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 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.