Dorsal ocellar system of the American cockroach: I. Structure of the ocellus and ocellar nerve

Abstract

The structure of the dorsal ocellus and ocellar nerve has been examined in the American cockroach. The ocellar retina contains more than 10,000 retinular cells whose axons converge synaptically upon three to five thick, second-order neurons in the ocellar neuropil. Efferent processes containing dark vesicles also synapse with second-order neurons in the ocellar neuropil. Additional synapses, i.e., feedback synapses of thick second-order processes, and reciprocal synapses between retinular axons, are also found, infrequently, in the proximal region of the neuropil. A large majority of retinular axons terminate within the ocellar neuropil, but some extend into the ocellar nerve. The ocellar nerve contains three to five thick, second-order processes, 20–30 retinular axons, a few efferent processes, a few cored-vesicle-containing processes, a distal extension of the thick third-order process, and through fibers extending to the ventral nerve cord. Synapses occur among these processes, the second-order processes being pre- and postsynaptic to surrounding thin processes throughout the ocellar nerve. These results suggest that the ocellar nerve is also a synaptic region, but differs functionally from the ocellar neuropil. The ocellar neuropil appears to be an input region for the second-order neurons, whereas the ocellar nerve appears to be an output region, where local interactions occur among many types of neurons.