Ultrastructural analyses of modular subunits in the mushroom bodies of thecockroach

The mushroom body is a higher centre of the insect brain involved in sensor y association, memory, and some forms of motor control. Recently we have re ported that 15 repetitive modular subunits are present in the output region s of the mushroom body in the cockroach. Each modular subunit consists of l ight and dark slabs. Seeking to clarify morphological organization and func tion of this unit structure, we compared the ultrastructure of the light an d the dark slabs. The pedunculus comprises a huge number of tightly packed Kenyon cell axons (KCAs), glial processes, and extrinsic fibres. The extrin sic fibres and synaptic vesicles in KCAs occur much less frequently in the pedunculus than in alpha and beta lobes. Nevertheless, the pedunculus also reveals light and dark slabs as well as lobes. Ultrastructural quantitative analyses of transversely sectioned KCAs in the pedunculus show that averag e diameters of KCAs in the light slabs are 5-18% larger than those in the a djacent dark slabs, which are significantly different. These results sugges t that the diameter of KCAs is the major factor contributing to the varied density of slabs, at least in the pedunculus. Difference in conduction velo cities due to varied diameters of KCAs may be utilized for signal processin g.