Collateralization in the spinothalamic tract: new methodology to support or deny phylogenetic theories

Components of the spinothalamic system that ascend in the anterolateral funiculus are reviewed. The presence of collateralization in this system in mammals is discussed with regard to theories of the phylogenetic development of pathways. The major theory investigated suggested that collateralization is an intermediate stage between a multisynaptic pathway and a direct non-collateralized lemniscus. The evidence and theories are reviewed. Methods for confirming or rejecting this theory are discussed. The literature reporting ascending spinal projections for non-mammalian vertebrates is reviewed. Certain reptiles have projections analogous to both the mammalian neospinothalamic and paleospinothalamic tracts. The presence of spinothalamic projections in elasmobranchs and amphibians is still controversial. Confirmation of earlier reports of projections in salamander and dogfish shark based on degeneration techniques have not been done. In addition, results from too few species of these classes have been reported. However, it is possible that paleospinothalamic connections are present in some species (e.g. salamander, nurse shark) and not in others (e.g. frog, dogfish shark) of the same class. Spinothalamic projections have not been reported for teleosts. A plea for new research in this area is made.