Odor coding in a model olfactory organ: The Drosophila maxillary palp

Odor coding relies on the activity of different classes of receptor neurons , each with distinct response characteristics. We have examined odor coding in a model olfactory organ, the maxillary palp of Drosophila. This organ c ontains only 120 olfactory receptor neurons, compartmentalized in sensory h airs called sensilla, and provides an opportunity to characterize all neuro ns in an entire olfactory organ. Extensive extracellular recordings from si ngle sensilla reveal that the neurons fall into six functional classes. Eac h of the 60 sensilla houses two neurons, which observe a pairing rule: each sensillum combines neurons of two particular classes, thereby yielding thr ee sensillum types. The sensillum types are intermingled on the surface of the palp, but their distribution is not random. The neurons exhibit diverse response characteristics, providing the basis for an olfactory code. A par ticular odor can excite one neuron and inhibit another and a particular neu ron can be excited by one odor and inhibited by another. Some excitatory re sponses continue beyond the end of odor delivery, but responses to most odo rs terminate abruptly after the end of odor delivery, with some followed by a period of poststimulus quiescence. The specificity of odor response is e xamined in detail for the neurons of one sensillum, which were found to dif fer in their relative responses to a homologous series of esters. Adaptatio n and cross-adaptation are documented, and cross-adaptation experiments dem onstrate that the two neurons within one type of sensillum can function ind ependently. The analysis of all neuronal types in this model olfactory orga n is discussed in terms of its functional organization and the mechanisms b y which it encodes olfactory information.