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When did you decide that you wanted to study the sense of smell?
REED: Strangely enough, I didn’t decide what to research as a faculty member until I arrived at Hopkins. I had no experience researching the sense of smell, and I just decided that I was going to become a neuroscientist even though I had no formal training in that area. In fact, I got a C in neuroscience at Johns Hopkins as an undergraduate.
Through financial support by the Howard Hughes Medical Institute, I set out on a mission to basically understand olfaction—the sense of smell. I was interested in the way that we can smell and detect differences between many thousands of compounds. I wanted to determine how the brain makes sense of these different smells and interprets them.
How does the sense of smell work?
REED: There are 1,000,000 neurons in each side of the nose. Each neuron picks just one receptor gene from a collection of 1000 receptors that detects smells, so there are 1,000 cells of each receptor type. All neurons start in the nose and lead all the way up to the brain. All cells with the same receptor have tails that lead to a tiny spot, smaller than a grain of sand. And other groups of cells that have different receptors lead to other points the size of a grain of sand somewhere else in the front of the brain. A particular odor activates a certain subset of receptors, and therefore a certain subset of neurons, in your nose. Those neurons send an electric impulse back to the brain that then causes electrical activity in a particular pattern in the front of the brain. The rest of your brain essentially reads the pattern of this activity to interpret the identity of the odors in your environment, such as a rose or an orange.
How much does smell actually contribute to our sense of taste?
REED: Almost everything that we think we are tasting is really what we are smelling. Either we breathe in the scent or when we are chewing food the scent wafts from the throat into the nose. You really can only taste sweet, sour, salty and bitter. I always walk with my coffee cup open without a lid so I can smell it better, which makes it “taste” better. Drinking coffee with a lid is like drinking wine with a straw and you would never drink good wine with a straw.
It is also why when you have a cold, you feel like you can’t taste anything. This is because your olfactory neurons—the ones in your nose that detect smell—are either blocked from the congestion or killed off by the virus, but your taste buds are actually left intact.
But after a cold, we can eventually taste again. Does that mean our olfactory neurons regenerate?
REED: Yes, they have to be able to. Olfactory neurons are the only neurons in your body that are exposed to the outside environment. Photoreceptor cells in the eyes are protected by your lens and cornea and the auditory cells in your ears are protected by the eardrum, but your olfactory neurons are out there detecting the cruel world at a chemical level. They are susceptible to damage by colds, environmental toxins or trauma. Olfactory neurons turn over continually in adults. In fact, they are the only neurons in your body that have the robust ability to completely replace themselves after significant damage.
Is having a good or bad sense of smell due to one’s genetics?
REED: In humans there is lots of genetic variation associated with altered perception in smell. When different individuals smell the same compound, they may have different thresholds as to whether they can smell it or not, and their perception of what that smell is like may be genetic. For example, humans differ by more than a billion-fold in their ability to detect a certain compound called androstenone, which is an airborne relative of testosterone. Which means some people can smell it easily while others can’t smell it at all, and not everyone likes the smell.
Androstenone is a pheromone — an airborne hormone — in pigs and it induces mating behaviors. Truffles release this chemical too, and this is why pigs are attracted to truffles.