The optic nerve fibres project on to the optic lobe in a regular manner, being precisely re-assorted after passing through a chiasma. In the outer plexiform zone the optic nerve fibres end in contact with the dendrites of second-order visual cells. These presumably serve to classify the visual input and four types can be recognized anatomically: (1) The smallest have minute circular dendritic fields, in contact with one or few optic nerve fibres. (2) There are also larger circular fields. (3) Many cells have very elongated narrow dendritic fields each running straight in one direction and thus perhaps sensitive to edges. (4) The largest second-order visual cells have enormous oval dendritic fields, several millimetres long, orientated in the long axis of the lobe. Each type of field occupies a different level, producing the characteristic layering of the outer plexiform zone. Numerous amacrine cell processes end in the outer plexiform layer, some are very small with restricted branches, others have wide trees with fibres passing first inwards then outwards several times. There are thus possibilities of establishing uniform conditions of excitation or inhibition over small or large areas of the visual field. The dendrites of the centrifugal cells with axons passing to the retina spread in the various layers of the plexiform zone. They could serve to project information of the areas excited, or inhibited, out to the retina. The axons of the second-order visual cells form radial columns in the outer part of the medulla of the optic lobe. Those with the smaller dendritic fields end more superficially, the largest ones about half-way through the lobe. Each column contains fibres and neuropil at its centre, surrounded by multipolar and bipolar amacrine cells, whose branches enter the neuropil among the endings of the second-order visual cells. Horizontal multipolar cells of various sizes link the columns. Third-order visual cells send dendrites into these columns and axons deeper into the lobe, some directly to the optic tract. The giant cells of the magnocellular lobe can thus be activated by a visual pathway involving only two previous synapses (as well as by a direct static pathway involving none). Central to the zone of radial columns is a zone where many of the connexions are tangential. There is an increasing number of large cells passing centrally, many being presumably fourth-order visual neurons. They send axons either elsewhere within the lobe or to the optic tract. Fibres reaching the lobe from the central brain or opposite lobe are distributed in this region and also reach out into the radial columns. In many of the tracts leaving the optic lobes for other centres the fibres maintain precise topographical relations, as also do those of the optic commissure. This regularity is especially clear in the bundles that pass to the motor centres (peduncle lobes and anterior basal lobes) but may be present in others. There is thus a regular mapping of the visual field throughout much of the system. Other pathways show complex interweaving, for instance those for colour control, where the response pattern is not topographically related to the visual input.
The exit of axons and glial membrane from the developing Drosophila retina requires integrins
