Parallel Mitral and Tufted Cell Pathways Route Distinct Odor Information to Different Targets in the Olfactory Cortex

Abstract

Odor signals are conveyed from the olfactory bulb to the olfactory cortex (OC) by mitral cells (MCs) and tufted cells (TCs). However, whether and how the two types of projection neuron differ in function and axonal connectivity is still poorly understood. Odor responses and axonal projection patterns were compared between MCs and TCs in mice by visualizing axons of electrophysiologically-identified single neurons. TCs demonstrated shorter onset latency for reliable responses than MCs. The shorter latency response of TCs was maintained in a wide range of odor concentrations, whereas MCs responded only to strong signals. Furthermore, individual TCs projected densely to focal targets only in anterior areas of the OC, whereas individual MCs dispersedly projected to all OC areas. Surprisingly, in anterior OC areas, the two cell types projected to segregated sub-areas. These results suggest that MCs and TCs transmit temporally distinct odor information to different OC targets.

Micro-, Meso- and Macro-Dynamics of the Brain

“…Although this does not rule out the possibility of some fine-scale topographical mapping of OB projections (e.g., mitral vs. tufted cell projections) (Igarashi et al 2012), it is now accepted that the glomerular clustering of olfactory receptor cells axons in OB is entirely discarded at the level of PCx (Wilson and Sullivan 2011). In DCx, early tracing studies from Ulinski and colleagues suggested that the visual field is projected onto the rostro-caudal axis of DCx in the form of iso-azimuth lamellae covering the naso-temporal dimension of the visual field (Mulligan and Ulinski 1990;Ulinski and Nautiyal 1988).…”

A Subtype of Olfactory Bulb Interneurons Is Required for Odor Detection and Discrimination Behaviors

“…A recent study reported that local dendrodendritic circuits within the EPL may segregate into parallel MC and TC (ETC) pathways (Fourcaud-Trocmé et al, 2014). Interestingly, Igarashi et al (2012) reported that TCs have odor-induced firing responses with shorter latencies at lower concentration thresholds than those of MCs. TCs (ETCs) may be specialized in the fast odor detection and fast behavioral responses that are required for simple discrimination between two distinctly different odors, whereas MCs may be specialized in performing the more difficult task of discriminating between two closely related odors such as enantiomers.…”

Olfactory projectome in the zebrafish forebrain revealed by genetic single-neuron labelling

“…Thus, our findings support the idea that the two distinct modes of connectivity patterns (stereotyped and random), which are suitable for innate and learned behaviours, are applicable to all animal species as a fundamental principle for olfactory representations in higher brain centres. As to the axon branching of output neurons innervating the same glomerulus, highly diverse patterns are observed in zebrafish and mouse 6,7 , compared with more stereotyped one in Drosophila 16,17 . Furthermore, we find in zebrafish that choice of target areas in the forebrain by output neurons innervating the same glomerulus is not always the same.…”

“…The odour map in the OB is transmitted by output neurons, mitral cells, to higher olfactory centres and eventually translated to elicit appropriate behavioural and physiological responses. Recent anatomical studies in mice showed that axons from identified glomeruli project diffusely throughout the piriform cortex [6][7][8][9] , the largest target area of the OB, and that piriform neurons receive convergent inputs from multiple mitral cells distributed throughout the OB 10 . These findings are consistent with an optical imaging study that found no apparent spatial organization of odour-evoked activity patterns in the piriform cortex 11 .…”