The first optic ganglion of the bee. V. Structural and functional characterization of centrifugally arranged interneurones.

The organization, characterization and connectivity patterns of four different interneurone types were studied with the use of Golgi light- and electron-microscopic techniques. All four cell types originate in the outer chiasma; they have an efferent end-branch in the lamina and an afferent one terminating in the distal region of the second optic ganglion, the medulla. These interneurones are referred to as: (i) Garland-cell: The efferent fibre has on its tangential branch numerous centripetal side branches, so-called "garlands", which synapse with first- and second-order visual cells. (ii) Y-cell: The lamina branch bifurcates before entering the lamina. It innervates two neighbouring cartridges. Synaptic contacts were seen in two different laminar strata where bottle-brush-like collaterals occurred. (iii) Single bottle-brush cell: The efferent part has only one centrifugal branch, which can be compared morphologically and in terms of synaptology with those of the Y-cell. (iv) Triptych-cell: The lamina component innervates three neighbouring cartridges at three different laminar layers interconnecting different first- and second-order visual neurones. The present study provides some essential qualitative and quantitative fine-structural information, which - when compared with adequate physiological data - may lead to a better understanding of the function of the first visual information-processing centre of the bee.