A search for corticospinal collaterals to thalamus and mesencephalon by means of multiple retrograde fluorescent tracers in cat and rat.

An attempt has been made to determine anatomically whether in rat and cat cortical projections to ventrolateral nucleus of thalamus and to mesencephalon are in part composed of corticospinal collaterals. For this purpose two different fluorescent tracers were injected: one in the spinal cord and the other contralaterally in the lateral thalamus and in the mesencephalon respectively. In these experiments Fast Blue and True Blue were used in combination with Nuclear Yellow. Evans Blue was used in combination with Granular Blue. After injections of the tracers into the thalamus and spinal cord two different populations of single retrogradely labeled cortical neurons were found, while after injections in mesencephalon and spinal cord double-labeled cortical neurons occurred. This has lead to the conclusion that in cat and rat corticospinal neurons do not distribute collaterals to specific thalamic nuclei, but do distribute collaterals to mesencephalon. Moreover, the preferential distribution of the double-labeled corticospinal neurons in cat suggest that the corticospinal neurons distributing collaterals to the mesencephalon in part are concentrated in those cortical areas which subserve the steering of movements of the head, neck and trunk.