TOPOGRAPHY OF MODULAR SUBUNITS IN THE MUSHROOM BODIES OF THE COCKROACH

The mushroom body (MB), a conspicuous neuropil structure in the insect brain, is implicated in associative memory and in some aspects of mot or control. Intrinsic neurons of the MB (Kenyon cells) extend dendrite s into the calyx, and their axons run through the pedunculus and then bifurcate to form the alpha and the beta lobes. At the pedunculus and the lobes, Kenyon cells make synaptic connections with dendrites of ex trinsic (output) neurons. Previously, we reported that the oc lobe of the cockroach MB consists of repetitive modular subunits (Mizunami et al. [1997] Neurosci. Lett. 229:153-156). Each subunit is composed of a dark layer and a light layer, and the layers are refereed to as slabs . Each slab is composed of axons of a specific subset of Kenyon cells. In the present study, we examined serial sections of reduced silver p reparations and found that each dark and light slab continues througho ut the length of the pedunculus and the alpha and beta lobes. We also found that Golgi-impregnated Kenyon cells often exhibit a characterist ic grouping, forming a thin sheet interlaced by dozens or hundreds of axons. The sheet is much thinner than the slab, and each sheet remains within a particular slab throughout the length of the pedunculus and the lobes. Thus, the sheet is a component forming the slab. In the ped unculus and the beta lobe, a class of Golgi-impregnated extrinsic neur ons exhibit segmented dendritelike arbors that interact with every oth er slab, i.e., either with only dark or light slabs. Because each neur on of this class interacts with each particular set of dark or light s labs, we conclude that the slabs are units for transmitting output sig nals from the MB. J. Comp. Neurol. 399:153-161, 1998. (C) 1998 Wiley-L iss, Inc.