Cellular Architecture and Connectivity of the Frog’s Optic Tectum and Pretectum

Abstract

The result of studies, in which horseradish peroxidase or cobalt was used to label neurons and fiber tracts in the frog’s brain are summarized in this paper. Focal cobalt injections into various layers of the optic tectum label small groups of neurons which appear as columns formed by dendrites oriented perpendicular to the surface. This arrangement of neurons resembles that of the cortex of mammals. In frogs tectal neurons projecting to the medulla oblongata and the spinal cord were identified as large ganglionic, pyramidal, and large piriform cells with widely arborizing dendritic trees. Large piriform neurons with narrow dendritic trees and small piriform neurons project to the isthmic nucleus. All tectal cell types which project to the medulla, are also efferents for the pretectum and other diencephalic nuclei, but the majority of neurons projecting rostrally are small piriform cells of layer 8. Afferents to the tectum originate bilaterally from the retina, pretectum, and the isthmic nucleus, and ipsilaterally from certain thalamic nuclei, the nucleus profundus lateralis, the anterodorsal and posterodorsal tegmental nuclei, and from the contralateral optic tectum. Retinal and isthmic afferents terminate in the superficial strata of the tectum (layers 8 and 9) and non-retinal afferents distribute in the deep layers (layers 2 to 6). Two nuclei receive retinal afferents in the pretectal area: the nucleus lentiformis mesencephali and the posterior thalamic nucleus. Both nuclei project to the ipsilateral medulla and to both tecta. Non-retinal afferents arrive at the pretectum from the ipsilateral striatum, anterior and lateral thalamic nuclei, pretectal grey, optic tectum and the medullary reticular formation, and from the contralateral superior vestibular nucleus and posterior thalamic nucleus.