Our sense of smell—like our sense of taste—is part of our chemosensory system, or the “chemical senses”. In order for us to smell a particular thing molecules from that thing must make it into our noses. We can smell things because those things that we smell are constantly giving off molecules. More precisely, molecules are escaping from that thing and the reason they’re escaping is because they are what we refer to as “volatile”. Volatile in this context means that they evaporate easily i.e., they go into a gaseous (vapor) state with little to virtually no energy input into them. A more scientific way to word it is to say that they have an extremely low boiling point. Once the molecules are in a vapor state they can make their way into our noses.
The ability to smell comes from specialized sensory cells, called olfactory sensory neurons, which are found in a small patch of tissue high inside the nose. These neurons are unique in that they are effectively out in the open and in contact with the air. These cells connect directly to the brain and each olfactory neuron has one odor receptor. Vaporized odor molecules (chemicals) floating in the air reach the nostrils and dissolve in a thin layer of mucous membrane called the olfactory epithelium. Once the neurons detect the molecules, they send messages to your brain, which identifies the smell. There are more smells in the environment than there are receptors, and any given molecule may stimulate a combination of receptors, creating a unique representation in the brain. These representations are registered by the brain as a particular smell.
An example of a smell that is easily appreciated is that of esters. Esters are chemical compounds responsible for many of the odors we encounter daily. We could say that the chemical compound ethyl formate (C3H6O2) sure reminds us of Hawaii… or we could just say it smells like Pineapple.
Smells reach the olfactory sensory neurons through two pathways. The first pathway is through your nostrils. The second pathway is through a channel that connects the roof of the throat to the nose. Chewing food releases aromas that access the olfactory sensory neurons through the second channel. If the channel is blocked, such as when your nose is stuffed up by a cold or flu, odors can’t reach the sensory cells that are stimulated by smells. As a result, you lose much of your ability to enjoy a food’s flavor. In this way, your senses of smell and taste work closely together.
Without the olfactory sensory neurons, familiar flavors such as chocolate or oranges would be hard to distinguish. Without smell, foods tend to taste bland and have little or no flavor. Some people who go to the doctor because they think they’ve lost their sense of taste are surprised to learn that they’ve lost their sense of smell instead.
Your sense of smell is also influenced by something called the common chemical sense. This sense involves thousands of nerve endings, especially on the moist surfaces of the eyes, nose, mouth, and throat. These nerve endings help you sense irritating substances—such as the tear-inducing power of an onion—or the coolness of menthol.
Our sense of smell is also deeply connected to our Limbic System. The olfactory bulb is one of the structures of the limbic system and a very ancient part of the brain. Information captured by the sense of smell goes from the olfactory bulb to other structures of the limbic system.
The limbic system is a network of connected structures near the middle of the brain linked within the central nervous system. These structures “work together to affect a wide range of behaviors including emotions, motivation, and memory” (Athabasca University-Advance Biological Psychology Tutorials). This system deals with instinctive or automatic behaviors, and has little, if anything, to do with conscious thought or will. It is through the limbic system that from day one of our lives we begin to associate feelings and emotions with smells.