Disorders of taste and smell can be persistently unpleasant, but only rarely is the loss of either of these modalities a serious handicap. Nevertheless, as all foods and inhalants pass through the mouth and nose, these two senses serve to detect noxious odors (e.g., smoke) and to avoid tainted food and potential poisons. The loss of these senses could then have serious consequences. Also, because a loss of taste and smell may signify a number of intracranial, neurodegenerative, and systemic disorders, they assume clinical importance.
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 receptor cells, which number between 6 and 10 million in each nasal cavity; sustentacular or supporting cells, which maintain the electrolyte (particularly potassium) levels in the extracellular milieu; 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 cilia. These hair-like processes, which lack motility, are the sites of olfactory receptors.
The central processes of these cells, or olfactory fila, are very fine (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. 11-1). Collectively, the central processes of the olfactory receptor cells constitute the first cranial (olfactory) nerve. Notably, this is the only site in the body 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 glands) and within which there are immunoglobulins A and M, lactoferrin, and lysoenzyme as well as odorant-binding proteins. These molecules are thought to prevent the intracranial entry of pathogens via the olfactory pathway (Kimmelman).
In the olfactory bulb, the receptor-cell axons synapse with granule cells and mitral cells (so-called because they are triangular, like a bishop’s miter), the dendrites of which form brush-like terminals or olfactory glomeruli (see Fig. 11-1). Smaller tufted cells in the olfactory bulb also contribute dendrites to the glomerulus. Approximately 15,000 olfactory cell axons converge on a single glomerulus. This high degree of convergence is thought to account for an integration of afferent information. The mitral and tufted cells are excitatory; the granule cells—along with centrifugal fibers from the olfactory nuclei, locus ceruleus, and piriform cortex—inhibit mitral cell activity. Presumably, interaction between these excitatory and inhibitory neurons provides the basis for the special physiologic aspects of olfaction.
The axons of the mitral and tufted cells form the olfactory tract, which courses along the olfactory groove of the cribriform plate to the cerebrum. Within the olfactory tract and posterior to the olfactory bulbs are groups of cells that constitute the anterior olfactory nucleus (see Fig. 11-1). Dendrites of these cells synapse with fibers of the olfactory tract, while their axons project to the olfactory nucleus and bulb of the opposite side; these neurons are thought to function as a reinforcing mechanism for olfactory impulses.