Cerebral activation during anal and rectal stimulation

While the rectum is innervated by visceral afferents, the anal canal is inn ervated by the somatosensory pudendal nerve. The representation of these tw o central domains of intestinal sensations in the human brain is largely un known. Nonpainful pneumatic stimulation of the anal canal and the distal re ctum using event-related functional magnetic resonance imaging (fMRI) was p erformed in eight healthy subjects. Subjective scaling of sensations reveal ed no differences in unpleasantness and pain during both stimuli. Both type s of stimuli revealed MM activation in secondary somatosensory, insula, cin gular gyrus, left inferior parietal, and right orbitofrontal cortex. Anal s timulation resulted in additional activation of primary sensory and motor c ortex, supplementary motor area, and left cerebellum. We concluded that vis cerorectal and somatosensory anal stimulation predominantly differ in their primary sensory activation and additional activation in motor areas. This motor response following aversive somatosensory stimuli may be caused by a reflexive avoidance reaction which is not observed after the more diffuse e xperienced visceral stimulation. (C) 2001 Academic Press.