Where do nociceptive fibers enter the spinal cord?
Nociceptive primary afferent fibers have their cell bodies in dorsal root ganglia (or trigeminal ganglia for the face). The central branches of these afferents enter the spinal cord through the dorsal root and ascend or descend a few segments in the tract of Lissauer. The central branches terminate predominantly in the superficial laminae of the dorsal horn, including lamina I, the marginal zone, and lamina II, the substantia gelatinosa. Some A delta primary afferent nociceptors also terminate more ventrally in the region of lamina V and around the central canal.
The fact that the level of analgesia observed after anterolateral cordotomy may be up to two segments below the segment at which the cordotomy was performed is presumed to reflect the anatomic course of axons in Lissauer tract. Some small-diameter primary afferents ascend the spinal cord one to two segments in the Lissauer tract, ipsilaterally, before entering the spinal cord and synapsing upon dorsal horn neurons, including cells at the origin of the spinothalamic, spinoreticular and spinoparabrachial pathways