Descending Visceral Pain Pathway calamus PAG Locus coeruleus Caudal raphe nucleus Amygdala Noradrenergic Rostral ven serotonergic medulla Colon OpioidergicDescending Visceral Pain Pathway Thalamus PAG Locus coeruleus Amygdala Colon Serotonergic Noradrenergic Caudal raphe nucleus Opioidergic Rostral ventral medulla