Effect of electrical stimulation of peripheral nerves on neuropathic pain.

STUDY DESIGN: Changes in the electrophysiologic response of spinal dorsal horn neurons elicited by peripheral electrical stimulation were examined. OBJECTIVE: To investigate whether the electrical stimulation of peripheral nerves causes an inhibition of pain at the spinal cord level. SUMMARY OF BACKGROUND DATA: The wide dynamic range neurons studied were known to be excited by primary afferent fibers, not only combined A (delta) and C nociceptive fibers, but also low-threshold mechanoreceptive A (beta) fibers and A (delta) fibers of down hairs. The wide dynamic range neurons are classified as nociceptive neurons. METHODS: Responses of wide dynamic range neurons in the lumbosacral dorsal horn to input from C fibers were studied in urethane chloralose-anesthetized cats. The posterior tibial nerve and sciatic nerve were stimulated simultaneously to examine the effect on the C fiber responses elicited by superficial peroneal nerve stimulation. RESULTS: Simultaneous stimulation of the posterior tibial nerve and sciatic nerve was performed with superficial peroneal nerve C fiber stimulation. CONCLUSIONS: This study demonstrated that electrical stimulation of peripheral nerves leads to inhibitory input to the pain pathways at the spinal cord level.

Changes in responses of wide dynamic range neurons in the spinal dorsal horn after dorsal root or dorsal root ganglion compression.

STUDY DESIGN: The electrophysiologic responses of wide dynamic range neurons in the spinal dorsal horn by compression of the dorsal root or of the dorsal root ganglion were investigated. OBJECTIVES: This study identified differences between the compression of the dorsal root against the compression of the dorsal root ganglion by examining the responses of wide dynamic range neurons. SUMMARY OF BACKGROUND DATA: The wide dynamic range neurons studied were known to be excited by primary afferent fibers, not only combined A delta and C-nociceptive fibers but also low threshold mechanoreceptive A beta fibers and A delta fibers of down hairs. Thus, the wide dynamic range neurons are classified as nociceptive neurons. METHODS: Extracellular activities of 32 wide dynamic range neurons were recorded from the laminae 4-6 of the seventh lumbar cord in anesthetized cats. A microvessel clip (40 g) was applied to compress the dorsal root or the dorsal root ganglion. RESULTS: Dorsal root compression produced only an initial burst (about 10-40 seconds). Prolonged repetitive firings were rarely maintained. In contrast, dorsal root ganglion compression resulted in a maintained repetitive firing throughout the period of compression. After release of compression of the dorsal root and the dorsal root ganglion, responses to brushing were facilitated, and the low threshold center of the receptive field expanded. CONCLUSIONS: These findings are consistent with the previous report that the radicular pain associated with a herniated intervertebral disc initially results from compression of the dorsal root ganglion.

C fiber responses of wide dynamic range neurons in the spinal dorsal horn.

Responses of wide dynamic range neurons in the spinal dorsal horn to input from C fibers with various conduction velocities were analyzed. The wide dynamic range neurons studied were located in the laminae IV-VI of the spinal dorsal horn. The C fiber response to stimulation of the superficial peroneal nerve consisted of three components: early, middle, and late. The separation into three components was found to be caused by asynchronous volleys in three different classes of C fibers in the superficial peroneal nerve. The phenomenon of windup was observed to occur always in the late component, frequently in the middle component, and to a far lesser extent in the early component. The early component was augmented by sciatic nerve compression, indicating that the initial part of the C fiber response is suppressed within the spinal cord by an inhibitory effect of A fiber afferent volleys. An intravenous administration of a specific N-methyl-D-aspartate receptor antagonist, MK-801 hydrogen maleate, significantly suppressed the middle and late components of the C fiber response, although the effect on the early component was insignificant. Thus, one can infer that the hyperexcitability of wide dynamic range neurons to input from early components of cutaneous C fiber stimuli will yield hyperpathic symptoms such as paresthesia, hyperalgesia, and allodynia associated with damaged peripheral nerves, and that learning how to inhibit the hyperexcitability of wide dynamic range neurons to input from early components of C fiber stimuli will lead to the curative treatment for neuropathic pain.