Researchers say that most smells hover about 10 inches off the ground, placing the human nose at a disadvantage among those of most other mammals. Nonetheless, when smells do reach the neurons inside the nose, the human brain can distinguish from among the thousands of chemicals that make up odors, and scientists are beginning to understand just how the process works.
In the last decade, the nose has been revealed as the site of a large family of sensory neurons, each of which specializes in a particular smell. Since this discovery, researchers have studied the olfactory system in rodents, following the axons that extend from neurons into the rodent brain. Their research shows that the axons from neurons with receptors for the same odor molecule congregate in the one or two glomeruli that are reserved for those axons. Glomeruli, which contain only axon terminals, are specialized structures in the olfactory bulb; the rodent brain has 2000 of them. By studying "odor maps" that show activity in certain glomeruli in response to different smells, Howard Hughes Medical Institute investigator Lawrence C. Katz of Duke University has found that each odor results in a pattern or "fingerprint," which humans and other mammals seem to use to distinguish from among different smells.
"We needed a way to 'see' which specific glomeruli were activated by different odorants," said Katz, a professor of neurobiology at Duke. "We accomplished this by using a technique called 'optical imaging of intrinsic signals,' which allowed us to take a picture of the bulb when it was being stimulated by a particular odor, and when it wasn't."
While the human brain's ability to distinguish different odors may be impressive, many mammalian brains can also identify an altogether different type of "smell." Mammals ranging from mice to elephants produce substances called pheromones, which communicate information about social status and when an individual is ready to reproduce. These signals are somehow picked up through the "accessory" olfactory system, which is separate from the "main" olfactory system that allows us to appreciate, for example, the scent of a rose bouquet.
Scientists have understood little about the accessory olfactory system, but a new mouse study by Katz and his colleagues, forthcoming in Science, should change that. It reveals that pheromone-detecting neurons in this system are carefully "tuned" to pick up on another mouse's sex and genetic makeup. This type of smelling may thus be a handy way for certain mammals to identify potential mates (and may also help mice get around the fact that they look pretty much alike). These findings may also open the way to a better understanding of how animals communicate dominance, mating receptivity, and individuality, according to Katz and his coauthors. The researchers implanted three microelectrodes into a specific region of the mouse's brain where they would record the activity of individual neurons responding to different pheromones. Then they added to the cage a second mouse, which was lightly anaesthetized, so that the test mouse could get close for a sniff. The test mouse's neurons each had highly selective responses to the pheromones of the introduced mouse, depending on the introduced mouse's genetic strain and sex. Interestingly, the face seemed to be the most important source of pheromones.
Other researchers are exploring how the axons from the sensory neurons negotiate "an extremely hostile environment" to make their way into the brain, according to Charles Greer, professor of neuroscience in the Department of Neurosurgery and Section of Neurobiology at Yale University School of Medicine. He points out that unlike the axons of any other neurons, axons from the sensory neurons travel in bundles that are wrapped in tentacle-like glial processes, offering a possible model for encouraging the growth of neurons in areas that have been damaged following spinal cord injury. "Think of the glial cells as a person with their arms wrapped around the axons," Greer said. "We've taken these glial cells, and have begun to make spinal cord cells functionally enervate areas."
During the AAAS Annual Meeting, a panel on olfactory research also will explore the question of "olfactory memories"-how the brain can retain memories of smells, despite rapid turnover in the sensory neurons that inhabit the nose. Katz notes that the one of the next breakthroughs in the science of the olfactory system will allow scientists "to unravel the links between olfactory perception and the formation of long-term olfactory memories."
Advance interviews possible upon request.
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MEDIA NOTE: Katz will take part in a newsbriefing during the AAAS Annual Meeting in Denver on Thursday, 13 February at 12:00 p.m. noon Mountain Time, Rooms C-110-112, in the Colorado Convention Center. Katz , Greer and other researchers will then participate in an Annual Meeting session titled, "How the Nose Knows: Neural Circuits for Chemical Detection," at 2:30 p.m. Mountain Time, Saturday, 15 February, in Room A-205 on the Main Level of the Colorado Convention Center. Reporters must register at the AAAS Press Center, Rooms C-101-103, Colorado Convention Center.