5.36 - Dorsal Columns and Visceral Pain

Abstract

This chapter reviews evidence that the dorsal column contains a visceral pain pathway. Clinical evidence is from the results of a variety of neurosurgical procedures to relieve visceral pain, including anterolateral cordotomy, commissural myelotomy, stereotactic C1 central myelotomy, and limited midline myelotomy. Experimental studies have shown that there are several visceral nociceptive pathways in the spinal cord, including the spinothalamic and spinoreticular tracts. However, a component of the postsynaptic dorsal column pathway appears to be the most significant. It has been known since the early 1950s that visceral information is conveyed by the dorsal column, but it was not evident until recently that this included nociceptive visceral signals. Studies have now shown that nociceptive visceral responses recorded from neurons of the rat and monkey ventral posterior lateral thalamic nucleus or of the rat dorsal column nuclei can be blocked by a lesion of the dorsal column. Similarly, behavioral responses of rats to noxious visceral stimuli are also prevented by such lesions, as are changes in regional cerebral blood flow in monkeys, as measured by fMRI. Evidence that postsynaptic dorsal column neurons conduct this visceral nociceptive information includes the observation that intrathecal or microdialysis administration of morphine or of a non- N-methyl- d-aspartic acid glutamate receptor antagonist into the spinal cord interferes with the responses of thalamic neurons to noxious visceral stimuli. The projections of the postsynaptic dorsal column neurons in the sacral spinal cord ascend near the midline of the dorsal column, as predicted from the clinical results. However, the axons of postsynaptic dorsal column neurons in the mid-thoracic spinal cord ascend near the dorsal intermediate septum. This means that although midline lesions of the dorsal column can relieve pelvic visceral pain, lesions designed to eliminate pain from abdominal organs would have to be placed more laterally in the rostral spinal cord or near the dorsal root entry zone at thoracic levels. Further evidence that noxious visceral stimuli have a powerful effect on postsynaptic dorsal column neurons includes the observation that colon inflammation results in the up-regulation of neurokinin-1 receptors in these neurons, which normally lack such receptors, and that distention of the ureter results in Fos expression in many postsynaptic dorsal column neurons, as well as in spinothalamic cells.