The Physics and Biology of Olfaction and Taste

Traditionally, the chemical senses include olfaction and gustation: olfaction (smell) reports on gaseous substances carried by air over long distances, and gustation (taste) provides information about food material already in the mouth. However, in recent times behavioral, physiological, histological, and molecular studies have increased our understanding of vertebrate chemoreception, and four chemical senses are recognized by some authors, reflecting the diversity in function and physiology (Dulac and Axel, 1995; Eisthen, 1997; Zufall and Munger, 2001; Mombaerts, 2004; Breer et al., 2005). For the purpose of this volume, three chemical senses are important: olfaction, vomeronasal sense, and gustation (Fig. 7.1 in this volume). It is difficult to distinguish sharp borders between the various chemosensory systems, especially in aquatic vertebrates. For example, the catfish Ictalurus natalis carries on its barbels densely packed taste buds. These taste buds appear to report on dissolved substances in the surrounding water (Caprio, 1988). Histologically, the barbels’ sensory buds serve taste, but functionally their role seems to overlap with olfaction: catfish successfully orient toward distant “olfactory stimuli” even after bilateral destruction of the olfactory tract (Bardach et al., 1967). A similar quasi-olfactory role has also been described for the tongue of the dolphin Tursiops truncatus (Kuznetzov, 1990), and Schmidt and Wöhrmann-Repenning (2004) have reported histological observations indicating a close interaction between taste buds and vomeronasal organ. Molecular transduction mechanisms of the sensory cells of the olfactory, vomeronasal, and gustatory system are also similar, as are the microstructures of the distal parts of the receptor cells (Eisthen, 1992, 1997; Reiss and Eisthen, chapter 4 in this volume). For mammals a classification based on the molecular genetics of the receptor proteins may be within reach (Mombaerts, 2004; Grus et al., 2005). However, this volume adheres to an anatomical basis for distinguishing between chemical sense organs, as it embraces the inclusion of