Cerebral responses following stimulation of unmyelinated C-fibers in humans: electro- and magneto-encephalographic study

There are two kinds of pain, a sharp pain ascending through Aδ fibers (first pain) and a second burning pain ascending though C fibers (second pain). By using a novel method, the application of a low intensity CO 2 laser beam to a tiny area of skin using a very thin aluminum plate with numerous tiny holes as a spatial filter, we succeeded in selectively stimulating unmyelinated C fibers of the skin in humans, and could record consistent and clear brain responses using electroencephalography (EEG) and magnetoencephalography (MEG). The conduction velocity (CV) of the C fibers of the peripheral nerve and spinal cord, probably spinothalamic tract (STT), is approximately 1–4 m/s, which is significantly slower than that of Aδ (approximately 10–15 m/s) and Aβ fibers (approximately 50–70 m/s). This method should be very useful for clinical application. Following C fiber stimulation, primary and secondary somatosensory cortices (SI and SII) are simultaneously activated in the cerebral hemisphere contralateral to the stimulation, and then, SII in the hemisphere ipsilateral to the stimulation is activated. These early responses are easily detected by MEG. Then, probably limbic systems such as insula and cingulate cortex are activated, and those activities reflected in EEG components. Investigations of the cortical processing in pain perception including both first and second pain should provide a better understanding of pain perception and, therefore, contribute to pain relief in clinical medicine.