Position of spinothalamic tract axons in upper cervical spinal cord of monkeys

Percutaneous upper cervical cordotomy continues to be performed on patients suffering from several types of severe chronic pain. It is believed that the operation is effective because it cuts the spinothalamic tract (STT), a primary pathway carrying nociceptive information from the spinal cord to the brain in humans. In recent years, there has been controversy regarding the location of STT axons within the spinal cord. The aim of this study was to determine the locations of STT axons within the spinal cord white matter of C2 segment in monkeys using methods of antidromic activation. Twenty lumbar STT cells were isolated. Eleven were classified as wide dynamic range neurons, six as high-threshold cells, and three as low-threshold cells. Eleven STT neurons were recorded in the deep dorsal horn and nine in superficial dorsal horn. The axons of the examined neurons were located at antidromic low-threshold points (<30 μA) within the contralateral lateral funiculus of C2. All low-threshold points were located ventral to the denticulate ligament, within the lateral half of the ventral lateral funiculus (VLF). None were found in the dorsal half of the lateral funiculus. The present findings support our previous suggestion that STT axons migrate ventrally as they ascend the length of the spinal cord. Also, the present findings indicate that surgical cordotomies that interrupt the VLF in C2 likely disrupt the entire lumbar STT.