Main olfactory bulb projections

The main olfactory bulb projects to a collection of structures referred to collectively as primary olfactory cortex (De Olmos et al. 1978). These structures may be usefully divided into three groups (A) the anterior olfactory nucleus (Fig. 15) (B) rostral olfac-... 

In its central projections the vomeronasal pathway, distinguished by a unique lectin-affinity, ascends to an accessory olfactory bulb, while dorsal and ventral pathways supply the dorsal and ventral regions of the main olfactory bulb (Saito and Taniguchi, 2000). The AOS (but not the MOS) of salamanders displays considerable diversity in the... 

Giannetti N., Saucier D. and Astic L. (1992). Organization of the septal organ projection to the main olfactory bulb in adult and newborn rats. J Comp Neurol 323, 288-298. 

Two olfactory systems have evolved in terrestrial vertebrates which differ in both their peripheral anatomy and central projections. The main olfactory system is usually conceived as a general analyzer that detects and differentiates among complex chemosignals of the environment (Firestein 2001). Odors are detected by olfactory sensory neurons located in the main olfactory epithelium (MOE) these neurons project to glomeruli in the main olfactory bulb (MOB). The mitral and tufted neurons abutting these MOB glomeruli then transmit olfactory signals to various... 

As with many macrosmatic mammals, rodents have two separate chemosensory systems, the main olfactory system (MOS) and accessory olfactory system (AOS), which respond to social odors. Importantly, these sensory systems differ not only in their peripheral morphology and central projections, but also in the types of chemosignals that they process (Meredith 1991). Sensory neurons of the MOS, which are located in the main olfactory epithelium and project to the main olfactory bulbs, process volatile chemicals and can detect odors at a distance. In contrast, sensory neurons of the AOS, which are located in the vomeronasal organs (VNO) and project to the accessory olfactory bulbs, primarily process large, non-volatile chemicals and require contact for stimulation (Meredith 1991). 

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.)... 

The septal organ is a small patch of sensory epithelium on the wall of the septum, in the anterior part of the nasal cavity, and ventral to the olfactory epithelium. It is found primarily in rodents, has chemical receptors similar to olfactory receptors, and is sensitive to volatile odorants. It projects into the main olfactory bulb, but not into the accessory olfactoiy bulb (Pedersen and Benson, 1986). Because of its forward location, the septal organ may serve as an early-warning system that arouses resting or sleeping animals when volatiles are present (Wysocki, 1989). 

Fig. 3 Vomeronasal system. Schematic representation of a rodent nasal cavity and brain (lateral view). Accessory olfactory bulb (AOB) mitral cells project to vomeronasal and extended amygdala. Inset The VNO is a bilateral tubular structure located at the base of the nasal septum. VSNs that express the same V1R or V2R converge on a small number of glomeruli in the AOB. Sensory neurons located in the apical layer of the epithelium project to the anterior part of the AOB, whereas those present in the basal layer project to the posterior part. MOE main olfactory epithelium, MOB main olfactory bulb, BSTMPM posteromedial bed nucleus of the stria terminalis, MEA medial amygdaloid nucleus, BACfF bed nucleus of the accessory olfactory tract, PMCO posteromedial cortical amygdaloid area...

Carson KA. 1984. Quantitative localization of neurons projecting to the mouse main olfactory bulb. Brain Res Bull 12 ... 

Schoenfeld TA, Marchand JE, Macrides E 1985. Topographic organization of tufted cell axonal projections in the hamster main olfactory bulb An intrabulbar associational system. J Comp Neurol 235 503-518. 

Schoenfeld TA, Clancy AN, Forbes WB, Macrides E 1994. The spatial organization of rhe peripheral olfactory system of the hamster. Part I Receptor neuron projections to the main olfactory bulb. Brain Res Bull 34 183-210. 

All subdivisions of the ipsilateral AON project to both the ipsilateral and contralateral main olfactory bulbs except the external division (AON pars externa, AONpE) which projects only to the contralateral MOB (De Olmos et al. 1978) (Fig. 18A). The AON contains the largest number of neurons projecting to the bulb from any one source (Carson, 1984a). 

The cytoarchitecture of NLOT (Fig. 17C) has been studied extensively by McDonald (1983). It is considered an anterior part of the amygdala. NLOT can be subdivided into 3 layers on the basis of Nissl preparations a superficial plexiform layer I which contains a few small and medium-sized cells, a layer II which contains many tightly packed cells, and layer III located dorsal to layer II and containing fairly large, loosely packed cells. Most cells of NLOT are medium-sized pyramidal shaped with extensive spines on secondary and distal dendrites. According to McDonald (1983), layers I and II appear similar in connections to the piriform cortex while layer III seems to be a closely related subcortical area. Many neurons of layers II and fewer neurons of layer III project to the olfactory bulb (de Olmos et al. 1978 Shipley and Adamek, 1984). In addition to olfactory bulb projections, many axons of NLOT neurons make up the stria terminalis and cross to the contralateral piriform cortex, olfactory tubercle, lateral nucleus of the amygdala, and bed nucleus of the stria terminalis (de Olmos, 1972). Afferent connections to NLOT arise mainly from olfactory related areas and the basolateral nucleus of the amygdala. 

The main olfactory bulb sends a projection to the entire extent of piriform, peri-amygdaloid and lateral entorhinal cortex (see above. Outputs of MOB). This projection terminates in the superficial half of layer I, layer la. Both mitral and tufted cells project to the rostral parts of AON and piriform cortex while the projection to more caudal parts of olfactory cortex becomes progressively dominated by mitral cells (Schoenfeld and Macrides, 1984). 

Davis, B.J. and Macrides, F. (1981) The organization of centrifugal projections from the anterior olfactory nucleus, ventral hippocampal rudiment, and piriform cortex to the main olfactory bulb in the hamster An autoradiographic study, J. Comp. Neurol. 203, 475-493. 

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,... 

Skeen L.C. and Hall W. (1977). Efferent projection of the main and the accessory olfactory bulb in tree Shrew (Tupaia glis). J Comp Neurol 172, 1-36. 

From the accessory bulb, projections lead to the medial nuclei and posteri-omedial portion of the cortical nuclei of the amygdala. The amygdala in turn is connected to higher centers via the stria terminalis, and hypothalamic structures. Thus, the accessory olfactory system represents a neural pathway separate from the main olfactory system. Both project into absolutely different parts of the amygdaloids (Powers and Winans, 1975 Powers etal, 1979). 

Wada E, Shigemoto R, Kinoshita A, Ohishi H, Mizuno N. 1998. Metabotropic glutamate receptor subtypes in ajoan terminals of projection fibers from the main and accessory olfactory bulbs A light and electron microscopic immunohistochemical study in the rat. J Comp Neurol 393 493-504. 

The axons of the external tufted cells project mainly to other sites in the same olfactory bulb (Schoenfeld et al. 1985). Middle and deep tufted cells also have local collaterals in the ipsilateral bulb but most of them appear to project out of the olfactory bulb to the anterior olfactory nucleus and other rostral olfactory cortical structures (Schoenfeld et al. 1985 Scott, 1986). The intrabulbar collaterals of the superficial tufted cells form... 

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