Mushroom Body

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Mushroom body structure

The mushroom body (corpus pendunculatus) is divided into three major subregions, the calyx, pendunculus, and the lobes. The calyx is connected to the lobes though the pedunculus. The lobes are divided into vertical and medial lobes and lobelet. The vertical lobe is also connected to the calyx though a separate structure, the Y tract. The calyx receives input via the antenno-cerebral tracts that connect to Kenyon cells. Processes of these cells are the major constituent of the calyces, pendunculi, and lobes. Kenyon cells synapse onto mushroom body extrinsic neurons, about which little is known in moths. Groups of Keynon cells form conspicuous units that are topographically arranged thoughout the pendunculi and lobes.

Mushroom body function

In the honey bee, retrograde amnesia can be induced by cooling. This information was used as a base for cooling experiments of very localised brain structures that have shown the mushroom bodies to be required for learning success in classical olfactory conditioning of the proboscis extension responses [Erber et al., 1980]．
Likewise, the mushroom bodies are involved in spatial learning tasks. Cockroackes with lesion in the mushroom bodies show performance deficiencies in spatial learning tasks [Mizunami et al., 1998]．
Extracellular recording has also revealed a possible role as a premotor control area. Units were found whose activity was preceeding turning behaviour during locomotion [Okada et al., 1999]．

Mushroom body neurons

Keynon cells have their dendrites in the calyx and send axons though the pedunculus and into the lobes. Their somata are generally very small but still large in moths compared to most insects. In the cockroach, it has been shown that the subunits formed by defined groups of Keynon cells also have in internal structure of repetitive slabs as seen by various classical staining methods. Each subunit contains to differently staining slabs As the subdivisions, the slabs run intact though the pedunculi into the lobes. The slabs are further subdivided into even thinner sheets, that are also componsed of defined sets of Keynon cells and may be elementary functional units of the MB. Extrinsic neurons have been intentified whose dendrites innervate the pedunculi and beta lobes, making connections only with slabs of the same staining type. Thus, slabs may be the elementary unit of mushroom body output [Iwasaki et al., 1999 IS THIS A JAPANESE PAPER? My info is from Mizunami et al 1998 JCN 399(2):153]. In Drosophila, it is known from experiments using genetic methods that aversive olfactory conditioning with electric shocks requires the function of Kenyon cells [Zars et al., 2000].
Besides olfactory input, other modalities are also present. This is well-known because mushroom body output neurons are multimodal in all species investigated (Homberg and others). to In the bee, it is believed that a tract from the SOG to the MB calyx carries gustatory information. In the cockroach, a segregation into input zones is seen at the level of the calyx, receiving AL PN input, input from the optic lobes, and from multimodal protocerebral interneurons [Nishikawa et al 1998; Strausfeld & Li 1999]. Another input is provided by GABAergic calycal giant interneurons as demonstrated in the cockroach. These cells, which also have processes in an olfactory area (the lateral horn) are inhibited by sensory stimuli of various modalities and may thus increase contrast in the detecting of stimuli by the mushroom body [Nishino et al., 1998]. In locusts, oscillatory phenomena in olfactory-induced activity have been intensively investigated and there is evidence that the mushroom body is optimised to extract relevant information from the oscillatory output of the antennal lobe [Perez-Orive et al., 2002].

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