Main olfactory organ

In the frog Rana temporaria, the VNO shares the nasal compartments with the main olfactory organ. The VNO has three cavities, and water enters from the external nares via two fissures. The VNO is used to sample water while the frog is submerged, while above water air is inhaled and the olfactory system stimulated (D0ving etal., 1993). 

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). 

Apart from taste, vertebrates have five different chemoreceptor systems for airborne chemosignals the main olfactory system, the vomeronasal organ (VNO), the trigeminal nerve, the septal organ of Masera, and the nervus termi-nalis. They each will be discussed in turn. All five are fully functional in most mammals (Fig. 5.1). 

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.)... 

Ultrasonic calling Vomeronasal organ and main olfactory system... 

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). 

Although the vomeronasal system is specialized to detect stimuli in a liquid environment, it probably is not functional in utero, at least in mice. Fluorescent microspheres were not taken up by the vomeronasal organ as the access canal is not open yet in utero. In rats, by contrast, the canal is open before birth and the microspheres can be taken up. The olfactory epithelium of the main olfactory system plays a greater role prenatally, as evidenced by the uptake of radiolabeled 2-deoxyglucose (Coppola and Coltrane 1994). Fetal mice respond to amyl acetate and isovaleric acid delivered into the nasal cavity through a tiny cannula (Coppola, 2001). In both rats and mice, the main olfactory system, and not the vomeronasal system, appears to mediate prenatal olfaction (Coppola, 2001). 

In vertebrates, most of the olfactory neurons of the nasal epithelium protrude from the apical dendritic knob a variety of cilia into the protective mucus layer, thus enlarging the sensory surface area of the cells. A subpopulation of sensory neurons in the main olfactory epithelium and all of the neurons in the vomeronasal organ (VNO) are structurally different their apical region of the dendrite extends in an array of microvilli. 

The same is true for odor memory, for it can also be acquired or at least trained. There are large individual differences here, it is certainly more a mental faculty than an attribute of the olfactory organ. An excellent odor memory is indispensable to the perfumer. The student perfumer who after six to nine months of training still has difficulties in recognizing the 200 or so main perfumery materials should seriously question whether it is wise to continue training. 

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... 

In most mammalian species, two functionally distinct classes of chemicals (odorants and pheromones) are detected and processed through anatomically segregated neural pathways the main olfactory system and the vomeronasal (accessory olfactory) system (Buck 2000 Mombaerts 2004). Volatile odorants are received by a large repertoire of ORs expressed on ciliated OSNs in the OE and the information is transmitted to the main OB. By contrast, pheromones are mostly received by two families of vomeronasal receptors (VIRs and V2Rs) expressed on microvillous sensory neurons in the vomeronasal organ that project their axons to the accessory OB. 

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). 

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.

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. 

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