Accessory neurons

Each sensory afferent neuron connects with an interneuron or accessory neuron. These interneurons are located entirely within the CNS, with the majority occurring in the cerebral cortex. They form numerous interconnections and are the means by which all cognitive information, thoughts and feelings, are processed. It should be emphasised that the main role of this processing of information is inhibitory. The sensory receptors provide the CNS with a massive amount of data. The interneurons process and filter this into a limited amount of useful and important informa tion. Conscious information processing forms just one part of this activity. A great deal of brain activity is concerned with routine processes, which continue without conscious awareness. 

The vertebrates show many morpho-functional variants on a basic theme (Chap. 2). Some of these, such as the pattern of distribution of the genetically distinct chemosensory neurones within die VN epithelium, will be related to the level of complexity of the animal. In some groups, the VNO can be equally complex, whilst the accessory areas of the brain will differ in complexity, as in the advanced reptiles and mammals. Eventually, detailed comparisons of the genomic repertoire of the various accessory systems should reveal the extent of the operational distinctions amongst them. Of particular interest would be the events which account for the suppression of AOS morphogenesis, and those which compensate for its absence. 

The chemosensory stem cells give rise to several types of neuronal and non-neuronal cell lines under the influence of multiple organisers. From a ventro-lateral infolding, the olfactory pit is produced and this invagination soon becomes separated into two areas which will produce the main and accessory olfactory neurones [Figs. 4.2(a)-(d)]. 

Dudley C.A. and Moss R.L. (1999). Activation of an anatomically distinct subpopulation of accessory olfactory bulb neurons by chemosensory stimulation. Neuroscience 91, 1549-1556. 

Inouchi J., Kubie J. and Halpem M. (1989). Accessory olfactory bulb neurones respond to liquid odourants delivered to vomeronasal organ. Chem Senses 14, 712. 

Jia C., Goldmann J. and Halpem M. (1997). Development of vomeronasal receptor neurons subclasses and establishment of topographic projection to accessory olfactory bulb. Dev Brain Res 102, 209-216. 

Jia C. and Halpem M. (1996). Subclasses of vomeronasal receptor neurons differential expression of G-proteins (Gi-alpha2 Go-alpha) and segregated projections to the accessory olfactory-bulb. Brain Res 719, 117-128. 

Kruzhalov N.B. (1980). Neuronal reactions of the accessory olfactory bulb in the frog, Rana temporaria, to chemical stimulation of the vomeronasal organ. Zh Evol Biokhim Fizio 16, 587-592. 

Kumar A., Dudley C. and Moss R. (1999). Functional dichotomy within the vomeronasal system distinct zones of neuronal activity in the accessory olfactory bulb correlate with sex-specific behaviors. J Neurosci 19, 1-6. 

Li C.S., Kaba H., Saito H. and Seto K. (1992b). Estrogen infusions into the amygdala potentiate excitatory transmission from the accessory olfactory bulb to tuberoinfundibular arcuate neurons in the mouse. Neurosci Lett 143, 48-50. 

Licht G. and Meredith M. (1987). Convergence of main and accessory olfactory pathways onto single neurons in the hamster amygdala. Exp Brain Res 69, 7-18. 

Llahi S. and Garcia-Verdugo J.M. (1989). Neuronal organization of the accessory olfactory bulb of the lizard Podarcis hispanica a Golgi study. J Morphol 202, 13-28. 

Reger R.L., Gerall A.A., et al. (1987). LHRH neuronal system in the accessory olfactory bulb of the prairie vole, Microtus ochrogaster. Neurosci Abs 13, 993. 

Schwarting G.A., Drinkwater D. and Crandall J.E. (1994). A unique neuronal glycolipid defines rostrocaudal compartmentalization in the accessory olfactory system of rats. Brain Res 78, 191-200. 

Takigami S., Mori Y. and Ichikawa M. (2000). Projection pattern of vomeronasal neurons to the accessory olfactory bulb in goats. Chern Senses 25, 387-393. 

Tanaka M., Treloar H., Kalb R.G., Greer C.A., et al. (1999). G(o) protein-dependent survival of primary accessory olfactory neurons. Proc Natl Acad Sci 96, 14106-14111. 

Inspiratory neurons of the VRG augment inspiratory activity. These neurons descend to the spinal cord where they stimulate neurons that supply the accessory muscles of inspiration including those that innervate the scalenus and sternocleidomastoid muscles. Contractions of these muscles cause a more forceful inspiration. 

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). 

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). 

Many pheromone-responsive protocerebral neurons have arborizations in the lateral accessory lobes (LALs), which are situated lateral to the central body on each side of the protocerebrum and appear to be important for processing of olfactory information (86). Each LAL is linked, by neurons with arborizations in it, to the ipsilateral superior protocerebrum as well as the lateral protocerebrum, where axons of AL... 

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.)... 

Sara Y, Virmani T, Deak F et al (2005) An isolated pool of vesicles recycles at rest and drives spontaneous neurotransmission. Neuron 45 563-73 Sato M, Blumer JB, Simon V et al (2006) Accessory proteins for G proteins partners in signaling. Annu Rev Pharmacol Toxicol 46 151-87... 

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