Olfactory axon convergence

Several hundreds of primary olfactory axons converge on a single mitral cell, located on an olfactory synapse of the olfactory bulb. 

Central/Tertiary structures The fish olfactory bulb is a fourlayered structure much as in higher vertebrates. Within the 2nd layer, the first synapse for olfactory input is on the dendrites of the mitral cells (MC). About 1000 ORN axons converge on one MC, a ratio similar to mammals. The MC output, from cells at various levels, leads into several glomeruli and receives (inhibitory) input from granule cells. The latter also innervate a distinct cell type in the MC layer of teleosts — the ruffed cells (RC), with which they have reciprocal synapses [Fig. 2.18(a)] both relay cells send ascending fibres to forebrain centres (Kosaka and Hama, 1982). The RC are unlike the MC since they are not stimulated by the ORNs directly. Their interactions (Chap. 5) may contribute to the processing of pheromonal stimuli (Zippel, 2000). The main bulbar pathways project to several nuclei in the forebrain via two ipsilateral tracts, the lateral and medial [Fig. 2.18(b)], the latter mediates sexual behaviour and the former probably other behaviours (Hara,... 

The olfactory epithelium of mammals contains many types of olfactory neurons, each expressing a specific odorant receptor. Linda Buck has shown that an odorant can activate multiple distinct receptors and that a receptor can be activated by multiple odorants. Thus, there must exist a combinatorial mechanism for odor detection some sort of pattern recognition. The axons of olfactory neurons converge on glomeruli in the olfactory bulb. There, incoming signals are integrated and the sense of smell is created. 

Chesler AT, Zou DJ, Le Pichon CE, Peterlin ZA, Matthews GA, Pei X, Miller MC, Firestein S (2007) A G protein/cAMP signal cascade is required for axonal convergence into olfactory glomeruli.Proc Natl Acad Sd USA 104(3) 1039-1044... 

FIGURE 5.7 Projection ofreceptor input from olfactory epithelium onto glomeruli in the main olfactory bulb in mice. The epithelium is organized into four zones defined by expression of odorant receptors. Olfactory neurons of a particular zone project to a corresponding zone in the bulb. Axons of these olfactory neurons that express the same odorant receptor (such as those shown in black) converge to a small number of glomeruli. AOB, accessory olfactory bulbs, NC, nucleus coeruleus. (From Mori etal, 1999.)... 

Figure 7 Schematic diagram demonstrating the connection system between the nasal odor receptors and the (main) olfactory bulb. Sensory neurons expressing identical odorant receptors converge their axons to a limited number of defined glomeruli. AOB, accessory olfactory bulb NC, neocortex. Reproduced from K. Mori H. Nagao Y. Yoshihara, Science 1999, 286, 711-715, with permission from AAAS.

Trinh K, Storm DR (2003) Vomeronasal organ detects odorants in absence of signaling through main olfactory epithelium. Nat Neurosci 6(5) 519—525 Tsuboi A, Miyazaki T, Imai T, Sakano H (2006) Olfactory sensory neurons expressing class I odorant receptors converge their axons on an antero-dorsal domain of the olfactory bulb in the mouse. Eur J Neurosci 23(6) 1436-1444... 

Del Punta K, Puche A, Adams NC, Rodriguez I, Mombaerts P (2002b) A divergent pattern of sensory axonal projections is rendered convergent by second-order neurons in the accessory olfactory bulb. Neuron 35 1057-1066... 

Olfactory receptor neurons project their axons towards the olfactory bulb, where they converge to terminate in several tens to several thousands of glomeruli, depending on the species. Injection of a single glomerulus with the fluorescent tracer Dil labeled widespread receptor neuron somata in the olfactory epithelium of zebrafish (Baier et al. 1994). Thus the projection pattern of receptor neurons is non-topological, i.e. neighborhood relationships are not retained. It follows that the spatial response pattern of the sensory surface is not maintained in the olfactory bulb. However, this does not allow us to infer whether the response patterns in the olfactory bulb are localized or non-localized. 

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