The organisation of the spinotectal projection. An experimental study in the rat

The spinal grey projects upon the contralateral superior colliculus via two overlapping pathways in the lateral funiculus. One, more ventrally placed, is the classical spinotectal tract, which crosses immediately in the spinal cord and remains crossed. The other, lying more dorsally, ascends homolaterally but is subject to delayed crossing in the brainstem (especially the intertectal) commissures to reach the contralateral colliculus. Both sets of fibers end only in the caudal half of the colliculus, predominantly in the stratum album intermedium. The projections from individual cord segments distribute in an ordered fashion as a series of transverse, overlapping bands, the cervical cord projecting most rostrally, the sacro-coccygeal most caudally. Additional fibers, which distribute with less marked topography, end in the lateral extreme of the stratum griseum profundum and stratum album profundum.