Why bad smells make you gag
Smells so irritating they make you cough or gag may act on a single type of cell in the nose that senses caustic chemicals and warns the brain of potential danger, US researchers say.
Scientists thought such smells acted directly on nerve endings in the nose.
But this study in mice suggests special cells in the tip of the nose act as air quality control sensors that protect the body from harmful chemicals.
"You can imagine walking into an environment where there is a lot of irritating dust in the area. This would give you pause," says Professor Thomas Finger of the University of Colorado Denver, whose study appears in the Journal of Neurophysiology.
Finger says these chemosensory cells are found in most aquatic vertebrates, including sharks, bony fish and lampreys.
He thinks they are part of an ancient sensory system and they are probably present in all mammals.
"The current study is the first in mammals that has a clear idea of what these cells are responding to," Finger says.
"Some fish use them to detect predators."
In people, the cells probably trigger a response to high concentrations of irritating chemicals.
Ammonia, paint thinner or even the spray from opening a carbonated soft drink can set off the alarm.
"That is the carbon dioxide triggering that little gasping response," Finger says.
Takes your breath away
University of Colorado Denver researcher Professor Diego Restrepo, who also worked on the study, says high concentrations of irritants can even trigger a reflex that causes you to stop breathing for a few moments.
"This is one of these really hard-wired reflexes. It gives you time to get out," Restrepo says.
The researchers used nasal tissue from mice to measure changes in chemosensory cells as they exposed them to low and high levels of several irritating, volatile chemical odours.
They saw evidence that the cells not only responded to the stimuli but that they were relaying that information to nerve fibres in the nose.
They say it takes more than a mere whiff of an offending odour to trigger the response.
Restrepo says only potentially dangerous levels of odours can set off the protective gagging-and-coughing response.
"There are some people who are especially sensitive to these irritants. This could have implications for their treatment," Restrepo says.
The study was funded by the National Institute on Deafness and Other Communication Disorders, part of the US National Institutes of Health.