In humans, the olfactory epithelium occu-pies a small region in both nasal cavities (ol-factory region) (A1) at the upper margin ofthe superior nasal concha and on the opposite surface of the nasal septum. The mul-tilayered sensory epithelium is composed of supporting cells (C2) and receptor cells (C3)that are characterized by pale, deep-lying cell nuclei. The olfactory region also con-tains small mucous glands, the olfactoryglands (Bowman’s glands). Their secretioncovers the olfactory mucosa as a thin film.
The apical part of the sensory cell tapers into a thin shaft that slightly extends be-yond the surface of the epithelium. This knoblike olfactory vesicle (C4) is occupied by a number of olfactory hairs. At the basal end, the ovoid cell body forms a fine process that, together with several other processes, is en-veloped by Schwann cells. The bundled processes (fila olfactoria) represent the ol-factory nerves (AC5) and extend through theopenings of the cribriform plate to the ol-factory bulb (A6). In the olfactorybulb, the processes terminate in the ol-factory glomeruli, where they form synapticcontacts with the dendrites of mitral cells. The epithelial sensory cells are bipolar neu-rons; the short dendrite represents the re-ceptor part and the long axon runs as centripetal fiber to the olfactory bulb.
Electron micrographs of olfactory cells (in the cat) show that the apical shaft of the cell (D7) terminates with a knob (D8) from which numerous long olfactory cilia (D9) ex-tend. The terminal parts of the sensory cilia lie in the mucous layer (D10) that seals the entire surface of the olfactory epithelium against the air space. Shaft and knob contain microtubules, numerous mitochondria (D11), and some lysosomes (D12). The knob extends above the surface of the supporting cells, which exhibit a dense border of micro-villi (D13).
How the sensory cells receive the different odors is currently the topic of intensive re-search. The odoriferous substances must be water-soluble in order to dissolve in thesuperficial mucous layer and reach the sensory cilia where they bind to specific membrane receptors. In sufficiently high concentrations, they induce depolarization of the membrane, which is conducted as an action potential in the cell’s axon. As in the case of taste, it is assumed that there are a few basic qualities of odor and that one sensory cell registers only one particular ba-sic quality via specific receptors. Since sub-stances belonging to a group of odors have roughly the same molecular size, it seems possible that the membrane of an olfactory cilium reacts to only one particular molecu-lar size. Indeed, recent studies suggest that each sensory cell expresses only one recep-tor type. In the mouse, the sensory cells of one receptor type project to only two of the 1800 glomeruli in the olfactory bulb (central olfactory pathway,).
Apart from the olfactory nerves, two other paired nerves run from the nasal cavity to the brain, namely, the terminal nerve and the vomeronasal nerve. The terminal nerve (B14) consists of a bundle of delicate nerve fibers that extends from the nasal septum through the cribriform plate to the terminal lamina and enters below the anterior com-missure into the brain. The bundle contains numerous neurons and is regarded as an au-tonomic nerve. The vomeronasal nerve (B15), which runs from the vomeronasal organ (Ja-cobson’s organ) to the accessory olfactorybulb, is well developed in lower vertebrates but can be demonstrated in humans only during embryonic development. In reptiles, the vomeronasal organ is a sensory epithelium in a pocket of the mucosa of the nasal septum; it is thought to play an impor-tant role in tracking down prey.