The Biomechanics of the Sense of Smell
Smells are detected when molecules of gas released into the air by some half million substances--everything from lemon pie to Limburger cheese, perfume to petunias--waft into a person’s nose and travel into the far reaches of the nasal cavity to a dime-sized patch made up of millions of olfactory cells.
There these gas molecules dissolve in the mucus that lines the nasal cavity, stimulating tiny hairlike projections attached to each olfactory cell. Impulses then move through the projections to the cells and to olfactory nerve fibers, which link the nose to the brain.
The nerve fibers transport the impulses to the olfactory bulb, a portion of the brain devoted to the perception of odor. From there, impulses travel to the front part of the cerebrum to be deciphered into information about the odor. However, scientists say it remains unclear precisely how the brain differentiates between smells. Individual receptor cells appear to respond to many odors. Some researchers believe the brain may distinguish among odors by how fast and precisely where their molecules adhere to receptor cells.
In people, the olfactory patch lies outside the main airstream, so generally only small amounts of stimuli filter up during light breathing. That’s why people sniff when a sensation occurs, increasing by as much as tenfold the molecules reaching the receptor cells. In dogs and other animals with notoriously good senses of smell, the olfactory patch is more centrally located in the nasal cavity.