- Human noses can distinguish between a multitude of smells
- It is known that receptors communicate these scents to the brain, but scientists were unsure of exactly how that happens
- Now, an experiment in mice provided a model which could shed light on the process
Think of all the smells you’ve ever encountered, whether pleasant or absolutely putrid. Our brains are designed to distinguish between as many as a trillion smells. But what exactly is the link between the brain and the nose and how does it work?
New research has just gained more insight.
How does smell work?
The ability to smell comes from many specialised sensory cells (olfactory sensory cells) high up in the nose, which are all connected to the brain. The olfactory sensory cells are known to contain as many as 350 odour receptors to help us distinguish between different smells.
Smells are released into the air through tiny molecules, which our noses pick up. The receptors communicate with the brain, and the brain in return “tells” the nose what the smell is. Besides your nose, your throat can also send messages to your brain to help register smells. While you chew food, the smell is released and communicated to the brain.
When we have a cold or flu, when the nose and throat are affected, we often experience a loss of smell and taste because the receptors are blocked and the odours therefore can’t reach the brain.
But the question remained exactly how the brain is able to detect so many smells and communicate them back to the nose. After all, there are many more smells in the world than odour receptors in the human nose.
What the new research entailed
A team of researchers from the New York University’s Grossman School of Medicine created an electric signature that is perceived as an odour in the part of the brain where smells are processed – the olfactory bulb. They then experimented on mice to manipulate the timing and order of the related nerve signalling to see what happens in the olfactory bulb.
"Decoding how the brain tells apart odors is complicated, in part because unlike with other senses such as vision, we do not yet know the most important aspects of individual smells," says study lead investigator Edmund Chong, MS, a doctoral student at NYU Langone Health in a news release.
"In facial recognition, for example, the brain can recognise people based on visual cues, such as the eyes, even without seeing someone's nose and ears," says Chong. "But these distinguishing features, as recorded by the brain, have yet to be found for each smell."
The study, published in the journal Science, takes a look at the olfactory bulb located behind the nose in animals and humans.
Each smell creates different timing and order in brain
According to the study, the timing and order of the patterns inside the olfactory bulb that help detect the smell (also known as the glomeruli activation) is known to be different for each unique smell. But smells can vary over time and mix with others, therefore scientists were not exactly sure how to track the exact pattern of a single smell inside the neurons.
Now, the experiment in mice showed through “synthetic” smells how they responded. The researchers used genetically engineered mice whose the brain cells were activated by shining a light on them.
The mice were then trained to recognise a signal generated when six glomeruli were activated by light – creating a pattern that is usually provoked in the brain by an odour – by being given water after perceiving the correct “odour” and pushing down on a lever.
The researchers used this model to change the timing and combination of activated glomeruli to see how the mice’s perception changed.
They then found that changing which of the glomeruli within each odour-defining set were first led to a drop of as much as 30% in the mice’s ability to correctly identify an odour signal.
Why is this research important?
According to study senior investigator and neurobiologist Dmitry Rinberg, creating this model was important to examine the minimum number and kind of receptors needed by the olfactory bulb to identify a certain smell.
"Our results identify for the first time a code for how the brain converts sensory information into perception of something, in this case an odour," Rinberg states in the news release. "This puts us closer to answering the longstanding question in our field of how the brain extracts sensory information to evoke behaviour."
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