Transducing and relaying odorant information centrally from olfactory receptor neurons are only the first steps in processing olfactory signals. As the olfactory receptor axons leave the olfactory epithelium, they coalesce to form a large number of bundles that together make up the olfactory nerve (cranial nerve I). Each olfactory nerve projects ipsilaterally. The target of the olfactory nerve on each side is the olfactory bulb, which lies on the ventral anterior aspect of the ipsilateral forebrain.

The most distinctive feature of the olfactory bulb is an array of more or less spherical accumulations of neuropil 100–200 μm in diameter called glomeruli, which lie just beneath the surface of the bulb and receive the primary olfactory axons (Figure 15.8A-C). In addition to these structures, the bulb comprises several cell and neuropil layers that receive, process, and relay olfactory information.

Within each glomerulus, the axons of the receptor neurons contact the apical dendrites of mitral cells, which are the principal projection neurons of the olfactory bulb (Figure 15.8B). The cell bodies of the mitral cells are located in a distinct layer deep to the olfactory glomeruli (Figure 15.8C) and, in adults, extend a primary dendrite into a single glomerulus, where the dendrite gives rise to an elaborate tuft of branches onto which the primary olfactory axons synapse. Each glomerulus in the mouse (where glomerular connectivity has been studied quantitatively) includes the apical dendrites of approximately 25 mitral cells, which receive innervation from approximately 25,000 olfactory receptor axons. This degree of convergence presumably serves to increase the sensitivity of mitral cells to ensure odor detection, and to increase the signal strength by averaging out uncorrelated noise. Each glomerulus also includes dendritic processes from two other classes of local circuit neurons: tufted cells and periglomerular cells (approximately 50 tufted cells and 25 periglomerular cells contribute to each glomerulus) (see Figure 15.8B). Although it is generally assumed that these neurons sharpen the sensitivity of individual glomeruli, their function is unclear. Finally, granule cells in the olfactory bulb synapse primarily on the basal dendrites of mitral cells within the external plexiform layer (Figure 15.8C,D). These cells make dendrodendritic synapses on mitral cells, and are important for establishing local lateral inhibitory circuits in the olfactory bulb.

Olfactory receptor neurons that express a distinct odorant receptor molecule project to bilaterally symmetrical subsets of glomeruli (Figure 15.8E). Thus there is a special zone-to-zone projection between individual glomeruli in the olfactory bulb and groups of olfactory receptor neurons. As already mentioned, however, there is no obvious systematic representation in this arrangement as there is, for example, in the somatic sensory or visual system. Rather, there is an affinity between widely distributed cells in the olfactory epithelium and ensembles of target glomeruli. This arrangement suggests that individual glomeruli respond specifically (or at least selectively) to distinct odorants.

Many investigations have confirmed the selective (but not uniquely specific) responsiveness of glomeruli to particular odorants using electrophysiological methods, voltage-sensitive dyes, and, most recently, intrinsic signals that depend on blood flow (Figure 15.9). Such studies have also shown that increasing the odorant concentration increases the activity of individual glomeruli, as well as the number of glomeruli activated.

Figure 15.9

Glomerular activity recorded by optical imaging (see Box C in Chapter 12). Dorsal surface of the olfactory bulb in a living rat monitored as increasing concentrations of amyl acetate are presented to the animal. The higher the concentration, the more (more...)