Horseradish peroxidase and Fast Blue were injected into the oculomotor and trochlear nuclei of rabbits so as to study the distribution of vestibular neurons that project to these nuclei. After the oculomotor nucleus was injected, labelled neurons were found in the superior, medial, and descending vestibular nuclei as well as in cell group Y. In the superior nucleus, most of the neurons (510 ± 46) were ipsilateral to the injection, although contralaterally labelled neurons were also observed (104 ± 19) more peripherally. In cell group Y, 186 ± 24 contralaterally labelled neurons were observed, whereas hardly any (8 ± 3) were found on the ipsilateral side. The largest group of labelled neurons (811 ± 65) constituted a neuronal band located contralaterally in the medial nucleus and rostral part of the descending nucleus. This band rostromedially continued with the caudal part of the group of internuclear neurons of the abducens nucleus. Only 190 ± 31 neurons were labelled in the medial and descending nucleus ipsilateral to the injected oculomotor nucleus. After injection of the trochlear nucleus, labelled neurons were found in the ipsilateral superior nucleus and contralateral medial and descending nuclei; a few labelled cells were also observed in the ipsilateral medial and descending nuclei as well as in the contralateral cell group Y.
In this study we have used retrogradely transported horseradish peroxidase (HRP) to investigate whether the nucleus prepositus hypoglossi of a lateral-eyed mammal projects to the oculomotor and trochlear nuclei. After the injection of HRP in the oculomotor nucleus of rabbits, labelled neurons were found bilaterally in the nucleus prepositus hypoglossi, though they were more numerous on the ipsilateral side. The majority of these neurons were labelled in the rostral part of the propositus nucleus and were dispersed predominantly in the lateral and ventral zone of the nucleus. Neurons were also labelled in the prepositus nucleus after injection of HRP in the trochlear nucleus. These data were compared with those for frontal-eyed mammals and birds and suggest that the said projections are less well developed in species that possess panoramic vision and a lesser degree of binocular yoking.
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