As we breathe in air through our nostrils, we inhale airborne chemical molecules, which are detected by the 10 million to 20 million receptor cells embedded in the olfactory membrane of the upper nasal passage. The olfactory receptor cells are topped with tentacle-like protrusions that contain receptor proteins. When an odor receptor is stimulated, the membrane sends neural messages up the olfactory nerve to the brain (see Figure 4.15).
We have approximately 1,000 types of odor receptor cells (Bensafi et al., 2004), 1 and it is estimated that we can detect 10,000 different odors (Malnic, Hirono, Sato, & Buck, 1999). 2 The receptors come in many different shapes and respond selectively to different smells. Like a lock and key, different chemical molecules “fit” into different receptor cells, and odors are detected according to their influence on a combination of receptor cells. Just as the 10 digits from 0 to 9 can combine in many different ways to produce an endless array of phone numbers, odor molecules bind to different combinations of receptors, and these combinations are decoded in the olfactory cortex. Women tend to have a more acute sense of smell than men. The sense of smell peaks in early adulthood and then begins a slow decline. By ages 60 to 70, the sense of smell has become sharply diminished.