Parallel tufted cell and mitral cell pathways from the olfactory bulb to the olfactory cortex

Abstract

In the mammalian olfactory system, sniff-induced odor signals are conveyed from the olfactory bulb to the olfactory cortex by two types of projection neurons, tufted cells and mitral cells. This chapter summarizes recent advances in knowledge of the structural and functional differences between tufted cell and mitral cell circuits. Tufted cells and mitral cells show distinct patterns of lateral dendrite projection and make dendrodendritic reciprocal synaptic connections with different subtypes of granule cell inhibitory interneurons. Tufted cells and mitral cells thus form distinct local circuits within the olfactory bulb: small-scale tufted cell dendrodendritic circuits and larger-scale mitral cell dendrodendritic circuits. In addition, tufted cells and mitral cells differ dramatically in their axonal projection to the olfactory cortex. Individual tufted cells project axons to focal targets in the olfactory peduncle areas, whereas individual mitral cells send axons in a dispersed way to nearly all areas of the olfactory cortex, including nearly all parts of the piriform cortex. Furthermore, tufted cells and mitral cells differ strikingly in how they respond to odor inhalation. Compared with mitral cells, tufted cells show earlier-onset, higher-frequency spike discharges. Tufted cells are activated at a much lower odor concentration threshold than activating mitral cells. During an inhalation-exhalation sniff cycle, tufted cell circuits generate early-onset fast gamma oscillation while mitral cell circuits give rise to later-onset slow gamma oscillation. From these structural and functional differences, we hypothesize that the two types of projection neurons play distinct roles in sending sniff-induced odor signals to the olfactory cortex. Specifically, tufted cells provide specificityprojecting circuits that send specific odor information to focal targets in the olfactory peduncle areas with early-onset fast gamma synchronization. In contrast, mitral cells give rise to dispersed-projection feed-forward "binding" circuits that transmit the response synchronization timing via their later-onset slow gamma synchronization to pyramidal cells distributed across all parts of the piriform cortex.