FoundationalConceptsNeuroscience

Some of these molecules are caught in the moist and mucousy tissue
lining the interior of the nasal passages, in the region called the nasal
or olfactory epithelium. (Epithelium is the name given to the types of
cells that line the surfaces of many structures throughout the body.)
There the molecules may come into contact with the cilia of olfactory
receptor cells, which are populated by olfactory receptor proteins.
Figure 12.1 depicts olfactory receptor cells embedded in the nasal
epithelium. The nasal passage (within the nose) would be just below
the diagram. The dendrites of the receptor cells branch into cilia con-
taining olfactory receptor proteins. These cilia extend into the mucus
lining the nasal passage, and their filamentous structure provides a
large surface area containing olfactory receptor proteins.
Within the nasal epithelium are also stem cells—not primordial
stem cells that can differentiate into any kind of cell, but cells with
enough flexibility to differentiate into the various different types of
olfactory receptor cells. These olfactory stem cells allow the receptor
cells to be regularly replaced, something that happens about every one
to two months. This is beneficial because direct exposure of the recep-
tor cells to potentially toxic substances from the environment results
in accumulation of cellular damage—the cost of chemoreception.