Afferents to the red nucleus in the lizard Podarcis hispanica: putative pathways for visuomotor integration

The afferents to the red nucleus from visual and nonvisual forebrain centers have been investigated in the lizard Podarcis hispanica by using both retrograde and anterograde transport of tracers. Because the red nucleus constitutes a key structure in the limb premotor system, these sensory afferents probably are involved in visuomotor and other forms of sensorimotor integration. After tracer injections aimed at the red nucleus, retrograde labeling was found in the reticular thalamus, the subthalamus, the nucleus of the posterior commissure, as well as in two retinorecipient nuclei, namely, the ventral lateral and pretectal geniculate nuclei, where labeled cells are especially abundant. These geniculorubral projections have been confirmed by means of anterograde tracing with dextranamine injections. On the other hand, small injections of tracers in the retina demonstrated that its projections to the ventral lateral and pretectal geniculate nuclei are organized in a point-to-point fashion. Moreover, small tracer injections into the optic tectum of Podarcis indicated that the ventral lateral geniculate nucleus also receives a precisely organized tectal afferent. Taken together, these results strongly suggest that geniculorubral projections might constitute the neuroanatomical substrate for the generation of quick locomotor responses to appropriate visual stimuli. Additional ventral thalamic, subthalamic, and pretectal afferents to the red nucleus are likely to subserve other kinds of sensorimotor integration. These results help to clarify the organization of the reptilian motor system, including the telencephalic control of motor responses, and to unravel some of the major trends in the evolution of the limb premotor network of tetrapodian vertebrates.