Conduction velocity of the spinothalamic tract following CO 2 laser stimulation of C-fibers in humans

Abstract

Pain-related somatosensory-evoked potential following CO 2 laser stimulation (laser-evoked potential (LEP)) is now used not only for research objectives, but also for clinical applications. Estimating the conduction velocity (CV) of the spinothalamic tract (STT) by analyzing LEP following activation of Aδ-fibers (Aδ-CVSTT) by CO 2 laser stimulation has been performed previously, but estimating the CV of STT following activation of C-fibers (C-CVSTT) has not. This is the first report to estimate the C-CVSTT in humans; by using the novel method of CO 2 laser stimulation applied to tiny skin areas. The calculation method was based on that of Kakigi and Shibasaki (Electroenceph clin Neurophysiol 80 (1991) 39) who measured Aδ-CVSTT by conventional CO 2 laser stimulation. The C-CVSTT ranged between 1.4 and 4.0 m/s, and its mean±SD was 2.9±0.8 m/s. This C-CVSTT was significantly slower than the Aδ-CVSTT, which ranged approximately from 10 to 21 m/s. The nociceptive signal of the C-fibers in STT is probably conveyed by unmyelinated axons of projection neurons to reach the thalamus. Our findings provide the first physiological evidence of the signals ascending through unmyelinated axons in the spinal cord in humans. In addition, estimating C-CVSTT and Aδ-CVSTT combined with conventional methods to measure the CV of the posterior column using electrical stimulation should be useful and have important clinical applications, particularly in patients with spinal cord lesions showing various kinds of sensory disturbances.