J Comp Neurol 1994 Aug 08;3462:306-20. doi: 10.1002/cne.903460209.

The contralaterally projecting neurons of the isthmic nucleus in five anuran species: a retrograde tracing study with HRP and cobalt.

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The morphology of projection neurons of the isthmic nucleus was studied in Rana esculenta, R. nigromaculata, Bufo marinus, B. bufo gargarizans, and Xenopus laevis from a comparative anatomical point of view. The main point of this work was to provide an anatomical basis for electrophysiological studies. Neurons projecting to the ipsilateral optic tectum were labeled by retrograde transport of horseradish peroxidase and cobaltous lysine complex injected into the optic tectum. Contralaterally projecting cells were filled by injecting the tracer substances into the crossed isthmotectal tract. Cells of the anterior nonrim cortex and the rostral part of the medulla project to the ipsilateral tectum. A band of cells in the middle of the medulla, a few cells in the caudal part of the medulla, and most of the neurons in the rim cortex project to the contralateral tectum. Five types of neurons were distinguished in the rim cortex of R. esculenta. Most of them have piriform perikarya and their dendrites arborize in the rim neuropil. In the medulla of the isthmic nucleus of R. esculenta, seven types of neurons were distinguished. Most of these neurons also exist in the other species. Medullary cells are piriform, fusiform, or multipolar, and are variable in size and in dendritic arborization. The isthmic neurons of the two Ranae and Bufo species are similar. The dominant cell types in Xenopus are multipolar with extensive dendritic arborization, which occupies more space in the nucleus than in the other species. Neurons with narrow dendritic trees may represent a system of fine resolution, and those neurons with extensive dendritic arborization may belong to a coarser system.

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