Researchers at the University of Maryland School of Medicine in Baltimore have discovered a new layer of circuitry in the brain that enhances our sense of smell. The findings, published in the December 11 issue of the journal Nature, show that odors are processed in the brain through a complex network of neural connections, and not through a simple pathway as previously thought.
"Because it's the oldest of the senses, the sense of smell was believed to be crude and straightforward compared to the senses of hearing, sight and touch," says Michael T. Shipley, Ph.D., professor and chairman of the Department of Anatomy and Neurobiology at the University of Maryland School of Medicine, and director of the Program in Neuroscience. "But the existence of a neural network in the olfactory system suggests that the brain uses similar decoding strategies for all of our primary senses. In fact, these fundamental neural mechanisms may have originated in the olfactory system," explains Dr. Shipley, the lead author of the study.
In the senses of hearing, sight and touch, the brain uses several circuit layers -- a neural network -- to enhance sensory information before it reaches our higher brain regions. "We believe a similar neural network may enhance our ability to process, recognize and respond to odors," says Adam C. Puche, Ph.D., assistant professor of anatomy and neurobiology, and co-investigator. "The way our brain understands the color purple, interprets the sound of a violin, and identifies the scent of lavender perfume may be much more similar than we previously thought," adds Dr. Puche.
So how do we know that a rose is a rose, and not a ripe banana? In the sense of smell, nerve cells in the nasal cavity detect inhaled molecules from the odor source, and electrical signals are sent along the olfactory nerve to the brain for interpretation. In the brain's olfactory region, sensory connections take place in small groups of nerve cells called glomeruli. In the study, investigators simulated odors by electrically stimulating nerve cells in tissue from the olfactory region of mice. High-speed cameras and tiny glass electrodes were used to record brain activity.
Using that process, the researchers detected a network of neurons whose primary function is to inhibit the response of other nearby nerve cells. "Some neurons exist only to keep other neurons quiet," explains Dr. Shipley. "This network helps the brain determine which odors are more important. If you are going to escape from a predator, you had better let the most important signal through first."
In addition to helping to explain how the sense of smell works, the research has important implications for the future treatment of neurological disorders. "By better understanding the organization and operation of neural networks, we will be in a better position to repair neural networks damaged by stroke, or brain injury," says Dr. Shipley, who adds that research into this realm of neuroscience is still in its infancy. "The next step is to look at additional networks in the olfactory system and how these networks cooperate to sharpen the sense of smell."
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