Propriospinal afferent and efferent connections of the lateral and medial areas of the dorsal horn (Laminae I-IV) in the rat lumbar spinal cord

The different subdivisions along the mediolateral extent of the superficial dorsal horn of the spinal cord are generally regarded as identical structu res that execute the function of sensory information processing without any significant communication with other regions of the spinal gray matter. In contrast to this standing, here we endeavor to show that neural assemblies along the mediolateral extent of laminae I-IV cannot be regarded as identi cal structures. After injecting Phaseolus vulgaris leucoagglutinin and biot inylated dextran amine into various areas of the superficial dorsal horn (l aminae I-IV) at the level of the lumbar spinal cord in rats, we have demons trated that the medial and lateral areas of the superficial dorsal horn sho w the following distinct features in their propriospinal afferent and effer ent connections: 1) A 300- to 400-mu m-long section of the medial aspects o f laminae I-IV projects to and receives afferent fibers from a three segmen t long compartment of the spinal dorsal gray matter, whereas the same lengt h of the lateral aspects of laminae I-IV projects to and receives afferent fibers from the entire rostrocaudal extent of the lumbar spinal cord. 2) Th e medial aspects of laminae I-IV project extensively to the lateral areas o f the superficial dorsal horn. In contrast to this, the lateral areas of la minae I-IV, with the exception of a few fibers at the segmental level, do n ot project back to the medial territories. 3) There is a substantial direct commissural connection between the lateral aspects of laminae I-IV on the two sides of the lumbar spinal cord. The medial part of laminae I-IV, howev er, does not establish any direct connection with the gray matter on the op posite side. 4) The lateral aspects of laminae I-IV appear to be the primar y source of fibers projecting to the ipsi- and contralateral ventral horns and supraspinal brain centers. Projecting fibers arise from the medial subd ivision of laminae I-IV in a substantially lower number. The findings indic ate that the medial and lateral areas of the superficial spinal dorsal horn of rats may play different roles in sensory information processing. (C) 20 00 Wiley-Liss, Inc.