Projections of the dorsal and lateral terminal accessory optic nuclei and of the interstitial nucleus of the superior fasciculus (posterior fibers) in the rabbit and rat.

The projections of the dorsal and lateral terminal accessory optic nuclei (DTN and LTN) and of the dorsal and ventral components of the interstitial nucleus of the superior fasciculus posterior fibers; inSFp have been studied in the rabbit and rat by the method of retrograde axonal transport following injections of horseradish peroxidase into oculomotor-related brainstem nuclei. The projections of the ventral division of the inSFp have been further investigated in rabbits with the anterograde axonal transport of 3H-leucine. The data show that the projections of the DTN, LTN, and inSFp are remarkably similar in rabbit and rat. The DTN projects heavily to the ipsilateral medial terminal accessory optic nucleus (MTN), nucleus of the optic tract, and dorsal cap of the inferior olive. The DTN projects sparsely to the ipsilateral visual tegmental relay zone and to the contralateral superior and lateral vestibular nuclei. The LTN and dorsal component of the inSFp are found to share the same basic connections; both project heavily to the ipsilateral nucleus of the optic tract and visual tegmental relay zone and send a moderately sized projection to the ipsilateral MTN. However, while the dorsal component of the inSFp sends significant ipsilateral projections to both rostral and caudal portions of the dorsal cap, only a few LTN neurons appear to follow this example and only by projecting to the rostral part of the dorsal cap. In addition, both the LTN and dorsal component of the inSFp send sparse contralateral projections to the MTN, nucleus of the optic tract, and visual tegmental relay zone; and the dorsal component of the inSFp also provides a sparse contralateral projection to both rostral and caudal portions of the dorsal cap. The ventral component of the inSFp projects heavily to the ipsilateral visual tegmental relay zone and moderately to the ipsilateral MTN and nucleus of the optic tract. The ventral inSFp projects sparsely to the contralateral MTN, the nucleus of the optic tract, and the visual tegmental relay zone. A few of its neurons target the ipsilateral dorsal cap of the inferior olive. Unlike the DTN (present study) and the MTN (Giolli et al.: J. Comp. Neurol. 227:228–251, '84; J. Comp. Neurol. 232:99–116, '85a), the LTN and the inSFp of the rabbit and rat lack projections to the superior and lateral vestibular nuclei. In contrast to the rabbit and rat MTN, no projections of the DTN, LTN, or inSFp could be identified to the interstitial nucleus of Cajal, the supraoculomotor-periaqueductal gray, the pars medialis of the deep mesencephalic nucleus, the pontine reticular formation, or the dorsolateral division of the basilar pons in the rabbit and/or the rat. As with the MTN projections in rabbits and rats, no evidence has been found of efferents from the DTN, LTN, or inSFp to the superior colliculus, the caudate-putamen, the nucleus reticularis tegmenti pontis, or the cerebellar flocculus in the rat and/or rabbit. (1) The functional significance of the prominent interneuron connections between the accessory optic nuclei (and between these nuclei and the nucleus of the optic tract) and (2) the probable role of the accessory optic nuclei in oculomotor control are issues addressed.