Localization of last-order premotor interneurons in the lumbar spinal cord of rats

There is strong evidence that neural circuits underlying certain rhythmic motor behaviors are located in the spinal cord. Such local central pattern generators are thought to coordinate the activity of motoneurons through specific sets of last-order premotor interneurons that establish monosynaptic contacts with motoneurons. After injections of biotinylated dextran amine into the lateral and medial motor columns as well as the ventrolateral white matter at the level of the upper and lower segments of the lumbar spinal cord, we intended to identify and localize retrogradely labelled spinal interneurons that can likely be regarded as last-order premotor interneurons in rats. Regardless of the location of the injection site, labelled interneurons were revealed in laminae V–VIII along a three- or four-segment-long section of the spinal gray matter. Although most of the stained cells were confined to laminae V–VIII in all cases, the distribution of neurons within the confines of this area varied according to the site of injection. After injections into the lateral motor column at the level of the L4–L5 segments, the labelled neurons were located almost exclusively in laminae V–VII ipsilateral to the injection site, and the perikarya were distributed throughout the entire mediolateral extent of this area. Interneurons projecting to the lateral motor column at the level of the L1–L2 segments were also located in laminae V–VII, but most of them were concentrated in the middle one-third or in the lateral half of this area. Following injections into the medial motor column at the level of the L1–L2 segments, the majority of labelled neurons were confined to the medial aspect of laminae V–VII and lamina VIII, and the proportion of neurons that were found contralateral to the injection site was strikingly higher than in the other experimental groups. The results suggest that the organization of last-order premotor interneurons projecting to motoneurons, which are located at different areas of the lateral and medial motor columns and innervate different muscle groups, may present distinct features in the rat spinal cord. J. Comp. Neurol. 389:377–389, 1997. © 1997 Wiley-Liss, Inc.