Neural Basis for Behavior - There's More to "Smell" Than Meets the Nose

March 20, 2001

Writer: Kara Nyberg

Using flying insects, scientists have discovered the details of how airborne odors dictate brain activity and behavior. Although moths have antennae that are a million times more sensitive to odor than the human nose is to scent, researchers believe that the basic principles of this model olfactory system apply to all animals, including humans.

They report it today (March 22) in Nature. The collaborating scientists are neurobiologists Neil Vickers, formerly of the University of Arizona and now at the University of Utah, Thomas Christensen and John Hildebrand of the University of Arizona, and Thomas C. Baker of Iowa State University.

Their article, "Odour-plume dynamics influence the brain's olfactory code", describes the long-time collaborators' multidisciplinary effort that combines behavioral and electrophysiological methods with chemical ecology for new insight into the complex functioning of the olfactory system.

Neurobiologists have long studied how animals detect odors and how these signals are represented in the brain. But their experiments are generally carried out in contained, isolated environments, not the conditions an animal would encounter when sniffing through its natural environment. And these studies typically focus on a single aspect of olfaction -- either testing how different odors affect patterns of activity in the brain or measuring the behavioral responses of the animal.

The UA scientists and their colleagues went a step farther by examining how odors in the environment influence BOTH brain activity AND animal behavior. Specifically, they studied how the physical characteristics of natural airborne odors shape the complex patterns of activity in neurons that encode different odors in its brain.

"We focus on behaviorally important olfactory stimuli and seek to understand how those natural odors are detected and analyzed as they are encountered in the environment," said John Hildebrand, Regents professor in the Arizona Research Laboratories Division of Neurobiology (ARLDN).

"Insects are telling us a lot about the basic principles underlying the organization and function of nervous systems," said ARLDN Research Scientist Tom Christensen. "And if we can understand these basic principles in other organisms, then perhaps we can gain insight into how the human nervous system responds to sensory stimuli."

Insects and other animals use odor as a primary means of communicating with one another and for basic survival skills, like finding food and places to lay eggs. One of the best-studied odors is the female sex pheromone recognized by male moths. Scientists have identified not only the specific chemical components of the pheromone, but years of previous study have provided detailed knowledge of how this chemical information is processed in the brain.

To examine how a plume of odor triggers complex patterns of activity in the brain and how this correlates with the moth's behavior, the researchers devised a method to monitor the olfactory response of a male moth in flight, as he maneuvered through a natural plume of female sex pheromone.

When the scent first hits a moth's antenna, specialized nerve cells along the antenna recognize the odor and produce small electrical signals that are then transmitted to other nerve cells in the animal's brain. Team scientists monitored odor detection by connecting tiny wires to the antenna. The wires registered an electric current every time the antenna contacted odor in the plume. The more odor molecules that hit the antenna, the larger the measured electrical signal.

The electrical signals that start in the moth's antenna then make their way to the brain, where information is processed. The researchers also monitored how the moth's brain processes information by inserting tiny recording electrodes in individual brain cells. The electrodes picked up electrical pulses each time the brain cells fired.

They discovered that some of the brain cells were very tightly synchronized to every pulse of odor. Every time odor hit the antenna, electrical impulses in antennal cells almost immediately registered as electrical responses in brain cells. That is, the brain very closely monitors when odor is present in the animal's environment and when it is not. And when the appropriate odor is present, activity in only the specific group of brain cells that is turned on results in behavioral changes in the animal, such as a change in flight direction.

This may explain why in the absence of pheromone, male moths simply fly back and forth across a windstream searching for the odor and make no upwind progress toward the odor source.

But each time his antenna gets a whiff of pheromone, stimulated nerve cells in the antenna send a brief signal to the appropriate subset of nerve cells in the male's brain. Excited brain cells then direct the moth to alter his flight path. The moth continues in the new direction until he again senses pheromone and the process is repeated. He makes an ever-tighter zigzag flight pattern as he nears the source of the pheromone, ultimately homing in on the female moth.

Now that Christensen and Hildebrand have a fairly good grasp of how these moths respond to the very specific pheromone odor, they intend to discover how the moth brain responds to a much wider variety of environmental odors. By learning how the moth responds to a great range of different odors, they will also shed more light on how other animals respond to the global spectrum of scents.

Ultimately, people may better comprehend why there is more to smell than meets the nose.


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