Ascending projections from the optic tectum in the lizard Podarcis hispanica.

The ascending projections of the optic tectum, including their cells of origin, have been studied in the lizard Podarcis hispanica by means of a two-step experimental procedure. First, tracers were injected in the tectum to study the anterograde labeling in the forebrain. Second, the cells of origin of these projections have been identified by analyzing the retrograde labeling after tracer injections in the thalamus, hypothalamus, and pretectum. Three main tectal ascending pathways have been described: the dorsal tecto-thalamic tract (dtt), the medial tecto-thalamic tract (mtt), and the ventral tecto-thalamic tract (vtt). The dtt originates in radial cells of layers 5 and 7 and bipolar cells of layers 8 and 10 that project to the lateral neuropile of the dorsal lateral geniculate nucleus (GLD), to the intergeniculate leaflet (IGL), and to the ventral lateral geniculate nucleus (GLV). The mtt arises from radial neurons of layers 3 and 5 and bilaterally reaches the putative reticular thalamus and its boundary with the hypothalamus, the rostral IGL, and the area triangularis (AT). The vtt is composed of fibers from ganglion and multipolar cells of the layer 7 that project bilaterally to the nucleus of the vtt, the ventrolateral thalamic nucleus, the medial posterior thalamic nucleus (MP), the nucleus rotundus (Rot), the IGL, and the cell plate of the GLD. Therefore, the GLD receives not only direct retinal afferents but also two different tectal inputs, thus constituting a convergence point in the two visual pathways to the telencephalon. Moreover, different tectal cells specifically project to the ventrolateral thalamus and to pretectal nuclei. These results are discussed from comparative and functional viewpoints.