Olfactory Bulb: Functional Architecture

Notes: (a) Illustration of distributed coding in olfactory sensory neurons. Three cilia arising from different OSNs each express a different odorant receptor protein. The three different receptors depicted are each activated to different degrees (vertical bars) by the three-carbon aliphatic odorant propanoic acid. (b) Responses of the same three ORs to the structurally similar four-carbon aliphatic odorant butanoic acid. Receptor I interacts more favorably with this ligand than with propanoic acid, so that the corresponding OSN responds more strongly to butanoic acid at the same concentration. Receptor II, in contrast, does not bind butanoic acid as well as it did propanoic acid, as the polar end of the larger molecule is too far away from its cognate receptor moieties to interact optimally (arrows). Nevertheless, butanoic acid weakly activates the receptor. Finally, butanoic acid cannot fit in the binding pocket of Receptor III (arrow illustrates impossible fit), and hence does not activate it at all. Reading the relational pattern of activation levels across all three receptors (a three-dimensional vector) enables identification of the odor ligand. (c) Illustration of major olfactory bulb circuit elements. OSNs (represented by their respective OR shapes) are distributed across the nasal olfactory epithelium (OE). The axons of OSNs that express the same OR cross the cribriform plate into the cranial cavity and converge together to form glomeruli (shaded ovals) on the surface of the OB. The OSN axon terminals form excitatory synapses (filled triangles) onto mitral (Mi), periglomerular (PG), and external tufted (ET) cells. External tufted cells in turn excite PG cells, superficial short axon (sSA) cells, and each other. Periglomerular cells inhibit mitral cell apical dendrites and OSN axon terminals (open circles). Mitral cell secondary dendrites extend laterally within the external plexiform layer (EPL, shaded box) and form reciprocal synapses with the dendritic spines of inhibitory granule cells (Gr), hence delivering recurrent inhibition onto themselves and lateral inhibition onto other mitral cells. The sSA cells are not affiliated with any given glomerulus, but extend between them, forming a lateral excitatory network consisting of themselves and the ET cells that proliferates in the deep glomerular layer. Shaded ovals connote the approximate physical boundaries of the glomerulus proper, whereas dotted boxes illustrate the group of neurons associated with a particular glomerulus. For visual clarity, only one cell of each type per glomerulus is depicted, and middle/deep (projecting) tufted cells, deep short axon (dSA) cells, and axons arriving from the rest of the brain have been omitted. Other areas shown: LOT = lateral olfactory tract. Olfactory bulb layers, surface to deep: GL = glomerular layer; EPL = external plexiform layer; MCL = mitral cell layer; IPL = internal plexiform layer; GCL = granule cell layer. See also color insert Figure 10.