Main olfactory bulb organization

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. 

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). 

Kosaka T, Kosaka K, Heizmann CW, Nagatsu I, Wu JY, et al. 1987d. An aspect of the organization of the GABAergic system in the rat main olfactory bulb Laminar distribution of immunohistochemically defined subpopulations of GABAergic neurons. Brain Res 411 373-378. 

Macrides F, Schneider SP. 1982. Laminar organization of mitral and tufted cells in the main olfactory bulb of the adult hamster. J Comp Neurol 208 419-430. 

Schoenfeld TA, Macrides F. 1984. Topographic organization of connections between the main olfactory bulb and pars externa of the anterior olfactory nucleus in the hamster. J Comp Neurol 227 121-135. 

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. 

The olfactory bulb is an allocortex that, like other cortical structures, has a characteristic laminar organization. The layers of the main olfactory bulb and their principal cell types are discussed next (Fig. 2). 

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. 

The peripheral organ for the olfactory system is the olfactory sensory epithelium, located in the dorsal-posterior portion of the nasal cavity. This epithelium, composed of several different types of cells, contains bipolar neurons (also known as olfactory receptor cells) whose axons extend to the brain and terminate in the main olfactory bulb. The VN organ is also situated in the periphery, and similarly contains several different types of cells, the most numerous being bipolar neurons whose axons terminate in the accessory olfactory bulb. The VN organ opens into the roof of the mouth via a very narrow channel, the VN duct. In snakes the tongue delivers odorants to the opening of the VN duct and, by a mechanism at present not understood, these odorants reach the dendritic tips of the bipolar neurons (Halpern and Kubie, 1980). 

Figure 1. A. Sagittal section of the anterior telencephalon stained with antibody to olfactory marker protein. Ant=anterior AOB, AOB=accessory olfactory bulb, FCx=frontal cortex, MOB=main olfactory bulb, Post=posterior AOB. Rostral is to the left. B. Vomeronasal organ stained by the NissI method. Bv=blood vessel, Lu=lumen, S=septum arrows point to sensory epithelium.

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... 

Fig. 20. Olfactory epithelium projections to the MOB. Photomicrographs of sagittal sections through the olfactory bulb In sections stained for Nissl (A) or with WGA HRP after injection of the tracer in the olfactory epithelium (B). Note that most of the olfactory bulb is comprised by the main olfactory system while a small portion of the dorsocaudal bulb is occupied by the accessory olfactory bulb in the rat. Note also in B that the WGA HRP did not transport to the glomeruli of AOB since the tracer did not gain access to the vomeronasal organ that is embedded in the nasal septum. Bar in B, 1 mm.

The accessory olfactory bulb (AOB) has some similar cytoarchitectural features to the MOB, but is much smaller (Fig. 20, 21). The AOB is located at the caudal-dorsal end of MOB. The vomeronasal nerve transmits information from the vomeronasal organ (VNO) to the glomeruli of AOB. The AOB does not receive projections from the main... 

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