We breathe in and out around 20,000 times a day and move more than 11,000 liters (380 cubic feet) of air. Each time odor molecules are sucked into our nostrils, they float through our nasal passages and into our nasal cavity, where they bind to cilia – microscopic hair-like projections on neurons. Millions of receptors react to the odor molecules and send signals to the olfactory bulb, where it is then transmitted to the olfactory cortex and limbic system – an ancient part of the brain involved in memory and emotion.
This happens thousands of time a day, yet we still know relatively little about how our brain manages this complex process. Now, neuroscientists are one sniff closer with the discovery of a type of neuron that seems to help tune, amplify, and dampen these neuronal responses. The work was published in Nature Neuroscience earlier this year and may shed light on how epileptic seizures occur.
“Our sense of smell is complex and involves many overlapping and interconnected neuronal circuits,” said lead author James Sturgill of the University of California, San Diego, in a statement. “More than hearing or sight, olfaction is based upon past experiences and associations.”
The results from the study suggest that there are specific neurons in the olfactory cortex that serve as tuner and volume controls for various types of input. In some sense, these neurons act like fine adjustments to the tuning dial on a radio – they are a way to reduce the “static” one picks up so the proper station can be clearly heard.
The study was conducted using optogenetics, a technology that uses light to control cell function in the brain. The scientists deactivated the inhibitory neurons in the olfactory cortex of mice and then presented them with different types and intensities of odors – such as lemon, pine, and banana. Their subsequent electrical activity was then recorded.
Without the use of inhibitory neurons, the researchers found an increase in background brain activity unrelated to the actual processing of odors. When reactivated, that very same activity decreased. The results suggest the inhibitory neurons somehow modulate the signal-to-noise ratio of brain activity.
“If you wonder how it is possible to smell a banana peel in a garbage can, it is because of this type of subtle neuronal control, achieved through inhibition,” said senior author Jeffry Isaacson of UCSD. Essentially, inhibitory interneurons work to keep noisy, "background" cortical activity (the overall smell of the garbage can in Isaacson's example) low while odor-evoked responses (to the recently thrown-in banana skin) are maintained.
It's also possible this study has implications for those who suffer from epileptic seizures, but more research is needed to confirm this association. “The olfactory cortex is the region of the brain most likely to experience epileptic seizures,” said Isaacson. “It’s likely that the cells involved in processing odors also prevent seizures. Epilepsy can [possibly] be recast as an abnormality in the function of these inhibitory neurons.”
Right image credit: Patrick J. Lynch, medical illustrator / Wikimedia Commons