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In 1991, Linda Buck and Richard Axel reported the discovery of odorant receptor genes and began a series of studies that clarified the foundation of odor perception (5). In rats, there appear to be about 1000 different receptors. Most olfactory sensory neurons in the nose appear to express only one of these genes, and while homologous sensory neurons expressing the same odorant receptor gene are distributed in fairly broad zones stretching from anterior to posterior within the main olfactory epithelium (52,71), they converge in their projections to the brain and connect to only a very few glomeruli in the main olfactory bulb (45,53,72). Glomeruli are roughly spherical masses of neuropil containing the synapses between sensory neuron axons and the dendrites of bulbar projection neurons and interneurons. The typical projection from homologous sensory neurons involves one glomerulus on the lateral aspect and one on the medial aspect of the bulb (45), and all synapses within a glomerulus appear to be from sensory neurons expressing a single receptor (69). The topography of the epithelium-to-bulb connection is such that sensory neurons located in the central channel of the olfactory epithelium project to dorsal parts of the bulb, whereas peripheral and ventral parts of the epithelium project more ventrally (60). There are also atypical sensory neurons that cluster in the epithelium and that make an unpaired projection to the extreme ventral part of the bulb (64,65).
Projection pattern of homologous sensory neurons.
This orderly pattern of projection, which is consistent across different animals, suggests that one can get a read-out of odorant receptor activation by studying the pattern of odorant-evoked neural activity across the glomerular layer of the rat olfactory bulb. These patterns of activity have been investigated using uptake of radiolabeled 2-deoxyglucose (2DG) as a metabolic marker, and this web site is intended to share these patterns as well as the insights that have been gained from them.
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