From "Brain Gut Microbiome Interactions and Functional Bowel Disorders":
In response to external and bodily demands, the brain modulates individual cells (ECC – enterochromaffin cells; SMC – smooth muscle cells; ICC – interstitial cells of Cajal) within this network via the branches of the autonomic nervous system (ANS) (sympathetic and parasympathetic/vagal efferents) and the hypothalamic pituitary adrenal (HPA) axis.
What is the meaning of "external demands"? What other possible demands can the brain strive to meet by modulating the activity of the neural network of the gut?