Early studies showed that different olfactory neurons respond to different odors and that odors stimulate specific patterns of activity in the olfactory bulb. Scientists also found that the olfactory cilia in fish contain binding sites, or receptors, for odor molecules. Recently, researchers identified a surprisingly large family of genes in rats that appears to code for odor receptors. Scientists think this large and diverse group of genes is what helps animals detect such a huge variety of odors. 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.
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. By studying how olfactory neurons establish connections with other neurons in the adult brain, researchers hope to learn how nerve fibers connect in other brain regions. Unlike most neurons, olfactory neurons that die can be replaced by new neurons that serve the same function. Learning how this process occurs may point to ways of making neurons regrow in other parts of the nervous system.