The planarian brain has an inverted-U shaped structure with functional regionalization. To investigate how each region in the brain connects to each other, we traced neural projections by microin-jection of fluorescence dye tracers. We found that external light and olfactory/taste signals received in the head region are conveyed in the main lobes (sponge region) of the brain. Chemosensory neurons distributed in the lateral branches project to the peripheral region of the sponge and visual neurons project to the medial region of the sponge. Parts of the sensory neurons project directly to the corresponding sensory neurons on the opposite side of the brain. However, all of the dye labeled brain neurons in the left and right lobes connect to each other via commissural neurons in the central region of the sponge. In addition to these observations, we detected regional differences in the planarian visual neurons. Posterior visual neurons have ipsilateral projection, but anterior visual neurons project to the contralateral side of the brain. A pair of longitudinal ventral nerve cords (VNC) connect to the brain on the ventral side, suggesting that they transmit signals which are integrated and processed in the brain. We also detected the direct connection of neurons in the brain and those of the pharynx, even though most pharynx neurons connect to VNC neurons. Here, we report for the first time on neural connections in the planarian central nervous system after overcoming technical difficulties specific to flatworms.
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