Neural Organization of the Protocerebrum of an Insect

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The aim of this study was to clarify basic neural organization of the protocerebrum of insects, where signals from various sensory systems are integrated and are converted into command signals for motor actions. Such signals are transmitted to thoracic locomotory centers via a number of descending neurons (DNs). At first, I examined the distribution of somata and dendrites of DNs of tne brain of the cockroach Periplaneta americana, by retrogradely filling their axons from the cervical connective. About 235 pairs of somata of DNs were stained, a majority of which is located in the protocerebrum. Dendritic arborizations of DNs are found in wide varieties of neuropil areas but not in the optic lobe, the antennal lobe, the mushroom body, the central complex, and ventral part of the lateral horn. Axon terminals of ascending neurons were also found in wide varieties of brain areas. Based on these findings, we propose that there is a number of parallel processing streams from cephalic sensory areas to thoracic locomotory centers, many of which are via various premotor areas of the brain, and also that pathways from higher association areas supply some premotor areas to form hierarchical processing streams. By examining documented findings in the cockroach brain, we propose that parallel processing streams function in various reflexive and instinctive behaviors, while hierarchical processing steams govern some forms of experience-dependent modification of behavior. Next, I examined neural organizations of the lateral protocerebrum of the cockroach, and found that this is a highly organized neuropil consisting of a number of functional subdivisions.