From the NYTimes:
How the Nose Sniffs Danger in the Air
The next time someone says, “I smell danger in the air,” that might literally be true — and the odor might be coming from you.
At the tip of the noses of mammals, including humans, is a ball of nerve cells known as the Grueneberg ganglion, named after Hans Grueneberg, the scientist who described the structure in mice in 1973.
Grueneberg thought it was just a nerve ending. Only in last few years, after scientists devised strains of mice that glow green under fluorescent light, did they deduce that the Grueneberg ganglion is a component of the olfactory system. But they still did not know what the ganglion smelled.
In the Aug. 22 issue of the journal Science, researchers at the University of Lausanne in Switzerland report that they have figured it out, at least for the green-glowing mice.
All sorts of organisms, including plants, insects and mammals, release “alarm pheromones” when they sense danger; the pheromones waft through the air to warn others. Very little is known about the alarm pheromones of mammals other than that they exist. Scientists have not identified the compounds; they do not know where in the body the pheromones are produced. Nonetheless, the Lausanne scientists could collect the pheromones by simply stressing mice and sucking up the air around them.
When other normal mice were exposed to the danger-scented air, they froze in their tracks. But mice whose Grueneberg ganglia had been excised did not notice anything wrong and continued to wander around their cages without a care in the world.