The tract of Lissauer and the dorsal root potential.
1. Intersegmental dorsal root potentials (d.r.p.s) have been recorded in the lumbar spinal cord of spinalized cats under Na pentobarbitone anaesthesia, to investigate the spinal cord structures involved in the intersegmental transmission of d.r.p.s. 2. A technique has been developed for restricted surgical isolation of Lissauer's tract between the segments with subsequent histological verification of the extent of the isolation. 3. Section of the ipsilateral dorsal column resulted in an increase in the latency of the intersegmental d.r.p. and a significant reduction in its amplitude. A further reduction in amplitude was achieved by section of the ipsilateral dorso-lateral funiculus. 4. Neither section of Lissauer's tract after the above lesions were performed nor restricted Lissauer's tractotomies abolished intersegmental d.r.p.s; only a small reduction in the amplitude of the d.r.p. was obtained. 5. It is proposed that intersegmental d.r.p.s are produced by the activaiton of a propriospinal system projecting through pathways other than Lissauer's tract and that primary afferent collaterals from the dorsal columns make a major contribtuion to their generation. The contribution made by Lissauer's tract is probably small.