|<<p> The brain can differentiate thousands of scents, should they happen to waft into our noses and tickle sensory neurons located there. But how those neurons pass information to the brain, and how the brain processes it, has been a mystery. Now, two studies appearing in the 8 November issue of Nature suggest that many odor-specific neural connections are hardwired, which may explain why smells trigger instinctual behavior.
The sensation of a smell begins when an odor molecule binds to a receptor protein on sensory neurons in the nose. Those neurons branch into a part of the brain called the olfactory bulb. What happens next--when information from the bulb travels to the olfactory cortex--has been "terra incognita," says neuroscientist Lawrence Katz of Duke University Medical Center in Durham, North Carolina.
To trace connections between the bulb and the cortex, Linda Buck and colleagues at Harvard Medical School in Boston marked neurons that received signals from specific smell receptors. The neurons transferred the marker to connecting neurons in the olfactory bulb and cortex. The stained pathway was clearly not random, the team found--for each type of odor, the same pattern of connections appeared.