ANATOMICAL EVIDENCE FOR IPSILATERAL COLLICULAR PROJECTIONS TO THE SPINAL-CORD IN THE CAT

Injections of WGA-HRP were made within the C1 segment of spinal cord i n cats with a midsagittal section of the midbrain. A small number of l abelled cells were found in the latero-caudal part of the deeper layer s of the superior colliclus (SC) ipsilateral to the injection sites. B ecause of the complete section of the dorsal tegmental decussation, th ese results definitively demonstrate the existence of an ipsilateral t ecto-spinal pathway projecting to upper cervical segments in the cat. Ipsilaterally projecting tecto-reticulo-spinal neurons represent about 5% of the total population of tectospinal neurons. They were exclusiv ely located in the deeper collicular layers and most of them were foun d in the latero-caudal part of the SC. Comparison with our previous st udies suggests that more ipsilateral tectospinal projections that foun d after the section of the dorsal tegmental decussation probably exist . They may arise from tecto-reticulo-spinal neurons recrossing the mid line in the brainstem or in the rostral part of C1. By analogy with th e cortico-spinal tract, we suggest that the existence of an ipsilatera l tecto-spinal pathway can be regarded as evidence for a substantial d evelopment of the cat tecto-spinal system as compared with other mamma ls.