PYRAMIDAL TRACT IN SPINAL CORD OF CAT

Abstract

There are 2 groups of fibers in the pyramidal tract of the cat. The 1st includes fibers with an average velocity range of 22-70 m.p.s. (peak 50 m.p.s.) and the 2d 8-22 m.p.s. (peak 14 m.p.s.). Both these groups can be traced down the cord in the contralateral corticospinal tract as far as the 3d lumbar segment, and the larger group to the 4th. There is no indication of a massive outfall of fibers in the segments giving rise to the brachial plexus, although some larger fibers appear to terminate in this region. No fibers appear to terminate between T3 and L1 segments. The 2 groups are not anatomically segregated in bulbar pyramid or lateral corticospinal tract. No electrophysiological evidence was found for the presence of unmyelinated fibers in the tract. It is suggested that the small fibers in the tract of the order of 1u. diameter may be finely myelinated. There are few, if any, homolateral fibers in the lateral corticospinal tract of the cat. A distinctive tract of large bulbospinal fibers, which runs with the medial longitudinal fasciculus and tectospinal tract in the medulla, and travels in the ipsilateral ventral funiculus to the lumbosacral region of the cord, was investigated. It contains fibers with a range in conduction velocity of 95-165 m.p.s. (peak 125 m.p.s.). Injury potentials, recorded with a microelectrode within the tract, and recordings from the surface of the cord are described and correlated. The advantages of deliberately achieving complete injury block (in recording tract activity with a micro-electrode) are stressed.