Functional organization of the main olfactory bulb.
Complete understanding of the role of the mammalian main olfactory bulb in sensory processing has remained elusive despite many detailed studies on its anatomy and physiology. Several lines of recent evidence viewed in the context of earlier knowledge have provided new insights into the bulbar mechanisms of olfactory coding. The output cells of the olfactory bulb receive a localized olfactory nerve input and interneuronal input via dendrodendritic synapses on distinct sets of dendrites. The spatial arrangement of granule cell contacts on output cell basal dendrites suggests that lateral inhibitory interactions may occur between neighboring output cells. The input from olfactory receptor cell axons to the bulb also has spatial order, but does not represent a precise map of the receptor surface. Recent studies with antibodies and lectins suggest that different groups of axons from chemically similar receptor cells collect into certain glomeruli, even if the axons originate from cells that are not contiguous in the mucosa. Electrophysiological studies have begun to explore the participation of spatially organized circuits in olfactory processing. The degree to which neighboring output cells respond similarly to odor stimulation, for example, depends on the distance between the cells, with those further apart showing complementary responses. Also, a single output cell can show 2 or more different temporal response patterns when different odors are presented. Intracellular recordings indicate that these responses are shaped by IPSPs. Electrical stimulation during such recordings shows that some mitral cells are excited by nerve inputs close to their glomerular tufts, while they are inhibited by nerve inputs to other parts of the bulb. Finally, recordings from granule and periglomerular cells indicate their potential in mediating components of output cell odor responses. These considerations suggest that the olfactory bulb performs a spatially based analysis on the information coming from the receptor cells. While the spatial organization of the olfactory bulb is probably not faithfully represented in the projections to the olfactory cortex, bulbocortical projections are not random. The fact that spatial factors exist at each of these levels in the olfactory system must be considered in developing models of central olfactory processing.