Your Sense of Smell Explained in Scientific Terms

Your Sense of Smell Explained in Scientific Terms

The sense of smell is one of the most fascinating of all of our sensual receptors. It is also one of the most critical for all animal species. As humans, we can detect and distinguish more than nine thousand odors. We use our sense of smell for any number of different things, including enjoying the aromas of our favorite foods and beverages as well as deciding whom we want to associate with based on smell. We can use it to detect danger as in a fire or a gas leak, and we can use it for fun like in reading scratch and sniff books to our children. Over the course of the past twenty years, science has made extensive study of the human sense of smell. The science community can now tell us both how our nose detects odor molecules and how the brain is able to deal with that information once it is detected.

Every odor your nose detects comes from molecules, tiny particles emitted from the object. Almost everything emits a smell, but some of them don’t reach as far as others. For example, bread, onions, perfume, fruit, and similar things give off many light, volatile molecules that are long range. They float through the space around the object and eventually end up in your nose. Other objects, like steel for instance, gives off molecules, but they are not long range and do not float as easily. As a result, we might say it has no smell.

Your sense of smell kicks in when the molecules enter your nose. At the very top of the nasal passage, there is an area, around the size of a postage stamp, which contains a large patch of neurons. Actually, it contains millions of neurons, called olfactory receptor neurons. The area is called the olfactory epithelium. The interesting, and unique, thing about these olfactory receptor neurons is that they are unprotected. This means they can come into direct contact with the air you breathe in. Their projects, a bit like hair, increase the surface area they can reach. The projections are called cilia. As an odor molecule comes up the nasal passage. The cilia will trap it. This forces the olfactory receptor neuron that the cilia are attached to, to send a message to your brain and cause you to perceive a smell.

Not everyone perceives the same smells, though. Recently, scientists discovered that the ability to smell certain odor molecules is genetic. Your individual genome encodes your olfactory receptor neurons. Each of the receptors has a place where an odor molecule can form a bond with it so your brain can perceive the smell correctly. If the right molecule falls into the right place, you get the smell. If, however, you are missing a genetic sequence, or the correct genetic sequence has been damaged in that area, your olfactory receptors are unable to accept the molecule. As a result, your brain is unable to receive the electrical impulse, and you never get the smell. It is possible to regrow these olfactory neurons if they are damaged.

There are four zones of olfactory receptor neurons within your brain. These four zones can help to distinguish the quality of the odor, the intensity of the odor, and the type of odor you are detecting. Even babies in the womb have a sense of smell. By nine weeks into gestation, the nasal cavity has separated from the mouth. It is at this point that the olfactory receptor neurons are formed. By the thirteenth week of gestation, the connections between the baby’s brain and the olfactory receptor neurons are formed. From that point onward, the baby can smell throughout the gestation. Smelling does not require air; it simply requires the odor molecules, which can be transmitted through the amniotic fluid. In some scientific studies, a baby can recognize its mother’s scent immediately after birth.

The sense of smell is often termed one of our chemical senses because it requires that we process the chemicals around us for use. Smell can alert us to danger, but it can also offer us a sense of recognition. Ever have someone wander by you wearing your mother’s perfume? You probably immediately recognized it as such. The sense of smell, as complicated as it is, is one of our most powerful.

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