Electrophysiologic recording and thermodynamic modeling demonstrate that helium-neon laser irradiation does not affect peripheral Aδ- or C-fiber nociceptors

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Abstract
The effect of helium-neon laser irradiation (632.5 nm) on A delta- and C-fiber sensory afferents was investigated in the rabbit cornea, to determine the physiologic basis for reports that low power (0.1-5 mW) helium-neon (He-Ne) lasers produce acute analgesia and alleviate chronic pain. Multiple and single unit extracellular recordings from nociceptive corneal afferent nerves (C-fiber cold, C-fiber chemical, A delta mechanical and A delta bimodal) were used to study the effects of He-Ne laser radiation upon the electrophysiologic responses to mechanical, thermal, chemical and electrical stimulation of the cornea. Action potentials were analyzed for latency, amplitude, rise time, duration and frequency. Exposure of the neural receptive field and/or nerve bundle to a 4-mm diameter He-Ne laser (0-5 mW; 0-1800 sec) did not alter spontaneous or evoked neural activity. In addition, single unit action potential parameters were not altered by laser irradiation. Modeling of thermal changes produced by He-Ne radiation on corneal nerves indicated that effects predicted for receptor and axonal depths in both skin and cornea were minimal (less than 0.15 degrees C) and unlikely to alter sensory transduction or transmission.

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