NADPH–diaphorase reveals presumptive sympathetic primary afferents in the developing human spinal cord

Numerous studies have elucidated two visceral afferent pathways in the spinal cord of mammals, the lateral collateral pathway (LCP) and the medial collateral pathway (MCP). The present study utilized NADPH-diaphorase histochemistry to visualize afferent pathways in the developing human thoracolumbar spinal cord. Diaphorase-positive fiber bundles, strikingly similar to the previously defined LCP and MCP, were observed coursing along the lateral and medial aspects of the dorsal horn to the base of the dorsal horn, the intermediate gray, and/or the dorsal commissure. Furthermore, some axons forming the MCP crossed in the dorsal commissure to the contralateral side of the spinal cord. In addition, axons projecting in the LCP often appeared to terminate within clusters of diaphorase-labeled sympathetic preganglionic neurons, supporting the concept that monosynaptic connections may exist between primary afferents and autonomic motor neurons. Thus, nitric oxide may be involved in both afferent and efferent neurons in reflex pathways of the human sympathetic nervous system.