Thursday, April 8, 2010
The Nose Knows
Even though the nose has been around a long time, little was known about HOW we sense scents, even with a nose as big as Jimmy Durantes. That is until 1991, when Richard Axel of Howard Hughes Medical Institute and Linda Buck at the Fred Hutchinson Cancer Research Center took the theory that different odors are attributed to different shapes and sizes of odor molecules that stimulate the olfactory organ and added the cloning of olfactory receptor proteins and pairing of odor molecules to specific receptor proteins. They discovered about 1,000 genes that encode for olfactory receptors inside the human nose. They found that each receptor is tuned for only a small number of odors. Odor receptor nerve cells function like a key-lock system. If the airborne molecules of a certain chemical can fit into the lock, the nerve cell will respond. They went on to become the 2004 Nobel Prize winners. Research is still being done to uncover the secrets of olfactory perception.
How do we smell?
Olfaction, or the sense of smell, is the detection and perception of chemical molecules floating in the air. When you take a breath air is sucked up into your nostrils over bony ridges called turbinates. They add more surface area. The molecules that enter the nose dissolve in mucous cells within a membrane called the olfactory epithelium (a tissue composed of cells that line the cavities and surfaces of structures throughout the body). It is a stamp size sheet of olfactory receptor neurons. In humans this is located up the nose from the nostrils on the roof of the nasal cavity. The cells have small hairs called cilia on one side and an axon on the other side. Humans have about 40 million olfactory receptors while a German Shepherd dog has about 2 billion olfactory receptors. The activity produced in these hair cells is transmitted to the mitral cells in the olfactory bulb, which sits underneath the front of your brain. From there the signals go to the brain areas that are part of the limbic system. When you smell something that reminds you of something it is because the limbic system is involved with emotional behavior and memory.
When your nose is stuffed up or you have a cold the molecules that carry smell cannot reach the olfactory receptors and your sense of smell decreases. Humans can recognize 10,000 different odors. No two people sense anything the same. Several factors, including genes, skin type, and diet are related to how smells smell. Even the weather can alter an odor. When we're hungry, our smell sense grows stronger. Women have better sniffers than men. Our sense of smell is at its worst in the mornings but improves as the day goes on.
And you thought smelling was something simple.
For more technical information go to http://en.wikipedia.org/wiki/Olfaction#Olfactory_epithelium
For a simpler explanation go to