Topography and Dynamics of the Olfactory System

The chemical senses—taste and smell—are evolutionarily the most ancient animal senses. They are characterized by a multidimensional and diverse stimulus space, consisting of many odorous molecules that cannot be classified along any small set of dimensions. Animals detect these odors with specialized olfactory sensory neurons that express one or a few ligand-binding odorant receptor (OR) proteins. Animals cope with the problem of recognizing an extremely large number of different odorants by programming a very large number of functionally different olfactory neurons. Odors activate these neurons and generate characteristic activity patterns across the population of these receptors, which are relayed to second-order olfactory neurons. The entire available raw information about the animal’s olfactory environment is present in these patterns; however, olfactory information is further processed before it is relayed to higher-order brain centers. This chapter reviews the role played by individual neurons in microcircuits in the first olfactory synapse in modulating this information. How is information extracted about odor quality, odor concentration, and scent components? Identity, spatial, and temporal coding mechanisms are discussed.