Using a fluorescence-based technique Luo developed to image small numbers of neurons (see sidebar, “Better Maps”), his lab group tracked subgroups of interneurons. Conventional wisdom held that these interneurons should connect all glomeruli into a large web, and researchers found that some groups of interneurons held this pattern, communicating among all olfactory channels. But others acted differently, connecting exclusive groups of glomeruli in private networks.

Luo’s group also found that the interneuron patterns weren’t the same from one fly to the next, suggesting variability in wiring that’s also been observed in vertebrates. Thus, Luo suggests, studies of the relatively simple fly brain might provide more direct insights into how the human brain works.

		A Better Maps Navigating the nervous system requires a good map. Read More

To complement those studies, Wilson’s team looked at how the interneuron mapping pattern related to the neural activity flowing into glomeruli. They found that the more active glomeruli tended to have more interneuron connections than their less active counterparts. That finding suggests that the ones with the most activity need more interneurons to dampen their output before sending information to other brain regions.

Wilson thinks this arrangement might provide a way for the olfactory system to monitor activity and keep it in the most useful range. By connecting all glomeruli “channels,” inhibitory interneurons can adjust overall activity so that the brain can make sense of the signals, much like a digital camera takes a light reading to make sure the subject of a photo is visible—neither too much nor too little light.

Wilson has also identified another type of interneuron that helps adjust olfactory signals. While most interneurons use chemical signals to dampen other neurons, Wilson and her colleague Emre Yaksi found that some interneurons use electrical communication to excite other neurons.

The work, published September 2010 in Neuron, showed that these special interneurons appear to have two jobs: They excite neurons in different glomeruli by linking directly to them, but they indirectly squelch the same neurons by prodding the inhibitory interneurons to push harder on the brakes. Fine-tuning the balance between these opposing forces helps keep olfactory signals in a range that’s just right, says Wilson.

The results are a glimpse at the complex way the nervous system interprets information about smells. “To understand what’s going on in higher brain regions is not just counting which receptors are active,” says Wilson, it’s also how active they are. Stay tuned for more advances as they sniff out the secrets of how we get a whiff of our world.

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