TOPOGRAPHY OF 4 CLASSES OF KENYON CELLS IN THE MUSHROOM BODIES OF THECOCKROACH

Mushroom bodies (MBs), which are higher centers in the insect brain, a re implicated in associative memory and in the control of some behavio rs. Intrinsic neurons of the MB, called Kenyon cells, receive synaptic inputs from axon terminals of input neurons in the calyx. Axons of Ke nyon cells project into the pedunculus and to the alpha and beta lobes , where they make synaptic connections with dendrites of extrinsic (ou tput) neurons. In this study, we examined the morphology of Kenyon cel ls in the cockroach by using Golgi stains and found that they can be c lassified into four classes (K1, K2, K3, and K4), according to the dia meter, location, and morphology of the cell bodies, dendrites, and axo ns. The somata of Kenyon cells of different classes occupy different c oncentric zones; K1 cells occupy the most central zone, and K4 cells o ccupy the most peripheral zone. The main processes of Kenyon cells of different classes also occupy different concentric zones in the calyx. Dendrites of K2 and K3 cells are distributed throughout the calycal n europil, whereas those of K1 and K4 cells cover the outer and inner ha lves of the depth of the neuropil, respectively. In the pedunculus and the alpha and beta lobes, axons of Kenyon cells of different classes occupy different zones, although the separation is not complete. A cla ss of extrinsic neurons in the alpha lobe has dendrite-like arbors tha t cover the zones where either K1, K2, or K3 are located. These neuron s probably transmit signals of each class of Kenyon cells. We conclude that, in the cockroach, four classes of Kenyon cells subdivide the ce ll body region, pedunculus, and lobes of the MBs, whereas subdivision is less prominent in the calycal neuropil. J. Comp. Neurol. 399:162-17 5, 1998. (C) 1998 Wiley-Liss, Inc.