The topographic organization of spinal afferents to the lateral reticular nucleus of the cat

The topographic organization of spinal afferents to the lateral reticular nucleus (LRN) has been reexamined in 34 adult cats. Two modifications of the Nauta technique were used to show secondary terminal degeneration resulting from circumscribed lesions at various levels of the spinal cord. The results demonstrate that the “inner segment” of LRN corresponding roughly to Brodal's magnocellular portion, receives fibers from spinal segments C 1 –D 3 , while the “outer segment” corresponding approximately to Brodal's parvocellular protion receives fibers from spinal levels below L 3 . The “middle segment” — a transitional zone between the two former portions — represents the spinal segments D 4 –L 3 . The ascending fibers terminate predominantly on the ipsilateral side; only few degenerated elements are noted within the contralateral nucleus. The subtrigeminal portion does not seem to receive afferent fibers from the spinal cord. This finding raises the question of nomenclature, which is briefly discussed in the light of the classical “nucleus funiculi lateralis” concept. Finally, the present data are consistent with electrophysiological data of Oscarsson, Rosén and collaborators according to which LRN represents spinal levels of convergent inputs from large as well as bilateral receptive fields rather than somatotopically arranged projections from peripheral sense organs.