Morphological and physiological characterization of descending ocellar interneurons in the American cockroach.

In the present study two types of descending ocellar neurons have been morphologically and physiologically identified in the cockroach Periplaneta americana: a descending ipsilateral ocellar neuron (DIO-neuron) and a descending contralateral ocellar neuron (DCO-neuron). Both DIO- and DCO-neurons possess a cell body near the ocellar tract in the protocerebrum and extend dendritic processes into the ipsilateral ocellar nerve. An axon of the DIO-neuron descends to the metathoracic ganglion, and its entire course is ipsilateral to the cell body. An axon of the DCO-neuron crosses the median plane of the protocerebrum and descends through the contralateral ventral nerve cord to the second abdominal ganglion. Both axons possess masses of branches in each ganglion as well as in the brain. Both DIO- and DCO-neurons showed no spontaneous spike discharges, and responded with a few off-spikes to ocellar illumination. They responded to various mechanical stimuli, like cercal stimulation with a train of spikes which was suppressed by ocellar illumination. Spikes of DCO-neurons caused postsynaptic potentials and spikes in some interganglionic metathoracic interneurons, and spikes in motor nerves. These data are discussed in relation to the ocellar function of the cockroach as well as in comparison with that of other insects.