Mammals main olfactory system

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). 

It is not always clear how pheromone signals are detected in mammals. Most vertebrates, mice for example, have a VNO in addition to the main olfactory system. The VNO has two separate families of olfactory receptors Vlr, 137 functional receptors in mice V2r, 60 functional receptors in mice. The genes for these are only distantly related to those for the main olfactory receptors, suggesting that these systems evolved independently. As a general rule, it is the VNO and not the olfactory epithelium that is responsible for detecting pheromone molecules. However, it has been demonstrated that mice whose VNO has been surgically removed can discriminate MHC-determined odor types. This finding clearly implicates the main olfactory system in the detection of pheromones. 

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). 

In mammals, the main olfactory system is the work horse in the perception of odors. Excellent detailed reviews of the mammalian olfactory system are available elsewhere. In brief, the olfactory epithelium is located on a portion of the scroll bones (endoturbinales and posterior part of nasoturbinales F ig. 5.4), in humans it is located about 1 cm beneath the bridge of the nose. Olfactory reception is affected by several factors, such as the size, shape, or wetness of the nasal passages. In the dog, the olfactory membrane extends over 75-150 cm depending on body size, while in humans it is only 2-4cm. It consists of three... 

In addition to the main olfactory system, most mammals possess a well-developed vomeronasal system that is commonly regarded to be specialized for the transmission of pheromonal information [241]. 

Fig. 4.4(a) Development of the N. terminalis in mammals and relationship to Vomeronasalis system (i) f = filia olfactoria (ii) induction of olfactory bulb and (iii) to (iv) separation of main/accessory systems (v,o) — from t, Nt system g, Nt ganglion + p/c, peripheral and t, central fibres (from Oelschlaeger, 1989). 

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