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
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:
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
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