Neural organization of ocellar pathways in the cockroach brain

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Abstract
A large number of photoreceptors of insect ocelli converge onto a smaller number of second-order neurons. Second-order neurons exit the ocellus and project into the ocellar tract neuropil of the brain. Here, the anatomy and physiology of ocellar interneurons of the ocellar tract neuropil of the cockroach are described. The total number and gross morphologies of ocellar tract neurons were examined by extracellular cobalt impregnations into the neuropil. Morphology and physiology of individual neurons were examined using intracellular recording and stainings. Each ocellar tract neuropil contains at least 25 interneurons comprising: (1) four second-order neurons, (2) 15 third-order neurons that receive synapses from second-order neurons at the ocellar tract and project into a number of target neuropil areas of the brain, (3) two possible efferent neurons, (4) three third-order or efferent neurons, and (5) one neuron still to be characterized. The projection areas of ocellar third-order neurons include (1) visual, olfactory, and mechanosensory centers; (2) the mushroom body (a higher associative center); (3) the posterior slope, a premotor center from which descending brain neurons originate; and (4) the thoracic motor systems. By comparing the present results to those reported from other insects, I conclude that the cockroach ocellar system has two distinctive features. (1) The ratio of convergence at synapses between photoreceptors and second-order neurons is higher than those reported for other insects so far studied. (2) Ocellar signals are transmitted to various target neuropils by third-order neurons, whereas ocellar systems of all other insects possess pathways in which ocellar signals are transmitted to target neuropils by second-order neurons. The functional significance of these features of the cockroach ocellar system is discussed.