Structure
Mushroom bodies are usually described as neuropils, i.e. as dense networks of neuronal processes (dendrite and axon terminals) and glia. They get their name from their roughly hemispherical calyx, a protuberance that is joining to the rest of the brain by a central nerve tract or peduncle. They were first identified in 1850 by the French biologist Félix Dujardin.
Most of our current knowledge of the mushroom bodies comes from studies of a few species of insect, especially the cockroach Periplaneta americana, the honey bee Apis mellifera, the locust and the fruit fly Drosophila melanogaster. Studies of fruit fly mushroom bodies have been particularly important for understanding the genetic basis of their functioning, since the genetics of this species are known in exceptional detail.
In the insect brain, the peduncles of the mushroom bodies extend through the midbrain. They are mainly composed of the long, densely packed nerve fibres of the Kenyon cells, the intrinsic neurons of the mushroom bodies. These cells have been found in the mushroom bodies of all species that have been investigated, though their number varies; for example fruit flies have around 2,500 whereas cockroaches have about 200,000.
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