Mushroom bodies

The mushroom bodies or corpora pedunculata are a pair of structures in the brain of arthropods (including insects and crustaceans), and polychaete annelids (notably the ragworm Platynereis dumerilii). They are known to play a role in olfactory learning and memory. In most insects, the mushroom bodies and the lateral horn are the two higher brain regions that receive olfactory information from the antennal lobe via projection neurons. They were first identified and described by French biologist Félix Dujardin in 1850.

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 joined to the rest of the brain by a central nerve tract or peduncle. Most of our current knowledge of 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 mushroom body functioning, since their genome has been sequenced and a vast number of tools to manipulate their gene expression exist. 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, first described by Frederick C. Kenyon in 1896. These cells have been found in the mushroom bodies of all species that have been investigated, though their number varies. Fruit flies, for example, have around 2,500, whereas cockroaches have about 200,000. A locust brain dissection to expose the central brain and carry out electro-physiology recordings can be seen here.

Evolutionary history Historically, it was believed that only insects had mushroom bodies, because they were not present in crabs and lobsters. However, their discovery in the mantis shrimp in 2017 lead to the later conclusion that the mushroom body is the ancestral state of all arthropods, and that this feature was later lost in crabs and lobsters. Using immunostaining, homologous lobate structures with a common ground pattern of chemosensory afferents supplying thousands of intrinsi/local neurons (the olfactory cascade) were found in arachnids and many groups of Lophotrochozoa: polychaete annelids, polyclad planarians, and nemerteans. Altogether this indicates a common origin, suggesting that this arrangement is present in the ancestral state. The annelid mushroom body is homologous to the vertebrate cerebral cortex, according to expression profiling by image registration. This likely indicates independent elaborations on a common simple ancestral structure.

Function Mushroom bodies are best known for their role in olfactory associative learning. These olfactory signals are received from dopaminergic, octopaminergic, cholinergic, serotonergic, and GABAergic neurons outside the MB. They are largest in the Hymenoptera, which are known to have particularly elaborate control over olfactory behaviours. However, since mushroom bodies are also found in anosmic primitive insects, their role is likely to extend beyond olfactory processing. Anatomical studies suggest a role in the processing of visual and mechanosensory input in some species. In Hymenoptera in particular, subregions of the mushroom body neuropil are specialized to receive olfactory, visual, or both types of sensory input. In Hymenoptera, olfactory input is layered in the calyx. In ants, several layers can be discriminated, corresponding to different clusters of glomeruli in the antennal lobes, perhaps for processing different classes of odors. There are two main groups of projection neurons dividing the antennal lobe into two main regions, anterior and posterior. Projection neuron groups are segregated, innervating glomerular groups separately and sending axons by separate routes, either through the medial-antenno protocerebral tract (m-APT) or through the lateral-antenno protocerebral tract (l-APT), and connecting with two layers in the calyx of the mushroom bodies. In these layers the organization of the two efferent regions of the antennal lobe is represented topographically, establishing a coarse odotopic map of the antennal lobe in the region of the lip of the mushroom bodies. Mushroom bodies are known to be involved in learning and memory, particularly for smell, and thus are the subject of current intense research. In larger insects, studies suggest that mushroom bodies have other learning and memory functions, like associative memory, sensory filtering, motor control, and place memory. Research implies that mushroom bodies generally act as a sort of coincidence detector, integrating multi-modal inputs and creating novel associations, thus suggesting their role in learning and memory. Recent work also shows evidence for the involvement of the mushroom body in innate olfactory behaviors through interactions with the lateral horn, possibly making use of the partially stereotyped sensory responses of the mushroom body output neurons (MBONs) across individuals. Although the connections between the projection neurons and the Kenyon cells are random (i.e., not stereotyped across individuals), the stereotypy in MBON responses is made possible by the dense convergence of many Kenyon cells onto a few MBONs along with other network properties. Information about odors may be encoded in the mushroom body by the identities of the responsive neurons as well as the timing of their spikes. Experiments in locusts have shown that Kenyon cells have their activity synchronized to 20-Hz neural oscillations and are particularly responsive to projection neuron spikes at specific phases of the oscillatory cycle.

Sleep The neurons which receive signals from serotonergic and GABAergic neurons outside the MB produce wakefulness, and experimentally stimulating these serotonergic upstream neurons forces sleep. The target neurons in the MB are inhibited by serotonin, GABA, and the combination of both. On the other hand octopamine does not seem to affect the MB's sleep function.

Drosophila melanogaster

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