Olfactory receptor neurons patterns

As is the case for all sensory pathways, the capacity to perceive and respond to olfactory cues (odorants) is the combined result of events that take place in both peripheral and central processing centers. These steps, which will be discussed in detail below, begin with the molecular transduction of chemical signals in the form of odorants into electrical activity by olfactory receptor neurons (ORNs) in the periphery whose axonal projections form characteristic synaptic connections with elements of the central nervous system (CNS). Within the CNS, complex patterns of olfactory signals are integrated and otherwise processed to afford recognition and ultimately, the behavioral responses to the insect s chemical environment. Within the context of pheromone recognition these responses would likely be centered on various elements of the insect s reproductive cycle. 

Merrill C. E., Riesgo-Escovar J. R., Pitts R. J., Kafatos F. C., Carlson J. R. and Zwiebel L. J. (2002) Visual arrestins in olfactory pathways of Drosophila and the malaria vector mosquito Anopheles gambiae. Proc. Natl. Acad. Sci. USA 99, 1633-1638. Moore P. A. (1994) A model of the role of adaptation and disadaptation in olfactory receptor neurons implications for the coding of temporal and intensity patterns in odor signals. Chem. Senses 19, 71-86. 

Hansson B. S., Almaas T. J. and Anton S. (1995) Chemical communication in heliothine moths. V. Antennal lobe projection patterns of pheromone-detecting olfactory receptor neurons in the male Heliothis virescens (Lepidoptera Noctuidae). J. Comp. Physiol. A 177, 535-543. 

B.S. Hansson et al., Chemical Communication in Heliothine Moths. 5. Antermal Lobe Projection Patterns of Pheromone-Detecting Olfactory Receptor Neurons in the Male Heliothis-Virescens (Lepidoptera, Noctuidae), Journal of Comparative Physiology a-Sensory Neural and Behavioral Physiology 177 (1995) 535-543. 

S.A. Ochieng, P. Anderson and B.S. Hansson, Antennal lohe projection patterns of olfactory receptor neurons involved in sex pheromone detection in Spodoptera httorahs (Lepidoptera Nocmidae), Tissue Cell. 27 (1995) 221-232. 

B.S. Hansson, Antennal lobe projection patterns of pheromone-specific olfactory receptor neurons in moths, in Insect pheromone research new directions, R.T. Carde, A.K. Minks (eds.), Chapman Hall New York, (1997), pp. 164-183. 

In insects, odorant molecules are captured by Olfactory Receptor Neurons (ORNs) (-90,000 in the locust) distributed on their antennae. A large number of ORNs that express the same odorant receptor genes converge onto many fewer glomeruli (-900 in the locust), presumably for improved sensitivity. The odorant identity is then robustly represented in the glomerulus layer as a spatially distributed pattern. In the following we will... 

Olfactory receptor neurons project their axons towards the olfactory bulb, where they converge to terminate in several tens to several thousands of glomeruli, depending on the species. Injection of a single glomerulus with the fluorescent tracer Dil labeled widespread receptor neuron somata in the olfactory epithelium of zebrafish (Baier et al. 1994). Thus the projection pattern of receptor neurons is non-topological, i.e. neighborhood relationships are not retained. It follows that the spatial response pattern of the sensory surface is not maintained in the olfactory bulb. However, this does not allow us to infer whether the response patterns in the olfactory bulb are localized or non-localized. 

If the odors of specific objects translate into unitary percepts, which constitute the basic entities in linguistic descriptions of olfaction, then the question follows as to whether these unitary percepts take shape at the level of the receptor neurons or in the olfactory bulb or elsewhere in the brain. That question remains unanswered, as of this writing. Because the sense of smell does not correlate perfectly with externally monitored patterns of electrical response from the receptor neurons or the olfactory bulb, the nature of olfactory coding remains unknown. Outside the laboratory unitary percepts rarely equate to pure compounds. Two vocabularies coexist, one of smells (which varies from individual to individual, and which refers to other inputs besides olfaction) and the other of chemical structures. 

The olfactory epithelium of mammals contains many types of olfactory neurons, each expressing a specific odorant receptor. Linda Buck has shown that an odorant can activate multiple distinct receptors and that a receptor can be activated by multiple odorants. Thus, there must exist a combinatorial mechanism for odor detection some sort of pattern recognition. The axons of olfactory neurons converge on glomeruli in the olfactory bulb. There, incoming signals are integrated and the sense of smell is created. 

J. Strottnann, I., Wanner, T. Helferich, A. Beck, and H. Breer. Rostro-caudal patterning of receptor-etqpressing olfactory neurones in the rat nasal cavity. CeU Tissue Res, 278, 11-20, 1994. 

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