By Ernesto Salcedo, PhD
Earl and Wim write "Is there an easy to understand schematic/diagram of how the brain processes scent/odor and what signals are involved? And when the brain stops perceiving odors (bad or good), what's happening?"
Dear Earl and Wim,
Smell is complex and fascinating, and I've been getting lots of great questions about it. So I contacted a local olfaction expert, Dr. Ernie Salcedo of the University of Colorado, to be our new resident expert on explaining just how smell works.
Cheers, Yo Pearl
P.S. If you've got a question, submit it to Yo Pearl at www.dmns.org/molecularcell under "Have a Question?"
Using our sense of smell, we can detect anywhere from 10,000 to 100,000 different odors. Since odors are typically made up of many different volatile molecules called odorants, our noses are essentially extremely sensitive chemical detectors that can detect all of these different molecules.
This remarkable ability depends on millions of little sensor cells found at the back of our noses called olfactory receptor cells. Each of these cells have hair-like structures, called cilia, which protrude into the mucosal lining of our noses. There, these cilia have direct contact with sniffed-in air, making olfactory receptors cells the only brain cells that directly interact with the outside world.
Image courtesy of the Nobel Prize Organization: http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/illpres/index.html
The cilia of the receptor cells are covered by little proteins, called odorant receptors (see diagram panel 1), which can bind odorants. If you think of each of these odorant receptors as a kind of lock, then the odorants act like keys. Each cilia in a given odorant receptor cell has hundreds of the same 'locks' that can all be opened by the same 'key', while a neighboring receptor cell will have a different set of 'locks' that can be opened by a different 'key.' When an odorant binds to the right odorant receptor, then, like turning the key in the ignition of a car, that olfactory receptor cell 'starts up' and begins to fire an electrical signal, which is detected by cells, called mitral cells, that are found in a brain structure called the olfactory bulb. Here, these mitral cells will process the incoming signal (see diagram panel 3) and then send that signal on to higher olfactory processing centers in the brain, and eventually (after several hundred milliseconds), you will perceive the smell (or stink, as the case may be).
Soon, however, the smell will fade away from your perception, even if the molecules that make up that smell are still in the air around you. This phenomenon is termed olfactory fatigue or olfactory adaptation and is the reason why some people don't realize that they are wearing WAY too much cologne or perfume.
Why does olfactory fatigue happen? Well, in fact, there several mechanisms at work here:
1. The odorants making up the smell can sometimes bind up all of the odorant receptors in a given cell so that it simply can't be activated any further.
2. Even if the odorants can find receptors to bind, the receptor cells themselves will begin to stop responding to continuous stimulation, almost as if the cell is getting tired.
3. The brain itself begins to tune out repetitive incoming signals over time. This phenomenon, called sensory adaptation, also occurs in our other senses, such as vision and hearing. People, for example, are pretty good at tuning out repetitive sounds, such as a fan or, perhaps, a droning professor.
A great reference for how our sense of smell works can be found on the reference page for the two Nobel Laureates, Linda Buck and Richard Axel, who were awarded the Nobel Prize in Physiology or Medicine in 2004 for their achievements in olfactory research. The image shown above is from those references pages.
~Dr. Salcedo, Resident Olfaction Expert of the Blue Tongue Blog
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