||Our sense of smell works
double duty, detecting both chemical and mechanical stimuli
to improve how we smell.
||When olfactory neurons respond to odor
molecules, they transmit chemical energy into electrical
signals, which then trigger a secondary molecular messenger
cascade that generates electrical impulses to the brain,
signaling that it is smelling something.
||Researchers at the University of Pennsylvania School
of Medicine discovered
that chemical and mechanical stimuli both resulted in the
same messenger molecule, cAMP, which acts like a gatekeeper
of reactions in the olfactory neurons.
||The research will be published in the
March issue of Nature Neuroscience.
(PHILADELPHIA) – Unlike most of our sensory systems that
detect only one type of stimuli, our sense of smell works double
duty, detecting both chemical and mechanical stimuli to improve
how we smell, according to University of Pennsylvania
School of Medicine researchers in the March issue of Nature
This finding, plus the fact that both types of stimuli produce
reaction in olfactory
nerve cells, which control how our brain perceives what we smell, explains why we sniff to smell something,
and why our sense of smell is synchronized with inhaling.
Cross section of olfactory sensory neurons
in a mouse nose
Click on thumbnail
to view full-size image
“The driving force for such synchronization remained a mystery
for more than 50 years,” says senior author Minghong
Ma, PhD, Assistant Professor of Neuroscience. “These results
help us understand how the mammalian olfactory system encodes and
decodes odor information in the environment.”
Researchers tested two different types of stimulation on olfactory
neurons in mice: chemical stimuli, such as those used in making
perfumes that have almond-like and banana-like scents, and mechanical stimuli, that is pressure carried by air flow to the nostrils while
The group did this first by puffing a chemical stimulus into the
nose. As expected, this produced a reaction in the olfactory neurons,
the primary sensory neurons in the nose that perceive odors. Researchers
then puffed a solution without the chemical stimuli into the mouse’s
nose. This also produced a similar, but smaller reaction in the
olfactory neurons. By decreasing pressure of the non-odor solution,
they also found that the reaction in the olfactory neurons was
less, confirming that it was sensitive to mechanical stimulation.
When olfactory neurons respond to odor molecules, they transmit
chemical energy into electrical signals, which then trigger a secondary
molecular messenger cascade that generates electrical impulses
to the brain, signaling that it is smelling something. The group
discovered that chemical and mechanical stimuli both resulted in
the same messenger molecule, cAMP, which acts like a gatekeeper
of reactions in the olfactory neurons.
Although this study was conducted on a mouse model, the researchers
tested two different parts of the nose, one that humans have and
one that humans do not. The first, the septal organ, is a patch
of smell-sensitive tissue on the septal wall of the nasal cavity.
The second, the main olfactory epithelium, is a smell-sensitive
tissue inside the nasal cavity.
The septal organ is only about 1 percent the size of the main olfactory
epithelium and isn’t shared by all mammals. Mice, for example,
have a septal organ. Humans do not. But in this study, Ma’s
group found that 50 percent of the cells in the main olfactory
epithelium are sensitive to physical stimuli, suggesting that
mechanosensitivity of the olfactory sensory neurons exists in
all mammals, even those like humans, without the septal organ.
The mechanosensitivity of our olfactory neurons has two possible
functions, suggest the investigators. The first is that it increases
our ability to smell, enhancing the detection of odorous molecules
in the air. The second is a peripheral drive in the brain to synchronize
rhythmic activity, which is the concurrent firing of neurons in
the olfactory bulb with breathing.
“The mechanosensitivity may increase the sensitivity of
our nose, especially when stimulated by weak odors,” says
Ma. “It helps the brain make better sense out of odor responses
when it integrates airflow information. We still don’t know
how it happens, but sniffing is essential for odor perception.”
Co-authors are Xavier
Grosmaitre and Lindsey C. Santarelli, both
from Penn, and Jie Tan and Minmin
Luo of the National
Institute of Biological Sciences (Beijing, China). The National
Institute on Deafness and Other Communication Disorders, the Whitehall
and the University of
Pennsylvania Institute on Aging provided
funding for this research.
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