How Do We Smell? 87
Odorant molecules can reach the olfactory epithelium via the nose, the
orthonasal olfaction process, and via the mouth, by the retronasal olfaction
process. The odorants dissolve into and pass through a layer of mucous overly-
ing the olfactory epithelium (Figure 3.2).
How Do We Smell?
Interaction of an odorant molecule with an olfactory receptor triggers a com-
plex molecular cascade within the olfactory receptor cell. This process, known
as transduction, decodes chemical information from the odorant into electri-
cal information that can be understood by the brain. As an odorant molecule
binds to cilia of the olfactory neurons (small protruding hair‐like structures) it
activates a particular odor recognizing neuron and a signal is sent to various
parts of the olfactory system. In this process, the primary neuron that recog-
nizes the particular odor “dies”. However, shortly after it is regenerated by an
identical neuron. Hence, regeneration of the olfactory system is essential for
the sense of smell. This explains why when a person uses a perfume, after a few
minutes the person cannot smell that particular fragrance. Also, we cannot
smell our own bad odor! The receptors involved have died and someone else
then has to tell us of our body odor (more later on this subject). Similarly,
people working with garbage no longer smell the intense odors of the trash and
the children, the so called “slumdogs,” who collect items from heaps of trash,
do not smell the awful stench. Simply, this is because the neurons that sensed
the odor have died and keep on dying and regenerating every day. In an adult
Odorant
Na+ Ca2+
Ca2+
OR
Golf AC
GRK APR cAMP
CI–
Figure 3.2 Axons from the millions of olfactory receptor cells in the nose pass through
a honeycomb‐like structure in the skull known as the cribriform plate as they travel to the
olfactory bulb. Interaction of an odorant molecule with an olfactory receptor triggers
a complex molecular cascade within the olfactory receptor cell. This process, known as
transduction, translates chemical information from the odorant into electrical information
that can be understood by the brain. Source:: Adapted from http://ars.els‐cdn.com/content/
image/3‐s2.0‐B9780123786302006769‐gr4.jpg.
