Anatomic and Physiologic Considerations

Anatomic and Physiologic Considerations Nerve fibers subserving the sense of smell have their cells of origin in the mucous membrane of the upper and posterior parts of the nasal cavity (superior turbinates and nasal septum). The entire olfactory mucosa covers an area of about 2.5 cm2 and contains three cell types—the olfactory or receptor cells (which number between 6 and 10 mil l ion in each nasal cavity); sustentacular or supporting cells; which maintain the electrolyte (particularly K) levels in the extracellular mil ieu; and basal cells, which are stem cells and the source of both the olfactory and sustentacular cells during regeneration. The olfactory cells are actually bipolar neurons. Each of these cells has a peripheral process (the olfactory rod), from which project 10 to 30 fine hairs, or ci l ia. These hair-l ike processes, which lack moti l i ty, are the sites of olfactory receptors. The central processes of these cells, or olfactory f i la , are very f ine (0.2 mm in diameter), unmyelinated fibers that converge to form small fascicles enwrapped by Schwann cells that pass through openings in the cribriform plate of the ethmoid bone into the olfactory bulb (Fig. 12-1). Collectively, the central processes of the olfactory receptor cells constitute the f irst cranial, or olfactory, nerve . Notably, this is the only site in the organism where neurons are in direct contact with the external environment. The epithelial surface is covered by a layer of mucus, which is secreted by tubuloalveolar cells (Bowman's glands) and within which there are immunoglobulins A and M, lactoferrin, and lysoenzyme as well as odorant-binding proteins. These molecules are