Smell is the most direct of the senses. Aromas must literally enter the body before they can be consciously identified. With every inhalation, molecules travel through the thin craggy pathways that begin at the nostrils and head toward the brain. They speed past the olfactory cleft, a narrow opening toward the top of the nose. They hit the olfactory receptors, which are housed on the hairlike tips of the millions of neurons that peek through a gold-hued mucous membrane called the olfactory epithelium.
Every human has around 350 different types of these receptors, which are unique proteins on both the left and the right side of the upper nostrils. These receptors are the gateway to the complex dance of perception. They connect to the smell molecules upon arrival and then transfer signals toward the brain by chemical impulse. Every human has between six and eight million neurons in the nose to do just that. These signals are fired rapidly, by many neurons at a time, forming a pattern not unlike a line of musical notes, or the HTML coding of a webpage. When combined, the brain interprets the signals as a smell, an “odor image”.
These patterns are both complicated and minute. Scientists have found that if the chemical structure of two smells are identical except for just one carbon atom, the patterns sent in response are nonetheless distinguishably altered. Nonanoic acid, for example, is a nine-carbon chain that yields the salty smell of cheese. Decanoic acid, with only one carbon atom added to its structure, however, smells rancid, like sweat.
These patterned signals travel on pathways made by neurons, which snake from the nose through a thin sheet of bone called the cribriform plate, and are deposited in the olfactory bulb, which lies toward the bottom of the brain. The bulb takes these patterns, like reading the score of a piano concerto or lyrics to a lullaby, and sends them farther on to the olfactory cortex. The cortex, in turn, relays an interpretation to other parts of the brain like the thalamus, which deals in conscious perception, and the limbic areas, for emotional response.
— Season to Taste: How I Lost My Sense of Smell and Found My Way (Molly Birnbaum)