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.