Olfactory bulb processing

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). 

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,... 

In fish, certain odorants activate specific glomeruli in the olfactory bulb. Amino acids stimulate glomeruli in the lateral region of the bulb, while glomeruli in the medial region of the bulb process information on bile acids (Yoshihara etah, 2001). 

The taste cells are situated in the lingual epithelium with the apical membrane exposed to the mucosal surface of the oral cavity and the basal surface in contact with the nerve [interstitial fluid] [FIGURE 10]. Within the basolateral surface are the nerves which respond to the chemestiietic stimulants, i.e. direct nerve stimulation. The microvilli at the apical membrane contain receptor proteins which respond to sweeteners, some bitters and possibly coolants. The olfactory cells are bipolar neurons with dendritic ends containing cilia exposed to the surface and axons linked to the brain, where they synapse in the olfactory bulb. The transfer of information from this initial stimulus-receptor interaction to the brain processing centers involves chentical transduction steps in the membrane and within the receptor cells. The potential chemical interactions at the cell membrane and within the cell are schematically outlined in FIGURE 10. 

It seems that the cortex contains a crude "map" that relates position to the type of smell.912 The neural processing involved in the discrimination of odors is not yet clear.912 913 Intemeurons of the olfactory bulb are unusual, being continuously discarded and replaced by new neurons that arise from neural stem cells.908 914 This process seems to be essential for odor discrimination but not for the sensitivity of odor detection. 

Mu receptors are almost always located proximally, on the presynaptic side of the synapse. The periaqueductal gray is the region containing the most mu receptors, but they are also found in the superficial dorsal hom of the spinal cord, the external plexiform layer of the olfactory bulb, the nucleus accumbens (an area deeply implicated in the process of addiction), in some parts of the cerebral cortex, and in some of the nuclei of the amygdala. Mu receptors avidly bind enkephalins and beta-endorphin, but they have a low affinity for dynorphins (primarily a kappa receptor agonist).6... 

Boeckh J., Distler P., Ernst K., Hosl M. and Malun D. (1990) Olfactory bulb and antennal lobe. In Chemosensory Information Processing, ed. D. Schild, pp. 201-228. Springer Heidelberg, Germany. 

Mitral and tufted cells in the olfactory bulb project their axons to the olfactory cortex, the site thought to integrate the signals from distinct glomeruli. The olfactory signals processed in the olfactory cortex are sent to a variety of higher centers of the brain, which include insular cortex, orbitofrontal cortex, amygdale, hippocampus, and the nucleus accumbens.205... 

One thousand or more different olfactory receptors transmit their signals through the axons of olfactory neurons. Olfactory neurons are clustered in the olfactory epithelium, and project their axons to a small number of topographically fixed loci, the omeruli, in the olfactory bulb of the brain, where the information is processed. Each individual glomerulus in the olfactory bulb apparently corresponds to a distinct type of receptor. Olfactory neurons have specific marker proteins (OMP, olfactory marker protein) which have facilitated the localization of olfactory neurons. 

K. Mori, H. Nagao, and Y. Vbshihata. The olfactory bulb Coding and processing of odor molecule infotination. Science, 2S6, 7II-7I5, 1999. 

Olfactory signals that leave the olfactory bulbs travel by several routes to the higher centers of the brain where the phenomenon of odor develops eventually. Remarkably, the architecture of the olfactory parts of the brain is consistent across all mammahan species research on other animals throws considerable fight on the function in humans. The entire process is well reviewed by Wilson and Stevenson (4) and by Delano and Sobel (10). 

How is the information about which receptors have been activated transmitted to the brain Recall that each neuron expresses only one OR and that the pattern of expression appears to be largely random. A substantial clue to the connections between receptors and the brain has been provided by the creation of mice that express a gene for an easily detectable colored marker in conjunction with a specific OR gene. Olfactory neurons that express the OR-marker protein combination were traced to their destination in the brain, a structure called the olfactory bulb (Figure 32.8). The processes from neurons that express the same OR gene were found to connect to the same location in the olfactory bulb. 

Figure 32.8. Converging Olfactory Neurons. This section of the nasal cavity is stained to reveal processes from sensory neurons expressing the same olfactory receptor. The processes converge to a single location in the olfactory bulb. [From P. Mombaerts, F. Wang, C. Dulac, S. K. Chao, A. Nemes, M. Mendelsohn, J. Edmondson, and R. Axel. Cell 87(1996) 675-689.]...

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