Neuronal Architecture and Functional Organization of Olfactory Glomeruli

In the antennal lobes of insects and olfactory bulbs of vertebrates, the primary processing of olfactory information occurs within specialized units, called glomeruli. Glomeruli are discrete areas of densely packed, fine neuropil, usually ensheathed in glia cells. Glomeruli are the sites of synaptic interaction between axons of olfactory receptor cells and dendrites of central olfactory neurons. This chapter reviews the functional significance of this neuronal architecture, the glomerulus, with particular emphasis on results obtained in the sphinx moth, Manduca sexta. How is neuronal circuitry of olfactory glomeruli functionally organized, what attributes of olfactory stimuli are analyzed in glomeruli and how are these attributes processed and encoded in them? Glomeruli have been found in different invertebrate groups, such as crustaceans and insects with the glomeruli in the antennal lobes and the deutocerebrum, and molluscs with subepithelial glomeruli in the tentacle, as well as in different vertebrate groups such as amphibians, birds, fish, and mammals with glomeruli in the olfactory bulb. The organization of primary olfactory centers into glomeruli in diverse species suggests that glomeruli have a common and fundamental function in the processing of information about chemosensory stimuli and that glomeruli across taxa may share similar means of processing olfactory input.