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How Does the Sense of Smell Work?

Olfactory cells are neurons.
The nose passes scent information to the olfactory cortex of the brain.
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The sense of smell, known as olfaction, uses a sensory organ called the nose to pass scent information on to the olfactory cortex in the brain. Diffuse suspensions of relevant molecules, called odors, are analyzed by the nose using a molecular lock-and-key scheme whereby odors are identified by their unique chemical signatures.

The sense evolved as a means of detecting survival-relevant information about the external world, especially appraising food. Smell is the oldest of the senses, with analogues dating all the way back to the first animals 600 million years ago. One of the five primary senses, smell is most intimately associated with the formation of memories.

Olfactory receptor neurons, the cells responsible for smell, are located on a 1-by-2 inch strip of tissue called the olfactory epithelium, located about 3 inches above and behind the nostrils. The human olfactory epithelium is about 16 cm², contrast with some dogs which have 150 cm².

With advanced color vision, sight can be considered the primary human sense: smell plays a more limited role, primarily related to food and sexual bonding. The olfactory receptor neurons are surrounded by supporter cells which excrete mucus, making it easier to pick up odor molecules. The nostrils are covered in hair to prevent the intrusion of unwanted organisms and inanimate material from the environments.

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The olfactory receptor neurons, each of which can detect several molecules, project axons into the brain via the olfactory nerve. These projections converge on a small (~50 micron) structure called the olfactory bulb, ultimately converging onto only 100 or so neurons. Considering the degree of convergence found in the olfactory portion of the nervous system, it may seem a wonder that we can distinguish between so many smells, but apparently arbitrary activation patterns among the 100 neurons is enough to do the job. Most humans can distinguish between several thousand smells and varying degrees of intensity and scent combinations therein.

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Discuss this Article

Izzy78
Post 8

@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
Post 7

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
Post 6

@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
Post 5

@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
Post 3

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
Post 2

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?

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