Figure 2.

Simplified diagram of olfactory processing in Drosophila melanogaster. Odors activate distinct sets of olfactory receptor neurons (ORNs) in the fly's antenna. ORNs which express the same receptor converge onto the same glomerulus in the antennal lobe. Thus, each odor induces a unique glomerular activity pattern. This pattern is modulated by the interaction between ORNs, excitatory and inhibitory local interneurons (LNs) and projection neurons (PNs). The PNs relay the odor information to the lateral horn (which is assumed to mediate the innate odor response) and to the mushroom body intrinsic Kenyon cells (KCs). Upon reinforcement, KCs receive input from modulatory neurons, signaling either punishment or reward. Coincident activation of these neurons and the odor-responsive KCs in delay conditioning is believed to modify the output in these KCs. This modification changes the fly's behavior to the previously reinforced odor stimulus. For trace conditioning the underlying mechanism is yet unknown, but it seems likely that the necessary modifications occur in the olfactory pathway and/or pathway-associated neurons.