The Journal of comparative neurology
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The peripheral and central aspects of the extraocular system were studied in the weakly electric fish Gnathonemus petersii. All six extraocular muscles show a similar composition of large and small fibers grouped characteristically in the proximal and distal regions respectively. The exit of the three extraocular nerves from the brain is similar to that in other vertebrates. ⋯ In all three nuclei the axon originates opposite to the main dendrite. Collaterals of the hairpin-bend abducens axons could be identified in a few cases. The oculomotor system of G. petersii appears basically similar to that of other teleosts; the differences observed concern mainly the structure of the abducens nucleus, the intracranial and intracephalic course of the trochlear nerve, and the relatively small number of axons in each nerve.
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Cells of origin of ascending afferents to the mammillary nuclei and the afferents' fields of termination within these nuclei were studied by using retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase in the rat. The pars compacta of the superior central nucleus projects bilaterally to the median region of the medial mammillary nucleus. The ventral tegmental nucleus projects ipsilaterally to the medial mammillary nucleus, except for its median region, in a topographic manner such that the rostrodorsolateral part of the ventral tegmental nucleus projects to the medial quadrant of the medial mammillary nucleus; the rostroventromedial part projects to the dorsal quadrant; the caudodorsolateral part projects to the ventral quadrant; and the caudoventromedial part projects to the lateral quadrant. ⋯ The periaqueductal gray around the dorsal tegmental nucleus projects bilaterally to the supramammillary nucleus. The pars alpha of the pontine periaqueductal gray projects bilaterally to the peripheral part of the lateral mammillary nucleus, whereas the pars ventralis of the dorsal tegmental nucleus projects ipsilaterally to the lateral mammillary nucleus. The results show that the tegmentomammillary projections are organized in a gradient fashion, with the rostral to caudal position of cells of origin within the tegmental nuclei of Gudden being reflected by the medial to lateral position of fields of termination within the mammillary nuclei.