Columns and Layers in the Second Synaptic Region of the Fly’s Visual System: The Case for Two Superimposed Neuronal Architectures

Combination of Golgi staining, reduced silver staining, normal and Golgi-E.M. has allowed estimates about the numbers of cells present in medullary columns. Up to the present time 120 cell types, classified according to orientation, depth, dendritic spread and projections, have been identified. Components of 34 cell types occur in each medullary column. In addition, each column contains the corresponding medullar components of elements destined for or derived from an optic cartridge, and the R7 / R8 endings. The basic three dimensional structure of the medulla, as defined by the 46 cells that contribute to each column, is columnar and stratified. Information flow must be both lateral and from periphery to centre or vice versa. This is in contrast with the lamina structure where most information flow must be along a cartridge. This structure is discussed with respect to the remaining cell types whose distribution per column is still unknown.