Our sense of smell — like our sense of taste — is part of our chemosensory system, or the chemical senses.

Specialized sensory cells, called olfactory sensory neurons, are found in a small patch of tissue high inside the nose. These cells connect directly to the brain. Each olfactory neuron expresses one odor receptor. Microscopic molecules released by substances around us — whether it's coffee brewing or a pine forest — stimulate these receptors. Once the neurons detect the molecules, they send messages to our brain, which identifies the smell. (Because there are more smells in the environment than there are receptors, a given molecule may stimulate a combination of receptors. This response is registered by the brain as a particular smell.)

Smells reach the olfactory sensory neurons by way of two pathways. The first pathway is through your nostrils. The second pathway is through a channel that connects the roof of the throat region to the nose. When we chew our food, aromas are released that access the olfactory sensory neurons through this channel. If the channel is blocked, such as when our noses are stuffed up from a cold or flu, odors cannot reach the sensory cells and much of our ability to enjoy a food’s flavor is lost. In this way, our 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. Some people who go to the doctor because they think they've lost their sense of taste are surprised to learn that they have a smell disorder instead.

Our 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 us sense irritating substances such as the tear-inducing power of an onion or the refreshing cool of peppermint.