Richard Axel and Linda Buck received the 2004 Nobel Prize in Medicine/Physiology for their important discoveries about the underlying neurobiology of our sense of smell, the olfactory sensory system that recognizes and interprets the various properties of airborne chemicals that enter our nose. Their discovery is especially noteworthy because it’s the only sensory system that was completely mapped by a single pair of collaborating scientists. They first reported their discoveries in 1991.
Touch and taste require direct physical contact with objects and fluids. Vision and hearing allow us to locate and mentally represent distant objects and events. Our sense of smell alerts us to nearby objects and organisms through molecules they release into the air that then attach to receptors in our nose. Because odors travel through the air, we know that the source of the odor is in the vicinity, but we may have trouble locating it by smell alone, and smell provides no information on size and shape. The various elements of our sensory system thus provide different kinds of information that our brain integrates into the unified perceptual map of our external environment that alerts us to dangers and opportunities.
Our nose (situated right above the entrance to our digestive system) processes our initial sense of smell through two postage-stamp sized mucus membranes at the top of the nasal air passages. Axel and Buck discovered the genes that code for the production of the 350 types of receptors located in these membranes. These receptors are located on five million olfactory neurons that send hair-like receptive projections into the mucus. Each olfactory neuron contains only one type of receptor.
These receptors interact with odor-bearing molecules that have entered our nose and are trapped and dissolved within the mucus. Smell receptors are located on bare neural endings, so when we smell something, our brain is in direct chemical contact with the outside environment. Perhaps because of this direct contact, the neurons projecting into the mucus membrane regenerate themselves every few weeks. Furthermore, each person has a unique combination of the strength of the various receptors, so we each take in a different bouquet of smells from the environment.
Only a small amount of the air we breathe passes over these receptors, but our sense of smell is powerful enough to respond to very light concentrations of odor-bearing molecules. Most animals have a much more powerful and directional sense of smell than we have. For example, the mouse system that Axel and Buck studied has about three times as many types of smell receptors as humans.
Our brain can recognize about 10,000 different odors that are created from perhaps 30 different kinds of molecules. Most odors are combinations of molecules that enter our nose in the air we breathe and then attach to receptors. Smell is the only sensory system that doesn’t pass through our brain’s thalamus for its initial processing.
In smell, the current combination of molecular inputs is sent to the olfactory bulb in the front of our brain, and it’s then relayed to various emotion and interpretive systems (principally in the amygdala and temporal lobes). Language functions similarly in that our brain can create words by differentially combining the few dozen different phonemes that our auditory system can recognize, and so can create languages of hundreds of thousands of words.
Smell plays an important role in the formation and recall of emotion-laden memories, but we can’t recall the odor of an absent object (such as the odor of an orange or a rose) in the same way that we can recall its shape and color, or in the same way that we can recall a melody that isn’t being currently played. Smell is thus more a phenomenon of recognition than of recall.
Smell is more important than taste in our recognition and selection of many foods, and we often use herbs and spices to enhance certain pleasant odors associated food. We similarly often use perfumes and lotions to enhance favorable body smells, and to mask unfavorable ones.
It’s difficult to classify smells, but seven primary odors have been identified: minty (peppermint), floral (roses), ethereal (unscented nail polish), musky (musk), resinous (camphor), acrid (vinegar), and putrid (rotten eggs).
So congratulations to Richard Axel and Linda Buck for explaining the neurobiology of something important that we often take for granted. Through their work, they’ve now added a Nobel quality to odors that stink!