The Sense of Smell Smell depends on sensory receptors that
respond to airborne chemicals. In humans, these chemoreceptors
are located in the olfactory epithelium — a patch of tissue
about the size of a postage stamp located high in the nasal
cavity. The olfactory epithelium is made up of three kinds of
cells: - sensory neurons each with a primary cilium - supporting
cells between them - basal cells that divide regularly producing
a fresh crop of sensory neurons to replace those that die (and
providing an exception to the usual rule that neurons seldom are
replaced). The sequence of events - The cilia of the sensory
neurons are immersed in a layer of mucus. Odorant molecules
(molecules that we can smell) dissolve in the mucus and - bind
to receptors on the cilia. These are "7-pass" transmembrane
proteins. - Binding of the odorant activates a G protein coupled
to the receptor on its cytoplasmic side. - This activates
adenylyl cyclase, an enzyme embedded in the plasma membrane of
the cilia. - Adenylyl cyclase ca