THE SENSE OF SMELL

Everyday experience tells us that the human nose can distinguish thousands of substances by their smell. Thus, the physiology of the olfactory system is important in the scope of sensory evaluation. Upon observation of the human nose it can be seen that the nasal cavity is divide into two spaces, left and right by a partition called the nasal septum. Each of these spaces folds to form ridges, called conchae , which project into the interior from the outside walls of the nose. There are three conchae arranged one above the other on each side of the nose. The entire nasal cavity is lined with a mucous membrane. The smell receptor cells, olfactory sensory cells, are restricted to a rather small area, the olfactory region. This region covers the whole upper conchae and part of the middle conchae. The part of the nasal mucosa that lacks olfactory cells is the respiratory region, which has a ciliated epithelium producing the mucus.

Olfactory Epithelium

The olfactory epithelium, as shown directly above, is essentially responsible for the sense of smell. There are two predominant cell types here, the olfactory sensory cells and the supporting cells. Basal cells are also present as in the taste buds, and serve the same function to regenerate the sensory cells. The olfactory sensory cells bear cilia towards the olfactory epithelium and are covered by a layer of mucus. In order for an odor to reach the sense cells,, the molecules of an odor substance must diffuse through the mucus layer. Unlike the taste receptor cells, the olfactory cells are primary sensory cells in that they send out axons from the basal pole. The primary process is poorly understood but it is thought that the molecules of an odor substance interacts with specialized molecules in the membrane of the olfactory cell. It is presumed that several related odor molecules react with a common receptor molecule, which the phenomenon of partial anosmia illustrates. There are so many (thousands) of odor substances that it is unlikely that there exists a particular sensory molecule for each odor substance. The known recognition of individual odor must nevertheless be coded somehow(Marieb, E.N.)

The coding of concentrations of odor substances and the response of receptor cells is not known. However, it is known that olfactory sensory cells exhibit characteristic response spectra in that each single receptor cell is excited to different degrees by many substances. The relative sensitivity to various substances, at given concentrations, varies from one cell to another.

Updated 8/29/97. Send mail to Food Resource, Oregon State University, Corvallis, OR.