Neuroanatomical studies on the information processing system in the primary facial taste center of fish

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This sutdy was undertaken to reveal neuronatomically the information processing system in the primary facial taste center, facial lobe (FL) of fish. The followings were mainly examined in this study ; 1)the projections of trigeminal fibers to the FL,2)cytoarchitecture of the FL,3)determination of the efferent fibers in the FL,4)projections of the efferent neurons in the FL to the brain stem, and 5)distribution of taste buds and nerve fibers in the barbels in catfish.The projection of trigeminal fibers onto the FL were found in 6 species of fish. This projection was especially noticeable in the catfishes ; the trigeminal fibers project somatotopically to the FL and distribute in the entire region of the FL except the trunk-tail lobule, indicating that trigeminal fibers contribute to make sharply defined somatosensory map in the FL of the catfish as well as the mechanosensory fibers of the facial nerve. Small (7-9 mum) and large sized (15-20 * 12-15 mum) neurons are present in the barbel … More lobules of Plotosus. Small neurons form cell clusters in the lobules and large neurons are scattered in the lobule. Golgi analyzes revealed that the small neurons are possessed of 50-60 mum dendritic fields around them, while the larger ones have thick dendrites of 150 mum in length. Large sized neurons and their dendrites were entirely surrounded by synaptic terminals of other fibers. Efferent neurons projecting to the superior (SSTN) or inferior (ISTN) secondary taste nuclei were determined as large types of neurons. Ascending neurons were numerous and scattered throughout the lobules, while descending neurons were small in number and located only in the peripheral margin of the lobules. Ascending neurons in each lobule projects to the entire regions of the SSTN located under the cerebellum. This shows that somatotopic organization is not present in the SSTN and support the previous electrophysiological findings that each neuron located in the SSTN has a wide receptive field in response to mechanical or taste stimulation of body surface.The taste buds were not uniformaly distributed in the barbels. They were most densely located in the rostral or caudal surface along the long axis of the barbels and few in the intermediate surface between the rostral and caudal surfaces. The nerve fibers make neutral networks under the epithelium and each network becomes smaller in the rostral or caudal surface than in the intermediate surface. Less