
Each of the hundreds of receptors are encoded by a specific gene. If your DNA is missing a gene or if the gene is damaged, it can cause you to be unable to detect a certain smell. For example, some people have no sense for the smell of camphor.
When you smell many fruits or flowers, what you are smelling is esters evaporating from the fruit or flower. Esters are organic molecules. For example, the ester that gives a banana its smell is called isoamyl acetate, and the formula for it is CH3COOC5H11. The primary smell of an orange comes from octyl acetate, or CH3COOC8H17. Esters can now be made artificially, and that is where artificial flavors come from.
In the battle of the sexes, women’s noses come out on top. When tested for odor detection and identification, women score consistently higher than men. This might have something to do with the size of their olfactory bulb, a structure in the brain that helps humans identify smells. One study found that women have, on average, 43 percent more cells in their olfactory bulb than men do—meaning they can smell more smells.
Think you like chocolate just because it tastes good? Think again. Smell is responsible for 75 to 95 percent of flavor, which explains why plugging your nose helps you swallow something unappetizing. More recently, chefs and neurologists have teamed up to create meals for cancer patients and others with a diminished sense of smell, such as the elderly. Cooking meals tailored to the smell-less could help stave off depression and improve the appetite without relying on sugar and salt.
The Science Of Smell: How Do Our Noses Identify Different Odors?
Does cheese smell the same to a mouse as it does to a human? Can a mouse identify cheese if it's mixed in with spaghetti sauce? A new study examining mouse brains, and specifically neural patterns associated with different smells, may shed light on how humans process smells, too.
Researchers created an algorithm that could learn to identify whether a specific odor was present in a mixture of scents. To "train" the algorithm to recognize those patterns, they gathered data from thousands of trials that looked at the neural activation patterns associated with various odors by brain imaging mice. Medical Xpress reported that the results of the study suggest these creatures may be employing a similar algorithm for identifying odors as humans do.
Essentially, one odor causes a particular neural activation pattern, and another odor causes a different pattern," said lead researcher and professor of molecular and cellular biology Venkatesh Murthy said, according to Medical Xpress. "When you start mixing odors, eventually those patterns will overlap. Mice have about 1,000 types of olfactory receptors, but a given odor only activates maybe 10 percent of them. That's sparse enough that, even if you have many scents mixed, they can still parse them out. What the algorithm does is look at those patterns, and even if they are partly occluded (by another odor), it can recognize that a particular pattern is there."
Results of a mice study have given us insight into odor recognition Photo courtesy of Pixabay
In the future, this study could serve as a model for conducting experiments virtually — like creating an algorithm — before carrying them out in the real world.
"Moving forward, we're excited about this because we want to design experiments for mice and humans that test new questions, for example, what odor experience will best improve smell detection skills, and is supervised learning necessary for improvement?" Murthy said, according to Medical Xpress. "The computer algorithms used in our work can generate strong hypotheses for testing."
Source: Alexander Mathis et al. Reading Out Olfactory Receptors: Feedforward Circuits Detect Odors in Mixtures without Demixing. Neuron . 2016.
By the way: the sensation of something as “hot” or “spicy” is quite often described as a taste. Technically, this is just a pain signal sent by the nerves that transmit touch and temperature sensations. The substance psaicin” in foods seasoned with chili causes a sensation of pain and heat.
There is a long-held misconception that the tongue has specific zones for each flavor where you can taste sweet or sour, for example, especially well. But this myth is based on an incorrect reading of an illustration of the tongue. You can still find these zones in many textbooks today.
Sweet, sour, salty, bitter and savory tastes can actually be sensed by all parts of the tongue. Only the sides of the tongue are more sensitive than the middle overall. This is true of all tastes – with one exception: the back of our tongue is very sensitive to bitter tastes. This is apparently to protect us so that we can spit out poisonous or spoiled foods or substances before they enter the throat and are swallowed.
Human Nose Can Detect a Trillion Smells
A rose, a fresh cup of coffee, a wood fire. These are only three of the roughly 1 trillion scents that the human nose and brain are capable of distinguishing from each other, according to a new study. Researchers had previously estimated that humans could sense only about 10,000 odors but the number had never been explicitly tested before.
