Primary Olfactory Neuron Subclasses

Abstract

Vertebrate nervous systems contain hundreds of groups of neurons that can be identified as distinct categories by their anatomical location and cellular morphology. In recent years, the increasing number of physiological and molecular parameters that can be determined has permitted many of these groups to be divided into subsets with discrete characteristics. On the basis of gross anatomical and cell morphological classifications, the peripheral olfactory system contributes only a single recognized cell type to neuronal diversity: the primary olfactory neuron or olfactory receptor neuron. In contrast to this single anatomical cell type, most theories of olfactory function suggest that multiple physiological groups of primary olfactory neurons are present. Individual theories imply that the actual number of functionally distinct categories of olfactory neurons is either 1 or 4 or 20 or 106. Using more recently developed morphological and biochemical criteria, about 101 different olfactory neuron groups can be defined. These groups are identified by dividing the olfactory neuron population into subsets with or without a particular characteristic. As these characteristics have been defined in several laboratories, the relationships among these olfactory neuron subsets are presently not known. Of more importance, the significance of these subsets for olfactory function is also unknown. This chapter reviews current information on olfactory neuron subclasses and then suggests additional approaches to determining whether the class of olfactory neurons is a unitary group of purebred and interchangeable units or whether like many other neuronal systems it is a mongrel pack of individuals with diverse capabilities.