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Mushroom Body
Structure of the mushroom body

The mushroom body consists of Calyx, Penduncle, and Lobes. The calyces receive the inputs from other protocerebral regions and the deutocerebrum, in particular the antennal lobe. Kenyon cells (KCs), the constituent neurons of the calyces, receive the inputs into the calyces. KCs axons project through the Pedunculi and establish contacts with fibers of mushroom body extrinsic neurons in the lobes. There are two lobes, medial (or a) lobe and vertical (or b) lobe. In the pecunculi and lobes, stratified compartments called slabs can be recognized. The compartments of slabs can be visualised with some staining methods as light and dark stripes and each of them shows different immunoreactivities. A specific subset of KCs and some extrinsic neurons interact within each slab. This finding suggests that the slab likely functions as a unit for information processing, but this has to be confirmed by further investigations (Iwasaki et al., 1999).



Function of mushroom body

Olfactory learning
Experiments in which localized areas of the brain of the honey bee were cooled revealed that the calyces as well as the antennal lobes are necessary for olfactory learning (Erber et al., 1980). Experiments using local injections of cholinergic antagonists into the mushroom bodies suggest that the cholinergic networks of the mushroom bodies play important role in the formation and recall of olfactory memory in the honey bee (Lozano et al., 2001). However, it should be kept in mind that most excitatory synapses in the insect brain are cholinergic and thus it is difficult to isolate learning-specific functions of cholinergic synapses. Consider that most if not all input from the antennal lobe to the calyces is cholinergic (Oleskevich 1998).

Place memory
In the cockroach (Periplaneta americana), behavioral tests, based on paradigms similar to those originally used to demonstrate place memory in rats, also showed a rapid improvement in the ability of individual cockroaches to locate a hidden target when its position can be inferred from the layout of distant visual cues. Bilateral lesions of selected areas of the mushroom bodies abolish this ability but leave unimpaired the ability to locate a visible target. These experimental results demonstrate that the integrity of the pedunclus and medial (a) lobe of a single mushroom body is required for place memory (Mizunami et al., 1998).

Motor control
Okada et al. (1999) recorded extracellulary neural activities in mushroom bodies of freely moving cockroaches and revealed that a cluster of extrinsic neurons acts synergistically to transmit a specific set of mushroom body output signals correlated to left-right turn movements. These results suggest mushroom bodies are involved in motor planning or actual motor command functions.



Neurons in the mushroom bodies

Kenyon cells
Kenyon cells have dendritic projections in the calyces and axonal projections in the pedunculus and lobes. Compared to most other brain neurons, the size of their somata is very small. Their axons run in the slabs of the pedunculi and lobes. Thicker axons project inside the light slabs and thinner ones in through dark slabs, forming stratified compartments (Iwasaki et al., 1999) [THIS is not so surprising because I think staining was OEG or methylene blue or silver, but in any case, either higher density of neurofibrils or membranes leads to darker staining]. In Drosophila, experiments using genetic methods revealed that Kenyon cells are required for successful aversive olfactory conditioning in which odor (CS) and an electrical shock (a not-so-well-defined US) are associated (Zars et al., 2000).

Mushroom body extrinsic neurons
Mushroom body extrinsic neurons have been electrophysiologically and morphologically investigated (Homberg around 1984!, Mauelshagen 1995, Strausfeld and Li, 1999; Nishino et al., 1998; Rybak & Menzel 1998). It has been reported that some of them receive inputs in (specific areas of?) the protocerebrum and have postsynaptic targets in the pedunculus and/or the lobes (Strausfeld and Li, 1999). Giant input neurons in the mushroom body of the cockroach are well investigated (Nishino et al., 1998). These inhibitory neurons exhibit GABA-like immunoreactivity and respond to olfactory, visual, and mechanosensory stimulus (Nishino et al., 1998). Locust extrinsic neurons respond to a broader range of olfactory stimuli. They have been suggested to be involved in the generation of oscillatory neural activities in the MB (Perez-Orive et al., 1998).



Reference

Erber J, Masuhr TH, Menzel R (1980) Localization of short-term memory in the brain of the bee, Apis mellifera. Physiol Entomol 5:343-358.

Iwasaki M., Mizunami M., Nishikawa M., Itoh T. and Tominaga Y. (1999) Ultrastructural analysis of modular subunits in the mushroom bodies of the cockroach. J. Electron Microsc, 48: 55-62

Mizunami M, Weibrecht JM, Strausfeld NJ (1998) Mushroom bodies of the cockroach: Their participation in place memory. J Comp Neurol 402:520-537.

Nishino H, Mizunami M. (1998) Giant input neurons of the mushroom body: intracellular recording and staining in the cockroach. Neuroscience Letters, 246: 57-60.

Okada R, Ikeda J, Mizunami M (1999) Sensory responses and movement-related activities in extrinsic neurons of the cockroach mushroom bodies. J Comp Physiol A 185:115-129.

Perez-Orive, J., Mazor, O., Turner, G. C., Cassenaer, S., Wilson, R. I. & Laurent, G. (2002) Oscillation and sparsening of odor representations in the mushroom body. Science, 297, 359-365.

Strausfeld NJ, Li Y. (1999) Organization of olfactory and multimodal afferent neurons supplying the calyx and pedunculus of the cockroach mushroom bodies. J Comp Neurol. 409:603-25.

Zars T, Fischer M, Schulz R, Heisenberg M (2000) Localization of a short-term memory in Drosophila. Science 288:672-675.


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