Irritating smells alert special cells, NIH-funded study finds

[Guest guest]

Though I don't understand this fully because of some medical terms,

but from what I can gather this is the way Sharon seems to react to

a chemical hit. Especially the gagging and coughing, that's a sign

for me that she just took a hit. We also know that this has effected

her trigeminal nerve.

Biology News Net (press release) - QC,Canada*

http://www.biologynews.net/archives/2008/03/04/irritating_smells_aler

t_special_cells_nihfunded_study_finds.html

If you cook, you know. Chop an onion and you risk crying over your

cutting board as a burning sensation overwhelms your eyes and nose.

Scientists do not know why certain chemical odors, like onion,

ammonia and paint thinner, are so highly irritating, but new

research in mice has uncovered an unexpected role for specific nasal

cavity cells. Researchers funded by the National Institute on

Deafness and Other Communication Disorders (NIDCD), part of the

National Institutes of Health, describe this work in the March issue

of the Journal of Neurophysiology, now available online.

Weihong Lin, Ph.D., of the University of Colorado Denver School of

Medicine and University of land, Baltimore County, led the study

which discovered that a particular cell, abundant near the entry of

many animal noses, plays a crucial and previously unknown role in

transmitting irritating and potentially dangerous odors. Dr. Lin and

colleagues from both universities plus the Mount Sinai School of

Medicine identified the role of this solitary chemosensory cell in

transmitting irritating chemical odors in the noses of mice.

Scientists have found similar solitary chemosensory cells in the

nasal cavities, airways and gastrointestinal tracts of many mammals

as well as fish, frogs and alligators; they think it is likely that

they are also present in humans, explains Finger, Ph.D., one

of the senior co-authors at the University of Colorado Denver.

Prior to this work, scientists who study smell and taste thought

that irritating odors directly stimulated the trigeminal nerve,

which senses touch, temperature and pain throughout the head region,

including the delicate membranes that line the inside of the nose.

The research team, under the guidance of Diego Restrepo, Ph.D.,

found that solitary chemosensory cells scattered in the epithelium

inside the front of the nose respond to high levels of irritating

odors and relay signals to trigeminal nerve fibers.

" This elegant research corrects an erroneous assumption about how

irritating odors are perceived and expands our understanding of

olfaction, " says F. Battey, M.D., Ph.D., director of

NIDCD. " With further investigation, it also might lead to a better

understanding of why some people are exceptionally sensitive to

irritating odors. "

Solitary chemosensory cells on the surface of the nasal cavity are

in close contact with trigeminal nerve fibers which end just below

the surface. Earlier research revealed that these cells contain

bitter taste receptors and that bitter substances applied to the

surface of the nasal cavity trigger a trigeminal nerve response.

Intrigued, Drs. Restrepo and Finger decided to explore whether

solitary chemosensory cells respond to irritating odors. Using nasal

tissue from mice, the scientists measured a variety of changes in

solitary chemosensory cells as they exposed the cells to low and

high levels of several irritating, volatile chemical odors.

Among their observations were changes in electrical activity in the

cells—which indicates a response to an outside stimulus—and changes

in intracellular calcium ion concentration—which indicates signaling

to other cells. Their measurements demonstrated that the solitary

chemosensory cells responded to the odors and relayed sensory

information to trigeminal nerve fibers.

Once stimulated, the trigeminal nerve will convey pain and burning

sensations and can trigger protective reflexes such as gagging and

coughing. The architecture of nasal tissue with solitary

chemosensory cells on the surface and trigeminal nerve fibers just

below allows the nose to detect a greater number of irritating

odors, the scientists explain.

Fortunately, the threshold for triggering a response is high, so

exposure to a small amount of an irritating chemical, as might

naturally emanate from some kinds of fresh fruit, will not bring on

gagging and coughing. For example, lemons contain the volatile

chemicals citral and geraniol but at levels too low to trigger a

trigeminal response. Only high, potentially dangerous levels of

odors will trigger the protective gagging-and-coughing response.

The researchers point out that their findings provide an example of

the Law of Specific Nerve Energies, conceived by Johannes

Muller in 1826. Muller said that the way we perceive a stimulus

depends on the nerve or sensory system that conveys it rather than

the physical nature of the stimulus itself. In the case of

irritating odors, we perceive them as irritating because they are

transmitted via the trigeminal nerve, leading the brain to interpret

the message as pain rather than as a smell.

The researchers say their findings raise new questions about how

irritating odors are detected. They say more research is needed to

explore whether solitary chemosensory cells are programmed to

recognize specific irritants, which receptors are involved, and what

steps a solitary chemosensory cell uses to convert a chemical

stimulus to a signal it relays to the trigeminal nerve.

Source : NIH/National Institute on Deafness and Other Communication

Disorders