How does the brain recognize, categorize and memorize the huge variety of odors? In 1991, Richard Axel and Linda Buck published a groundbreaking paper that shed light on olfactory receptors and how the brain interprets smell. They won the 2004 Nobel Prize in Physiology or Medicine for the paper and their independent research.
Axel and Buck discovered a large gene family -- 1,000 genes, or 3 percent of the human total -- that coded for olfactory receptor types. They found that every olfactory receptor cell has only one type of receptor. Each receptor type can detect a small number of related molecules and responds to some with greater intensity than others. Essentially, the researchers discovered that receptor cells are extremely specialized to particular odors.
Axel and Buck also found that each olfactory receptor type sends its electrical impulse to a particular microregion of the olfactory bulb. The microregion, or glomerulus, that receives the information then passes it on to other parts of the brain. The brain interprets the "odorant patterns" produced by activity in the different glomeruli as smell. There are 2,000 glomeruli in the olfactory bulb -- twice as many microregions as receptor cells -- allowing us to perceive a multitude of smells.
Image courtesy Nobelprize.org
Another researcher, however, has challenged the idea that humans have a large number of receptor types that respond only to a limited number of molecules. Biophysicist Luca Turin developed the quantum vibration theory in 1996 and suggests that olfactory receptors actually sense the quantum vibrations of odorants' atoms. While molecular shape still comes into play, Turin purports that the vibrational frequency of odorants plays a more significant role. He estimates that humans could perceive an almost infinite number of odors with only about 10 receptors tuned to different frequencies.
Next, we'll learn about how smells trigger memory and find out how much cognition actually influences perception.