Differences In Spinal Pathways

Once transmission has occurred at a spinal level, the information must be passed to higher sites of processing. There is good evidence that visceral pain follows pathways that are different from those used for the perception of superficial pain. There now exist at least 10 clinical reports from six different neurosurgical groups in the United States, Europe, and Asia who have demonstrated that a midline myelotomy of the spinal cord (ablation of dorsal midline region) produces analgesia for visceral pain related to pelvic and lower abdominal organs (30-37) and for upper abdominal organs such as the stomach, pancreas, and hepatobiliary systems (38,39). Traditionally, it has been taught that the primary pathways for pain-related information from the dorsal horn of the spinal cord to the brain are via the ventrolateral quadrant white matter of the spinal cord. Tracts located within the ventrolateral quadrant include the classic spinothalamic and spinoreticular tracts as well as the spinomesencephalic and spi-nohypothalamic tracts. The ventrolateral quadrant of the spinal cord is clearly important for cutaneous pain sensation because lesions of those areas of white matter lead to pinprick analgesia in contralateral dermatomes below the level of the lesion. It is for this reason that the observation that surgical lesions of the dorsal midline of the spinal cord produce clinical analgesia was considered so contrary to dogma. Fortunately, there are good basic science data to support these clinical observations. In primates, dorsal midline lesions reduce the activity of thalamic neurons evoked by colorectal distension (40). In rats, effects of similar lesions have been demonstrated to reduce or abolish thalamic neuronal responses and/or behavioral responses to colorectal distension (30,41), duodenal distension (42), pancreatic stimulation (43), and hypersensitivity following lower extremity osteotomy (44). Whereas dorsal midline lesions affect visceral inputs to the nucleus gracilis of the medulla (45), these lesions do not affect visceral inputs to the ventrolateral medulla (41). Hence, it would appear that the dorsal midline pathway is one of at least two ascending pathways important to the perception of visceral pain. Spinal neurons with viscerosomatic convergence and axonal extensions into the dorsal columns have been demonstrated for primates (46) and rats (30).

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