PHYSICOCHEMICAL BASIS OF OLFACTION

Within the last three decades, many characteristics of olfactory sensing in organisms have been established. Most of this information is semiquantitative at best, and further analysis of the physicochemical processes involved in the sensing sequence is necessary to pinpoint directions for future studies and to develop even a crude model of olfactory chemosensors. It is generally accepted that odorivector molecules must reach chemosensors physically; liaison by radiation through distance has never been proved and seems highly un1ikely.I The following scheme of the olfaction process is consistent with the known facts, e.g., in tests by olfactory panels. When maximum sensitivity is desired, relatively odorless air in the nasal cavity is suddenly replaced by air containing the odorivector. The replacement brings in a volume of air with n, molecules of the odorivector per ~ m . ~ Initially, this bulk volume is separated from the sensors by a boundary layer of odorivector-free air and by a film of mucus which bathes the olfactory cilia. The first steps in olfaction must, therefore, include diffusion of the odorivector through the air and the mucus layers. The concentration of odorivector in the sensor plane then builds up, the odorivector molecules attach to the sensor, and the electrical activity in the nerve leading to the olfactory bulb changes. Various fundamental and empirical physicochemical relations can be applied to this general scheme of the olfaction process.