16.4: Olfactory Anatomy

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Rosanna Hartline
West Hills College Lemoore

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Olfactory Anatomy

All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules into the air from its surface, it has no smell. And if a human or other animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell.

Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity. The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb (part of C.N. I olfactory nerve) of the brain. Humans have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense of smell—have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans.

Diagram of the olfactory epithelium and olfactory bulb

Above: In the human olfactory system, (a) bipolar olfactory neurons extend from (b) the olfactory epithelium, where olfactory receptors are located, to the olfactory bulb.

Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like olfactory cilia that trap odorant molecules. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Unlike any of the following special senses, neurons from the olfactory bulb bypass the thalamus and synapse directly with the olfactory cortex.

Diagram of the structures of olfaction. olfactory bulb (C. N. I olfactory nerve) cribriform plate (with olfactory foramina) olfactory epithelium mitral cell olfactory foramen olfactory sensory neuron cribriform plate olfactory epithelium olfactory axon dendrite olfactory cilia

Above: Diagram of olfactory anatomy. Olfactory sensory neurons embedded in the olfactory epithelium have olfactory cilia with receptors that attach to odor molecules and send a signal along the olfactory sensory neuron from 1. the olfactory cilia, 2. to the dendrite, 3. to the olfactory axon, and then 4. the signal is passed to another neuron called a mitral cell in the olfactory bulb of C.N. I. The signal is then transfered to the olfactory cortex in the brain.

Attributions

"Biology 2e" by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax is licensed under CC BY-NC 4.0
"Human Anatomy Lab Manual" by Malgosia Wilk-Blaszczak, Mavs Open Press, University of Texas at Arlington is licensed under CC BY 4.0
"Olfactory system.svg" by Chabacano is licensed under CC BY-SA 2.5