Primary afferent fibers in the tract of Lissauer in the rat

More than two‐thirds of the axons in the tract of Lissauer at mid‐thoracic and lumbosacral levels of the rat spinal cord are primary afferent fibers. The proportions of primary afferents in the tract are approximately the same at the two spinal levels. A slightly higher percentage of the unmyelinated, as opposed to the myelinated, fibers are primary afferents. There is a somewhat greater percentage of primary afferent axons in medial parts of the tract in mid‐thoracic levels, but all areas of the tract that were examined contain a majority of primary afferent fibers. The primary afferent axons appear to travel less than a segment in the tract at mid‐thoracic levels but for several segments in the tract at lumbo‐sacral levels. These data indicate that the tract of Lissauer is predominately a primary afferent fiber system in these segments of the rat.