Brain processing of the signals ascending through unmyelinated C fibers in humans: an eventrelated functional magnetic resonance imaging study

Event-related functional magnetic resonance imaging was used to investigate brain processing of the signals ascending from periph-eral C and Ad fibers evoked by phasic laser stimuli on the right hand in humans. The stimulation of both C and Ad nociceptors activated the bilateral thalamus, bilateral secondary somatosensory cortex, right (ipsilateral) middle insula, and bilateral Brodmann’s area (BA) 24/32, with the majority of activity found in the posterior portion of the anterior cingulate cortex (ACC). However, magnitude of activity in the right (ipsilateral) BA32/8/6, including dorsal parts in the anterior portion of the ACC (aACC) and pre-supplementary motor area (pre-SMA), and the bilateral anterior insula was significantly stronger following the stimulation of C nociceptors than Ad nociceptors. It was concluded that the activation of C nociceptors, related to second pain, evokes different brain processing from that of Ad nociceptors, related to first pain, probably due to the dif-ferences in the emotional and motivational aspects of either pain, which are mainly related to the aACC, pre-SMA, and anterior insula