Interneurons in the Ventral Nerve Cord of Insects

Abstract

The majority of neurons in the nervous systems of vertebrates and invertebrates are interneurons (INs). Despite the enormous diversity in the anatomical properties of INs, in mammals at least, most can be readily classified as either relay INs or local-circuit INs (Rakic, 1975). Relay INs are those which function to transmit information from one discrete region in the nervous system to another, while local-circuit INs are those contained wholly within a localized region of the nervous system which function to integrate neural information within these regions. In addition to the anatomical and functional differences in these two classes of INs, there may be significant differences in their electrical properties since many local-circuit INs do not generate action potentials (Rakic, 1975; Pearson, 1976). Although this division of many types of INs into two classes seems reasonable, there are some obvious limitations to this classification. Some relay INs (e.g., mitral cells in the olfactory bulb) interact reciprocally with local-circuit INs and are therefore also involved in local integrative processes. Thus these INs are not strictly relay INs. Furthermore, to define an anatomically discrete region within certain parts of the central nervous system is often not possible, which means that some INs within these areas cannot be classified. To obviate these difficulties, it is probably better not to regard individual neurons as the only functional units within the nervous system (Shepherd, 1972, 1974). Spatially separated regions of certain neurons are now known to have different functions, and each region can interact locally with adjacent neurons (Nelson et al., 1975; Shepherd, 1972). Thus, in some cases, it is better to regard a part of a neuron, or a part of a neuron plus parts of other neurons, as the functional unit (Shepherd, 1972).