Evidence for morphologically, neurochemically and functionally heterogeneous classes of mitral and tufted cells in the olfactory bulb

Abstract

A growing body of evidence supports the view that the mitral and tufted cells of the olfactory bulb are not a homogeneous population of principal neurons but instead are comprised of several morphologically and physiologically distinct categories. Furthermore, the mitral and tufted cells and their central circuits appear to be organized as functionally defined parallel pathways. We review some of the evidence for heterogeneity among mitral and tufted cells and discuss its functional implications. This evidence indicates that mitral cells and deeply situated tufted cells of the main olfactory bulb (MOB) participate in broadly convergent and divergent patterns of intrabulbar and more central interactions involving reciprocal interconnections with large rostral-caudal expanses of the olfactory cortex. These interconnections lack a prominent sector-to-sector topographic organization. In contrast, superficially situated tufted cells appear to integrate information over more spatially restricted regions of the MOB, utilize different transmitters or neuromodulators, and participate selectively in interbulbar or intrabulbar interactions that are rhinotopically organized. The superficially situated tufted cells are likely to function in ways that tend to preserve and possibly enhance spatially and temporally distributed information across populations of neurons. The topographic organization of their intra- and interbulbar connections is complementary to the spatial patterns of 2-deoxyglucose labeling that have been reported in the MOB in response to stimulation with particular odorants or animal scents. The superficially situated tufted cells thus may play a key role in processing the sensory information that is reflected in these patterns of labeling.