Pain and itch perception in human limbic system

Abstract

Both electrophysiological studies such as magnetoencephalography (MEG) and hemodynamic studies such as functional magnetic resonance imaging (fMRI) are intensively being used to elucidate underlying mechanisms of human pain and itch perception. MEG following A-delta (first pain) and C fiber stimulation (second pain) were similar except for a longer latency for the latter. At first, primary somatosensory cortex (SI) contralateral to the stimulation is activated and then secondary somatosensory cortex (SII), insula, amygdala and anterior cingulate cortex (ACC) in the bilateral hemispheres are activated sequentially. As for findings using fMRI, the stimulation of both C and A-delta fibers activated the bilateral thalamus, bilateral SII, right (ipsilateral) middle insula, and bilateral Brodmann's area (BA) 24/32, with the majority of activity found in the posterior portion of the ACC. However, magnitude of activity in the BA32/8/6, including ACC and pre-supplementary motor area (pre-SMA), and the bilateral anterior insula was significantly stronger following the stimulation of C nociceptors than A-delta nociceptors. Findings following itch stimulation were similar to those following pain stimulation, but the precuneus may be itch selective brain region. This unique finding was confirmed by both MEG and fMRI studies.