The very dense synaptic region between olfactory neurons and mitral cells is called glomerulus . It is in the glomeruli that odors are actually first represented through a great degree of signal convergence. An additional computational element is provided by granule cell inhibitory interneurons, which laterally connect different mitral cells. All projections eventually converge onto the secondary olfactory cortex, where olfactory perception is represented non-topographically. In rats and mice, the olfactory lining is divided into four zones, each containing neurons with different odor receptors.
Neurons expressing the same receptor genes within each zone appear to be randomly arranged.
In the olfactory bulb, however, fibers from neurons with the same receptor converge on just one or a few glomeruli. Research suggests that an individual odor molecule stimulates several types of receptors, each of which responds to a part of the molecule's structure. Brain mapping techniques have shown that the pattern of glomeruli activated by each odor forms a map or code that the brain may recognize as a unique scent. Recently, researchers identified a surprisingly large family of genes in rats that appears to code for odor receptors. Olfactory receptors are very specific.