Smell's path to the brain

Study: Researchers say a communication system high in the nose increases the contrast in odors.

Medicine & Science

December 15, 2003|By Jonathan Bor | Jonathan Bor,SUN STAFF

When molecules of freshly ground coffee waft into your nose through a kitchen full of aromas, what makes you notice the coffee? And what, to the trained nose, suggests gourmet rather than canned, espresso rather than regular?

Though the complete answer might be years off, smell researchers at the University of Maryland School of Medicine say they've found an important clue: a communications system high in the nose that heightens the contrast between odors so the brain can make better sense of them.

Nerve bundles that capture particular scents relay their information to the brain, but not before negotiating with each other to decide what odors should be turned up and down, they said. The system evolved not simply to enhance our appreciation of aromas but, on the most basic level, to help animals survive in the wild.

"If you're going to escape from a predator, you had better let the most important signals go through first," said Dr. Michael T. Shipley, chairman of the school's department of anatomy and neurobiology and a co-author of a paper in the latest issue of the journal Nature.

This process is similar to the way the retina distinguishes shades of gray in a darkening sky, the way the inner ear singles out footsteps in a windy forest, the way the skin detects a pin prick on a frigid day.

"The road map the brain is using is very similar for all these sensory circuits, whether it's vision, sound or olfaction," said Adam C. Puche, a colleague of Shipley's and co-author of the paper.

Odors originate from molecules that slough off substances - from fine wines to raw sewage, sizzling steaks to rancid milk - and float through the air into the nose. These molecules (called odorants) find their way to a postage stamp-sized patch of tissue covered with millions of odor-sensing nerve cells at the top of each nostril.

The neurons have fine hairs, called cilia, that capture the molecules and send impulses communicating "coffee" to a higher region of the brain. The neurons come in about 1,000 types, each equipped with a unique docking station - or receptor - for an odorant of a particular size or shape.

Nerve cells sensitive to the same odorant are scattered randomly across the tissue, but they are wired together in capsules - called glomeruli - just millimeters away. The glomeruli sit in a structure called the olfactory bulb, located in the region where the nose merges into the brain.

Scientists used to believe that the glomeruli simply gathered and relayed the information directly to brain for further processing. But in studies of laboratory mice and rats, the Baltimore scientists found that they also are wired to each other so they can first sort out and accentuate their differences.

"We had no idea the glomeruli talk to each other over long distance," said Shipley, meaning a few millimeters. "`Whoops, my message is more important than yours.' It's democratic."

In this way, the stronger messages inhibit the weaker ones, and keep the brain from being overwhelmed with information that is too "raw and disorganized" to process, said Shipley.

Smells, of course, are complex, consisting of many notes of differing intensities. The aroma of coffee comes from an assortment of molecules that stream into the nose, striking the olfactory equivalent of a musical chord. About 1,000 different notes are on the smell scale, which makes for an almost infinite number of possible chords.

The glomeruli not only increase the contrast between coffee and, say, bacon, but also the contrasts within the coffee aroma itself. The scent might have a darker or lighter quality depending on the brew.

Most everyone has the same olfactory equipment, but some can distinguish finer shades than others through practice and experience.

"Look at wine tasters," said Shipley. "They can tell the difference between umpteen different kinds of cabernet sauvignon. To the untrained person, it might smell like sweet vinegar. It involves some learning, but the basic apparatus is there."

Understanding how smell works isn't just of academic interest, said Shipley. If the circuits involved in various senses are indeed similar, scientists might eventually learn how to use circuitry from one sense to compensate for deficits in another.

Allowing his mind to roam a bit, Shipley imagined a scenario in which someone could repair an animal's retina by transplanting into the eye a piece of olfactory circuitry. In searching for therapies like this, scientists would be borrowing a lesson from nature.

"Nature is not stupid," said Shipley. "It does not form a new circuit every time it has to perform a new operation. That makes the search more rational."

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