Mushroom bodies

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The mushroom bodies or corpora pedunculata are a pair of structures in the brain of insects and other arthropods. They are usually described as neuropils, i.e. as dense networks of neurons 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.

Mushroom bodies are known to be involved in learning and memory, particularly for smell. They are largest in the Hymenoptera, which are known to have particularly elaborate olfactory control over behaviour. In larger insects, studies suggest that mushroom bodies have other learning and memory functions, like associative memory, sensory filtering, motor control, and place memory. Comparisons between genes early in mushroom body development show a homology with similar developing genes in the forebrain of mammals. (graded junction) The mushroom bodies are currently the subject of intense research. They have been compared to the cerebral cortex of mammals. Because they are small compared to the brain structures of vertebrates, and yet many arthropods are capable of quite complex learning, it is hoped that investigations of the mushroom bodies will allow a clear view of the neurophysiology of animal cognition. The most recent research is also beginning to reveal the genetic control of processes within the mushroom bodies.

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.