In addition to perceive odours, smell is also a mode of communication. Chemicals released by
the body, called pheromones, are important signals for reproductive behaviours, and they may
also be used to mark territories, identify individuals, and indicate aggression or submission.
The Organs of Smell
We smell with a small, thin sheet of cells high up in the nasal cavity called the olfactory
epithelium (Fig. 8.9). This epithelium has three main cell types:
Olfactory receptor cells are the site of transduction. These are genuine neurons, with
axons of their own that penetrate into the nervous system
Supporting cells are similar to glia and help produce mucus
Basal cells are a source of new receptor cells
Olfactory receptors continually grow, die, and regenerate, in a cycle that lasts about 4-8 weeks.
Sniffing brings air through the nasal passages, but only a small percentage of that air passes
over the olfactory epithelium. The epithelium exudes a thin coat of mucus containing antibodies,
enzymes, and odorant binding proteins. Chemical stimuli in the air (odorants) dissolve in the
mucus layer before they reach receptor cells.
The antibodies are critical because olfactory cells can be a direct route by which some
virus and bacteria enter the brain.
Odorant binding proteins are small and soluble and may help concentrate odorants in
The size of the olfactory epithelium is one indicator of an animal’s olfactory acuity. Humans are
relatively weak smellers.
Olfactory Receptor Neurons
Olfactory receptor neurons have a single, thin dendrite that ends with a small knob at the
surface of the epithelium (see Fig. 8.9). These knobs have several long, thin cilia. Odorants
dissolved in the mucus bind to the surface of the cilia and activate the transduction process. The
opposite side of the olfactory receptor cell is a very thin, unmyelinated axon. These olfactory
axons constitute the olfactory nerve (cranial nerve I). After leaving the epithelium, small
clusters of the axons penetrate a thin sheet of bone called the cribriform plate then course into
the olfactory bulb.
Although taste receptor cells use several different molecular signalling systems for transduction,
olfactory receptors probably use only one (see Fig. 8.10). The olfactory pathway:
o Binding to membrane odorant receptor proteins