Cell olfactory bulb

Hippocampus (pyramidal cells) olfactory bulb (granule cells, periglomerular cells) cerebral cortex amygdala hypothalamus superior colliculus superior olivary neurons spinal trigeminal neurons spinal cord... 

Many brain areas are innervated by neurons projecting from both the locus coeruleus and the lateral tegmental system but there are exceptions (Fig. 8.3). The frontal cortex, hippocampus and olfactory bulb seem to be innervated entirely by neurons with cell bodies in the locus coeruleus whereas most hypothalamic nuclei are innervated almost exclusively by neurons projecting from the lateral tegmental system. The paraventricular nucleus (and possibly the suprachiasmatic nucleus, also) is an exception and receives an innervation from both systems. 

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,... 

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Zippel H., Gloger M., Luthje L., Nasser S., et al. (2000). Pheromone discrimina ability of olfactory bulb mitral and ruffed cells in the goldfish. Chem Senses 339-349. 

FIGURE 29-5 In the adult rodent brain, dividing cells in the subventricular zone adjacent to the lateral ventricle migrate in the rostral migratory stream (RMS) and differentiate into olfactory neurons in the olfactory bulb (OB). This is one site of neuronal turnover in the adult that appears to result from persistent generation of neurons from adult CNS stem cells. CIS, cerebellum NC, neocortex. 

Astic, L., Saucier, D. and Holley, A. Topographical relationships between olfactory receptor cells and glomerular foci in the rat olfactory bulb. Brain Res. 424 144—152, 1987. 

Ressler, K. ]., Sullivan, S. L. and Buck, L. B. Information coding in the olfactory system evidence for a stereotyped and highly organized epitope map in the olfactory bulb. Cell 79 1245-1255,1994. 

Vassar, R. et al. Topographic organization of sensory projections to the olfactory bulb. Cell 79 981-991,1994. 

By contrast, the accessory olfactory system is thought to be involved in the detection of odors that influence a variety of reproductive and aggressive behaviors (Keverne 1999). Sensory neurons are located in the vomeronasal organ (VNO) and detect pheromones which gain access to the VNO by a pumping mechanism (Meredith and O Connell, 1979). VNO neurons send projections to the accessory olfactory bulb (AOB). Mitral cells of the AOB project in turn to the medial nucleus of the amygdala olfactory information is then dispatched to several hypothalamic regions such as the bed nucleus of the stria terminalis, the medial preoptic area and the ventromedial hypothalamus (Scalia and Winans 1975). 

Olfactory glomeruli must have evolved early, because these characteristic structures are present in the "olfactory brains" of modem representatives of ancient marine groups including molluscs (5) and crustaceans (6). Likewise the lampreys, which are extant representatives of the most primitive vertebrates, have relatively large olfactory bulbs with glomeruli and conspicuous mitral cells not unlike those of more advanced vertebrates (7). 

An important insight from many studies (28) is that the response patterns—the molecular images—at various levels in the central olfactory pathway are set up by the differential responses of the ORCs in the peripheral receptor epithelium. These studies also suggest that functional modules, which may correspond to recognizable structural units such as individual glomeruli with their associated cells, in the olfactory bulb or lobe participate in the analysis of olfactory information conveyed to them... 

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