THE MUSHROOM BODY
The mushroom bodies are clusters of neurons in the insect brain that look, well, like a pair of mushrooms. The number of neurons forming each of the mushroom bodies — called the Kenyon cells — varies across species. In the fruit fly Drosophila melanogaster, there are about 2,000 Kenyon cells per hemisphere. A large body of evidence collected over the past decades demonstrated that the mushroom body is the brain center where associative memories are formed and stored. In Drosophila, most Kenyon cells receive their input through the projection neurons of the antennal lobe, an olfactory processing center.
The antennal lobe consists of 50 or so glomeruli and is connected to the antennae, the nose the fly. Each sensory neuron that covers the antennae expresses only one of about 50 olfactory receptor genes and the neurons expressing the same receptor converge in the antennal lobe where they innervate a single glomerulus. The antennal lobe projection neurons relay information from individual glomeruli to the mushroom body and other brain centers. Because all sensory neurons that express the same olfactory receptor converge into one glomerulus, each odorant the fly encounters elicits a specific pattern of neural activity among the glomeruli. This is sometimes called an ‘odor-evoked map’.
This map is highly stereotypical and does not vary much between individual flies. In the mushroom body, however, we have found that this stereotypical organization is lost. Individual Kenyon cells receive input from a random set of about seven projection neurons each connected to different glomeruli. The glomeruli connected to a given Kenyon cell do not share any meaningful biological features and are often tuned to radically different kinds of odors. We believe that such random wiring, which may also be playing a role in the brains of vertebrates, serves to maximize the memory capacity of the fly brain.