All the molecules that are involved in olfactory transduction are located within long cilia that arise from the olfactory bulb. The chemical signals (the odorants) are detected and selectively bind to the odorant-binding proteins (OBPs) that are concentrated in the olfactory mucus (Fig 19.4). The OBPs are small (10 to 30 kilodalton), water-soluble proteins that are synthesized and secreted by supporting cells. First, incoming odorant molecules are solubilized in the olfactory mucus, and then OBPs act as molecular carriers that transport odorants and deliver them to the olfactory receptors (ORs) located in the plasma membrane of the cilia. Olfactory receptors are specific for the olfactory receptor cells and belong to the family of G protein–coupled receptors (known as Golf ). When stimulated by odorant molecules, Golf ORs activate enzyme adenylyl cyclase and initiate the cyclic adenosine monophosphate (cAMP) cascade of evens (see Fig 19.4). These include binding of cAMP to specific Na and Ca2 channel proteins and influx of Na and Ca2, which is responsible for plasma membrane depolarization that generates action potential. To accurately detect several thousands of known odor molecules by only 350 known different ORs in humans requires a special coding system for different impulses. This is achieved by a population coding scheme, in which each OR protein binds to different odorants with different sensitivity. Thus, the olfactory system must decode olfactory impulses not from only one cell, but from the entire population of cells within the olfactory epithelium.
Supporting cells provide mechanical and metabolic support for the olfactory receptor cells.
Supporting cells are the most numerous cells in the  olfactory epithelium. The nuclei of these tall columnar or sustentacular cells occupy a more apical position in the epithelium than do those of the other cell types, thus aiding in their identification in the light microscope (see Fig. 19.3 and Plate 69, page 688). They have numerous microvilli on their apical surface, and abundant mitochondria. Numerous profiles of smooth endoplasmic reticulum (sER) and, to a more limited extent, rough endoplasmic reticulum (rER) are observed in the cytoplasm. They also possess lipofuscin granules. Adhering junctions are present between these cells and the olfactory receptor cells, but gap and tight junctions are absent. The supporting cells function in a manner comparable to that of neuroglial cells, providing both metabolic support (secretion of OBP molecules) and physical support to the olfactory  receptor cells.
Brush cells are columnar cells specialized for transduction of general sensation.
The olfactory epithelium also contains cells present in much smaller numbers, called brush cells. As noted, these cells are present in the epithelium of other parts of the conducting air passages. With the electron microscope (EM), brush cells exhibit large, blunt microvilli at their apical surface, a feature that gives them their name. The basal surface of a brush cell is in synaptic contact with nerve fibers that penetrate the basal lamina. The nerve fibers are terminal branches of the trigeminal nerve (cranial nerve V) that function in general sensation rather than olfaction. Brush cells appear to be involved in transduction of general sensory stimulation of the mucosa. In addition, presence of a microvillous border, vesicles near the apical cell membrane, and a well-defined Golgi apparatus suggest that brush cells might be involved in an absorptive as well as a secretory functions.
Basal cells are progenitors of the other mature cell types.
Basal cells are small, rounded cells located close to the basal lamina. Their nuclei are frequently invaginated and lie at a level below those of the olfactory receptor cell nuclei. The cytoplasm contains few organelles, a feature consistent with their role as a reserve or stem cell. A feature consistent with their differentiation into supporting cells is the observation of