“People have been talked into this idea that humans are bad at detecting smells,” says neurobiologist Leslie Vosshall of Rockefeller University in New York City, who led the new work. “So these findings should give the whole human race a confidence boost.”
Humans detect smells by inhaling air that contains odor molecules, which then bind to receptors inside the nose, relaying messages to the brain. Most scents are composed of many odorants a whiff of chocolate, for example, is made up of hundreds of different odor molecules. Understanding how people process the complex information contained in scents—or memories of smells—offers a window into how the human brain functions.
Vosshall says she and others in the field had long guessed that the number of detectable scents often cited in the literature, based on rough calculations made in the 1920s of the known groups and ranges of smells—claiming that humans could distinguish 10,000 odors—was way off. So her lab decided to test it once and for all. They took 128 odor molecules that represented a wide range of smells and started combining them into unique mixtures containing 10, 20, or 30 different components. Then, they recruited volunteers from the community, aged 20 to 48, to start sniffing the mixtures. “The people we invited to do this study were not professionals they were not wine tasters or perfumers,” Vosshall says.
Each volunteer was given three smell-containing vials at a time—two that were identical and one that was a slightly different mixture—and then was asked which was the odd one out. On average, if the components varied by more than 50%, the scientists found, people could distinguish the smells as different. When Vosshall’s team crunched these numbers, extrapolating how many different combinations of the 128 odorants an average person could differentiate, they arrived at an average of 1 trillion smells.
Individual performance, however, varied, they report online today in Science. The researchers calculated that the least successful smeller in the study would be able to smell only 80 million unique scents. And the best performer had a far more sensitive sense of smell, likely able to distinguish more than a thousand trillion odors.
The ability to distinguish a trillion scents from one another when they’re paired up, though, doesn’t mean that humans can identify a trillion different scents, says neurologist Jay Gottfried of the Northwestern University Feinberg School of Medicine in Chicago, Illinois. “Even if humans can distinguish that many odors based on these projected mixtures, I don’t know if there are really 1 trillion unique odors in the world that we would need to be discriminating.”
Gottfried adds, however, that the study brings up interesting questions regarding how complex smells are sensed by the nose and brain. “In general, it highlights a growing interest in how combinations of odors—rather than single odor molecules at a time—are sensed and processed.”
Vosshall and her colleagues are pursuing some of these questions, including whether certain combinations of odors are indistinguishable despite being very different at a molecular level. But for now, she just hopes the new findings encourage people to take another sniff at the world around them.
“Knowing we have these capabilities, I hope people, as they go about their business, start saying, ‘Hey, I can smell all these things.’ Maybe the companies that make scented products will start making greater use of the human capacity and develop cleaners and perfumes with new, more interesting scents,” she says. “Maybe we’re going to start using those corners of our smell capacity that have just not been exercised lately.”
Molecules And Olfactory Receptors
All around us, things like coffee or gasoline emit tiny molecules that can enter our olfactory system in two ways: either through our nostrils or the back of the throat (mostly everything emits molecules, from perfume to bread). We’re mainly familiar with smelling through our nostrils, although eating food which releases molecules into the back of the throat can also cause us to smell.
Once inside your nostrils, these air molecules land on the olfactory epithelium — a tissue covered in mucus that lines the nasal cavity. The epithelium contains millions of olfactory receptors, or neurons that are capable of binding with specific odor molecules. These are the “locks and keys” of the olfactory system, which help identify certain smells. An odor molecule from a cup of coffee floating up into your nose will find and bind to an olfactory receptor that’s specifically designed to identify that molecule. This notion was uncovered by Richard Axel and Linda Buck, who won the 2004 Nobel Prize in Physiology or Medicine for their discovery.
Once the olfactory receptors bind with a specific odor, they send their electrical impulses to a certain microregion, also known as the glomerulus (of which there are some 2,000 in the olfactory bulb), which then passes it along to other parts of the brain. The “odorant patterns” that are released from the glomerulus are interpreted in the brain as smell.
