Olfactory Receptors

Olfactory receptor cells may be considered specialized, primitive-type, bipolar neurons. Their nuclei are located at the base of the epithelial layer. Basal stem cells located along the basement membrane differentiate into olfactory receptors or supporting cells, replenishing the olfactory epithelium about every 2 weeks. From the nuclear region of the olfactory receptor cell, a thin dendritic process extends toward the surface of the epithelium. At its apical end, this process widens into an olfactory rod, or vesicle, from which 10 to 15 motile cilia project into the mucous layer covering the epithelium. Desmosomes at the base of the olfactory vesicle provide a tight seal between the membranes of olfactory and sustentacular cells, thus preventing external substances from entering the intercellular spaces. At its base, the olfactory receptor cell narrows and gives rise to a fine (0.2 to 0.3 µm) unmyelinated axon. Large numbers of these axons converge to run together within a single Schwann cell sheath. The fibers then penetrate the cribriform plate to collectively form the olfactory nerve. In humans, this nerve contains on the order of 100 million axons.

Odorant Transduction. The cell membranes of the olfactory receptor cells are able to convert chemical odorants into an electrical signal by activation of a G-protein–coupled protein receptor cascade that activates the enzyme adenylate cyclase, which produces cyclic adenosine monophosphate (cAMP) as a second messenger. cAMP then changes the structure of the cell membrane channel proteins to an open state. The channel is permeable to cations that flow from the nasal mucosa into the cell. The negative resting membrane potential (−70 mV) is shifted to a more positive value. Once a certain threshold is reached, the analog sensor potential is converted to a digital action potential, which is conducted via the axon of the olfactory cell to the brain.

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