[Mechanisms of intracerebral pain and itch perception in humans]

Electrophysiological studies involving techniques such as magnetoencephalography (MEG) and hemodynamic studies involving techniques such as functional magnetic resonance imaging (fMRI) have recently been intensively used to elucidate the mechanisms underlying pain and itch perception in humans. The MEG results obtained after A-delta fiber (first pain) and C fiber (second pain) stimulation were similar, except for longer latency in the case of C fibers. Initially, the primary somatosensory cortex (SI) contralateral to the stimulation is activated, and the secondary somatosensory cortex (SII), insula, amygdala, and anterior cingulate cortex (ACC) in both hemispheres are then activated sequentially. The fMRI findings obtained after the stimulation of C fibers and those obtained after the stimulation of A-delta fibers both showed activation of the bilateral thalamus, bilateral SII, right (ipsilateral) middle insula, and bilateral Brodmann's area (BA) 24/32, with most of the activity being detected in the posterior region of the ACC. However, the magnitude of activity in the anterior insula on both sides and in BA 32/8/6, including the ACC and pre-supplementary motor area (pre-SMA), after the stimulation of C nociceptors was significantly stronger than that after the stimulation of A-delta nociceptors. We have recently developed a new stimulation electrode that causes an itching sensation via electrical stimulation applied to skin. The conduction velocity (CV) of the signals caused by this stimulation is approximately 1 m/sec in a range of CV of C fibers. The findings obtained after itch stimulation were similar to those obtained after pain stimulation, but the precuneus may be an itch-selective brain region. This unique finding was confirmed by both MEG and fMRI studies.