Brain imaging of mechanically induced muscle versus cutaneous pain

This study aimed to investigate the differences in the brain responses between muscle versus skin pain, both of which were caused by tonic mechanical stimuli. Using local anesthesia (LA), we induced muscle pain without any accompanying cutaneous sensation. Subjects underwent functional magnetic resonance imaging while tonic pressure was applied to the right calf under the following four conditions: (1) non-painful pressure without LA (causing mechanoreceptive skin and muscle stimulation); (2) painful pressure without LA (causing nociceptive skin stimulation and mechanoreceptive skin and muscle stimulation); (3) non-painful pressure with LA (causing mechanoreceptive muscle stimulation); (4) painful pressure with LA (causing nociceptive and mechanoreceptive muscle stimulation). Although there was no brain region specifically activated by nociceptive muscle stimuli, activation in the following regions was observed specifically during nociceptive muscle stimuli: anterior midcingulate cortex, anterior and posterior insular cortex, lentiform nucleus, thalamus, pre-supplementary motor area, dorsolateral prefrontal cortex, and inferior parietal lobule. This indicates that there is no region specific for muscle pain but activation pattern or network specific for muscle pain. Furthermore, secondary somatosensory cortex (S2) was found to be responsive to cutaneous pain, not muscle pain, because S2 was specifically activated by nociceptive cutaneous stimuli.