The efferent projections of the pretectal complex: an autoradiographic and horseradish peroxidase analysis.

Anterograde autoradiographic data reveal that neurons within the pretectal complex of the tree shrew possess axons which terminate within three general categories of targets. First, there are targets of a major ipsilateral descending pathway which include: the dorsal cap of Kooy of the inferior olivary complex, the dorsolateral and dorsomedial regions of the griseum pontis, the mesencephalic reticular formation which lies immediately dorsal and lateral to the red nucleus, the medial terminal nucleus and the superficial layers of the superior colliculus. A second category of targets receive their pretectal input from a large ascending bundle which projects ipsilaterally to: the reticular and lateral nuclei of the thalamus, the zona incerta, the central lateral and paracentral intralaminar nuclei of the thalamus and bilaterally to the ventral lateral geniculate nucleus. A third category of targets include cranial nerve and closely associated nuclei which play a role in eye movements. Pretectofugal fibers projecting to nuclei in this third category terminate ipsilaterally within the nucleus of Darkschewitsch, bilaterally within the nucleus of the posterior commissure and the interstitial nucleus of Cajal, and contralaterally within the somatic cell column of the oculomotor and trochlear nuclei. There are also commissural projections to contralateral pretectal cell groups. Injections of horseradish peroxidase were placed within 11 pretectal targets. These data, which confirm and extend our autoradiographic findings, show that the majority of pretectal targets receive input from several pretectal nuclei, and that the size of pretectal neurons, rather than the cell groups in which they are located, dictates the termination site(s) of their axons.