Primary afferent and spinal sensory neurons that respond to brief pulses of intense infrared laser radiation: A preliminary survey in rats

Thermal stimulation with intense pulses of CO₂ laser radiation has recently come into use as a method for generating robust cerebral evoked responses in man. Because the heat transient involved (at least 200°C/s) is at least an order of magnitude greater than that of most conventional thermal stimulators, we checked whether or not there might be anomalous activation of fiber types other than the well known cutaneous thermoreceptors. Recordings were made from primary afferent fibers in the rat sciatic nerve and second order somatosensory cells in the dorsal horn. Most of the laser-sensitive afferent fibers sampled were C polymodal nociceptors with lesser representation of other thermoreceptor types. There were no instances in which low threshold mechanoreceptors or other nonthermal afferent fibers were engaged. We conclude that the advantages of infrared laser stimulation are not compromised by a loss of receptor specificity.