A study of fiber systems within the spinal cord of the domestic pig that subserve pain

A total of 21 domestic pigs were used in a study of spinal cord fiber systems which subserve a pain response in the pig. Distribution of fiber systems were determined by the placement of various lesions into the spinal cord and examining the animal for its ability to give a complete pain response. The Swank‐Davenport modification of the Marchi technique for staining degenerating myelnated fibers was used to determine the pattern of trajectory of myelinated fibers along the spinal cord. Lesions were made at the cervical level in three animals and resulting pattern of degenerating fibers was compared with those of the other animals in which the lesions had been placed at lower thoracic levels. In most animals the lesions were limited to a single site on the spinal cord, but in six animals staggered, bilateral hemisections were made to determine whether or not these fiber systems are as diffuse in the pig as has been reported for the cat. The spinal cord fiber systems which subserve a pain response in the pig are located generally within the ventrolateral quadrant of the spinal cord. It was found that only a small portion of one lateral funiculus just ventral to the dorsal and ventral spinocerebellar tracts, if left intact, was sufficient for the elicitation of a pain response from either rear limb. The results presented here indicate that the fiber systems in the spinal cord of the pig which subserve pain responses are more diffuse than in carnivora or in primates.