Activation of neurons of secretomotor pathways by noxious stimulants (e.g., bacteria, viruses, and bacterial toxins). The noxious
agents activate neurons of secretomotor pathways to provide a physiological response, stimulation of secretion that is accompanied by propulsive
activity to remove the noxious agent. Sustained activation of these pathways produces a pathological overresponse resulting in secretory diarrhea.
Noxious stimuli also activate neurons indirectly through the release from the epithelium of 5-hydroxytryptamine (5-HT) and PG that act on their
respective receptors (5-HTr and PGr) on nerve endings. Entrochromaffin cells are shown in green (dark gray); enterocytes (mucosal epithelial
cells) are shown in pink (light gray). Color version available in the online PDF