For a while, it was assumed that the human nose was capable of only smelling 10,000 different scents. It was only recently that scientists overturned that notion with a new theory: that the nose was capable of much, much more, smelling up to one trillion scents. This is because odor molecules have a myriad of different shapes that can fit into several receptors at once, making it possible for the nose to identify more smells than the number of receptors available.
The nose is also considered the “guardian of your lungs,” acting as a filter that retains tiny particles, and humidifies and warms the air you breathe to keep the bronchial tubes moist.
References: Widespread receptor-driven modulation in peripheral olfactory coding. Xu L, Li W, Voleti V, Zou DJ, Hillman EMC, Firestein S. Science. 2020 Apr 10368(6487). pii: eaaz5390. doi: 10.1126/science.aaz5390. PMID: 32273438.
Funding: NIH’s National Institute of Neurological Disorders and Stroke (NINDS), National Institute on Deafness and Other Communication Disorders (NIDCD), and National Cancer Institute (NCI) Department of Defense the Simons Foundation the Kavli Institute for Brain Science and the National Science Foundation.
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@Emilski - I'm not an animal expert, but your reasoning makes sense. I do know cats have pretty good smell, though. Sometimes if they get the sniffles, they won't eat, because they can't smell the food.
As far as I know, smell is still one of the senses that we don't understand very well, at least as far as knowing how the chemicals react in our bodies to produce the sensation of smell. With touch and sight it's a little easier to understand how the nerve impulses could give the different sensations. It's odd the think that those same impulses have to be generating smell.
One of the few things I remember from taking organic chemistry in college is that esters are responsible for a lot of fruity smells. I don't remember what their exact structure is, but I remember mixing different chemicals to produce esters. Some of the combinations we made smelled like bananas and apples. Emilski April 9, 2012
I always wondered how smell worked. This was really interesting. I don't know that I ever really made the distinction between human noses and dog noses either. Dogs typically have much longer noses, so it makes since they could have a larger smell reception area. As far as dog smell or any animal smell goes, though, does the length of the nose always predict smell?
I'd say it's probably true for dogs, at least. If you think about a basset hound or dachshund or any other dog bred to hunt or track, they have longer snouts. Pugs and dogs not meant to hunt generally don't have long noses. As far as other animals go, though, what about horses, do they have good smell? Can't a cat smell well, though? They don't have very long noses. jcraig April 9, 2012
@parsleypea - I would think that remembering smells just depends on the memory associated with it. I have smelled thousands of things in my life, but there are only a few smells that I specifically associate with a certain memory. Of all the ones I can think of, they are all associated with good things, which is unsurprising.
Whenever I smell those few smells, I automatically get a picture of a certain scene in my life. In contrast, if I smell chili powder or something, I recognize what it is, but I don't link it to anything significant, just food.
There are even some great moments in my life that probably had distinct smells, but that I don't remember. It guess our minds or just selective about what it chooses. cardsfan27 April 8, 2012
@parsleypea - I think the opposite would usually be true as far as smelling food. At least when I smell food, I get the urge to want to eat. I think that is just a basic human instinct. At least for me, it doesn't even have to be real food cooking that makes me hungry sometimes. There have been instances where I have been out in the woods and smelled a flower or something that kind of smelled like a spice, and it reminded me of a food, and I was hungry.
Thinking about it more, though, I think smelling food might have a brief effect. There have been times when I was really, really hungry to the point of feeling sick. Whenever it came time to finally eat, and I knew I was going to be getting food, the smells actually calmed my stomach down. I guess it is because my body new it would be getting food in a couple of minutes. parsleypea September 2, 2008
How long does the memory of a scent remain with the person. Can a scent spur the mind to remember such details/memories that were not retrievable prior to the "re-living" of that particular scent? Does the memory of a scent linger within the person infinitely. does the memory never leave the person? parsleypea September 2, 2008
Can the sense of smell, in a kitchen, where a cook is cooking something delicious, create an appetite and can the sense of that smell create a sense of being full as well? In other words, can the very scent of food that stimulates appetite also give a feeling of satisfaction that would suffice as much as the actual consumption of that food in their presence?