Topography of four classes of Kenyon cells in the mushroom bodies of the cockroach.

Mushroom bodies (MBs), which are higher centers in the insect brain, are implicated in associative memory and in the control of some behaviors. Intrinsic neurons of the MB, called Kenyon cells, receive synaptic inputs from axon terminals of input neurons in the calyx. Axons of Kenyon cells project into the pedunculus and to the alpha and beta lobes, where they make synaptic connections with dendrites of extrinsic (output) neurons. In this study, we examined the morphology of Kenyon cells in the cockroach by using Golgi stains and found that they can be classified into four classes (K1, K2, K3, and K4), according to the diameter, location, and morphology of the cell bodies, dendrites, and axons. The somata of Kenyon cells of different classes occupy different concentric zones; K1 cells occupy the most central zone, and K4 cells occupy 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 neuropil, whereas those of K1 and K4 cells cover the outer and inner halves 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 class of extrinsic neurons in the a lobe has dendrite-like arbors that cover the zones where either K1, K2, or K3 are located. These neurons probably transmit signals of each class of Kenyon cells. We conclude that, in the cockroach, four classes of Kenyon cells subdivide the cell body region, pedunculus, and lobes of the MBs, whereas subdivision is less prominent in the calycal neuropil.