Fig. 10

The anatomy of the Drosophila olfactory nervous system. The number of neurons or processes is given in the brackets. Nearly 1200 olfactory receptor neurons (ORNs) are of 50 different types based on the presence of odorant receptors. Their axonal terminals end in 50 different glomeruli. All of the ORNs that express one odorant receptor converge on to the same glomerulus in the antennal lobe. The presynaptic terminals from a rough average of 25 ORNs synapse with the postsynaptic terminals of nearly three sister projection neurons (PNs) within a single glomerulus. ORNs spike continuously at rest at the rate of an average eight spikes per second in the absence of ligands. Approximately three synchronous unitary ORN synaptic inputs drive one PN spike. In the absence of any odor, activity spread between the glomeruli through excitatory cholinergic local interneurons (ELN). Most odors cause ORN to fire at a rate of less than 50 spikes per second. Since all the ORNs that express a given odorant receptor converge onto the same glomerulus, one specific odorant is expected to activate PNs within one glomerulus. This leads to activation of inhibitory local interneurons (ILN) and  results in the inhibition of remaining glomeruli. PN axons innervate the mushroom body (MB) by terminating in large boutons and synapses with one claw each of many KCs. A single bouton connects to multiple Kenyan cell (KC) claws; but each claw synapses with only one PN bouton. PNs from different glomeruli synapse to the claws of the KCs by chance alone. The oscillatory potentials between the glomeruli is shown as a waveform through the ELNs