What is the role of the olfactory system in the pathophysiology of taste and smell disorders?

Updated: Jan 08, 2021
  • Author: Eric H Holbrook, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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The sense of smell is mediated through stimulation of the olfactory receptor cells by volatile chemicals. To stimulate the olfactory receptors, airborne molecules must pass through the nasal cavity with relatively turbulent air currents and contact the receptors. Odorants can also be perceived by entering the nose posteriorly through the nasopharynx to reach the olfactory receptor via retronasal olfaction. This mechanism is thought to play a key role in the sensation of flavor during eating and drinking. Odorants diffuse into the mucous and are transported to the olfactory receptor. [7] Important determinants of an odor's stimulating effectiveness include duration, volume, and velocity of a sniff.

Each olfactory receptor cell is a primary sensory bipolar neuron. The average nasal cavity contains more than 100 million such neurons. The olfactory neurons are unique because they are generated throughout life by the underlying basal cells. New receptor cells are generated approximately every 30-60 days.

Each regenerating receptor cell extends its axon (CN I) into the CNS as a first-order olfactory neuron and forms synapses with target mitral and tufted cells in the olfactory bulb.

The bipolar olfactory neurons have a short peripheral process and a long central process. The peripheral process extends to the mucosal surface to end in an olfactory knob, which has several immobile cilia forming a dense mat at the mucosal surface. The cilia express the olfactory receptors that interact with odorants. The family of odor receptor proteins are G-protein coupled receptors (GPCRs) associated with adenylate cyclase. The genes that encode them were discovered in 1991 by Linda Buck and Richard Axel, culminating in the Nobel Prize awarded in 2004.

This family encompasses approximately 900 genes, the largest in the genome. It has been found in mice that each neuron expresses only one gene, so odorants are recognized by the complex binding pattern they create. This is most likely the case in humans as well. Interestingly, these genes have been recently discovered to exist in nonolfactory tissues such as sperm and the gut. The function of these genes outside of their role in olfaction is under investigation. [7]

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