The transduction of olfactory information occurs in the olfactory epithelium, the sheet of neurons and supporting cells that lines approximately half of the nasal cavities. (The remaining surface is lined by respiratory epithelium, which lacks neurons and serves primarily as a protective surface.) The olfactory epithelium includes several distinct cell types (Figure 15.5A). The most important of these is the olfactory receptor neuron, a bipolar cell that gives rise to a small-diameter, unmyelinated axon at its basal surface that transmits olfactory information centrally. At its apical surface, the receptor neuron gives rise to a single process that expands into a knoblike protrusion from which several microvilli, called olfactory cilia, extend into a thick layer of mucus. The mucus that lines the nasal cavity and controls the ionic milieu of the olfactory cilia is produced by secretory specializations (called Bowman's glands) distributed throughout the epithelium. Two other cell classes, basal cells and sustentacular (supporting) cells, are also present in the olfactory epithelium. This entire apparatus—mucus layer and epithelium with neural and supporting cells—is called the nasal mucosa.

Figure 15.5

Structure and function of the olfactory epithelium. (A) Diagram of the olfactory epithelium showing the major cell types: olfactory receptor neurons and their cilia, sustentacular cells (that detoxify potentially dangerous chemicals), and basal cells. (more...)

The superficial location of the nasal mucosa allows the olfactory receptor neurons direct access to odorant molecules. Another consequence, however, is that these neurons are exceptionally exposed. Airborne pollutants, allergens, microorganisms, and other potentially harmful substances subject the olfactory receptor neurons to more or less continual damage. Several mechanisms help maintain the integrity of the olfactory epithelium in the face of this trauma. The respiratory epithelium, a non-neural epithelium found at the most external aspect of the nasal cavity, warms and moistens the inspired air. In addition, it secretes mucus, which traps and neutralizes potentially harmful particles. In both the respiratory and olfactory epithelium, immunoglobulins are secreted into the mucus, providing an initial defense against harmful antigens, and the sustentacular cells contain enzymes (cytochrome P450s and others) that catabolize organic chemicals and other potentially damaging molecules that enter the nasal cavity. The ultimate solution to this problem, however, is to replace olfactory receptor neurons in a normal cycle of degeneration and regeneration. In rodents, the entire population of olfactory neurons is renewed every 6 to 8 weeks. This feat is accomplished by maintaining among the basal cells a population of precursors (stem cells) that divide to give rise to new receptor neurons (see Figure 15.5A). This naturally occurring regeneration of olfactory receptor cells provides an opportunity to investigate how neural precursor cells can successfully produce new neurons and reconstitute function in the mature central nervous system, a topic of broad clinical interest. Recent evidence suggests that many of the signaling molecules that influence neuronal differentiation, axon outgrowth, and synapse formation in development elsewhere in the nervous system (see Chapters 22 and 23) perform similar functions for regenerating olfactory receptor neurons in the adult. Understanding how the new olfactory receptor neurons extend axons to the brain and reestablish appropriate functional connections is obviously relevant to stimulating the regeneration of functional connections elsewhere in the brain after injury or disease (see Chapter 25).