Researchers at the University of Pittsburgh have unraveled one of the longstanding mysteries of how our sense of smell works.
In the back of your nose is a 3-square-centimeter piece of specialized tissue called the olfactory epithelium; that’s where your olfactory neurons live. When you smell something, odor molecules enter your nose and connect to the neurons’ receptors, which send signals to your brain telling you that you’ve smelled a lilac or fresh fish.
Each neuron has only one type of receptor, and each receptor responds to a handful of different types of odor molecules with varying degrees of sensitivity. That means we need to have hundreds of individual neurons responding to different molecules in order to differentiate between the 10,000 to 1 trillion different odors that scientists believe humans are capable of detecting. The exact number is widely disputed, but rest assured the human nose can identify a lot of distinct smells.
Dr. Jianhua Xing, lead author of the paper published this week in the Proceedings of the National Academy of Sciences, said for decades, researchers have wondered about the cellular mechanism that allows olfactory neurons to specialize in this way, while simultaneously ensuring that all approximately 400 types of receptor are represented.
“This is a long standing puzzle and we showed it can be explained by very simple physics,” he said.
The secret, said Xing, lies in the concept of cooperativity. Put simply, each neuron doesn’t work independently. As neurons develop and specialize, they are communicating with each other, saying “I’ll take fish” and “I’ll do lilacs.”
Xing said the advanced science underpinning this process is difficult to explain, but provided a metaphor.
Perhaps you have been at an event where random applause eventually coalesces into synchronized clapping. This sometimes happens at sporting events or political rallies.
“The clapping is random initially, then becomes synchronized,” Xing said. “You may notice, first, the transition is spontaneous; no one orchestrates it. Second, the actual transition is fast.”
There isn’t a period of time when some people are clapping together and others are applauding randomly. It appears to happen almost instantaneously. This is due to cooperativity, Xing said, and this is what happens when olfactory neurons are developing their unique specialties.
Cooperativity is the same concept that keeps schools of fish swimming together and Xing said it’s no surprise it has other applications in the natural world.
“Mother nature is an experimental engineer,” he said. “Through evolution and natural selection, she has tried many possible designs and kept those that work well for certain …. objectives.”
As computational and systems biologists, Xing and his team discovered this not through lab experiments, but through computer modeling and theoretical analysis using existing experimental data. The computer model they developed correctly predicted findings from other research groups working on the olfactory system.