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

The morphology and physiology of small multimodal ocellar interneurons (SM-neurons) have been examined in the cockroach Periplaneta americana. The SM-neuron is monopolar with its cell body in the tritocerebrum near the esophagus. Its axon ascends to the protocerebrum and extends into the ocellar neuropil through the ocellar tract and the ocellar nerve. The axon sends a long collateral process towards the optic tract. The SM-neuron responded with spike discharges to various sensory stimuli. The collateral and many side branches along the axon in the deutocerebrum appear to be input regions from those sensory afferents. Cercal stimulation triggered most effectively a train of spikes in the SM-neuron: some of seven giant axons in the ventral nerve cord were involved in this pathway. Cercal stimulation also evoked depolarization in the large second order ocellar neurons (L-neurons). A possible neural connection between SM-neurons and L-neurons was examined pharmacologically. Interaction of ocellar illumination and cercal stimulation in the L-neuron was also examined. These data are discussed in relation to the ocellar function of cockroaches and in comparison with data obtained in other insects.