FoundationalConceptsNeuroscience

Olfactory nerve f i

fibers to brain Ss Olfactory stem cells

Nasal

Olfactory epithelium

receptor < structural

cells cells

a Mucous layer

Cilia y

Figure 12.1. Olfactory epithelium.

Like many of the receptor proteins found in animals, olfactory
receptors are G-protein-coupled receptors (GPCRs). Among the ver-
tebrate animals, the number of different types of olfactory receptor
proteins varies widely. A typical fish, for example, might have around
one hundred different olfactory receptor proteins, coded for by one
hundred different genes. Mammals may typically have ten times this
number of different olfactory GPCRs, around a thousand. A mouse is
on the high end of the mammalian range, with about thirteen hun-
dred different olfactory receptor proteins. Humans are on the low end,