Scientists Decode Connection Between Sense of Scents: Smell & Brain

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Whenever you walk by a bakery or a restaurant, a cloud of chemicals and scent comes swirling up your nose. Identifying the smell as freshly baked bread is a complicated process. But, compared to the other senses, the sense of smell is often not appreciated. Scientists from New York University (NYU) studying olfaction have shed light on how our sense of smell works and provided compelling evidence that it’s more sophisticated than previously thought.

Olfaction is a chemoreception that, through the sensory olfactory system, forms the perception of smell. Olfaction has many purposes, such as the detection of hazards, pheromones, and food. Olfaction occurs when odorants bind to specific sites on olfactory receptors located in the nasal cavity

In a survey of 7,000 young people around the world, about half of those between the age of 16 and 30 said that they would rather lose their sense of smell than give up access to technology like laptops or cell phones.

“We’re not that sharply aware of our use of olfaction in daily living,” explains Noam Sobel, a scientist who studies smell at the Weizmann Institute of Science. But five percent of our DNA is devoted to olfaction, a fact that emphasizes how important our sense of smell is, he said.

Smell begins at the back of nose, where millions of sensory neurons lie in a strip of tissue called the olfactory epithelium. The tips of these cells contain proteins called receptors that bind odor molecules. The receptors are like locks and the keys to open these locks are the odor molecules that float past, explains Leslie Vosshall, a scientist who studies olfaction at Rockefeller University.

People have about 450 different types of olfactory receptors. (For comparison, dogs have about two times as many.) Each receptor can be activated by many different odor molecules, and each odor molecule can activate several different types of receptors. However, the forces that bind receptors and odor molecules can vary greatly in strength, so that some interactions are better “fits” than others.

“Think of a lock that can be opened by 10 different keys. Two of the keys are a perfect fit and open the door easily. The other eight don’t fit as well, and it takes more jiggling to get the door open,” explains Vosshall.

“The complexity of receptors and their interactions with odor molecules are what allow us to detect a wide variety of smells. And what we think of as a single smell is actually a combination of many odor molecules acting on a variety of receptors, creating an intricate neural code that we can identify as the scent of a rose or freshly-cut grass.

“This neural code begins with the nose’s sensory neurons. Once an odor molecule binds to a receptor, it initiates an electrical signal that travels from the sensory neurons to the olfactory bulb, a structure at the base of the forebrain that relays the signal to other brain areas for additional processing.

“The olfactory system is critical when we’re appreciating the foods and beverages we consume,” says Monell Chemical Senses Center scientist Charles Wysocki. This coupling of smell and taste explains why foods seem lackluster with a head cold.

“You’ve probably experienced that a scent can also conjure up emotions and even specific memories, like when a whiff of cologne at a department store reminds you of your favorite uncle who wears the same scent. This happens because the thalamus sends smell information to the hippocampus and amygdala, key brain regions involved in learning and memory.

“Although scientists used to think that the human nose could identify about 10,000 different smells, Vosshall and her colleagues have recently shown that people can identify far more scents. Starting with 128 different odor molecules, they made random mixtures of 10, 20, and 30 odor molecules, so many that the smell produced was unrecognizable to participants. The researchers then presented people with three vials, two of which contained identical mixtures while the third contained a different concoction, and asked them to pick out the smell that didn’t belong.

“No longer should humans be considered poor smellers. In fact, new research suggests that your nose can outperform your eyes and ears, which can discriminate between several million colours and about half a million tones. “It’s time to give our sense of smell the recognition it deserves,” said Vosshall.